Patent Application
20090273825
1. Field of the Invention
The present invention relates to a mirror device (also called “digital micromirror device” or “micromirror device”) comprised in a projection apparatus, et cetera.
2. Description of the Related Art
Even though there are significant advances made in recent years on the technologies of implementing electromechanical micromirror devices as spatial light modulators, there are still limitations and difficulties when they are employed to provide high quality image displays. Specifically, when the display images are digitally controlled, the image qualities are adversely affected due to the fact that the image is not displayed with a sufficient number of gray scales.
Electromechanical micromirror devices have drawn considerable interest because of their application as spatial light modulators (SLMs). A spatial light modulator requires an array of a relatively large number of micromirror devices. In general, the number of devices required ranges from 60,000 to several million for each SLM. Referring to
The on-and-off states of micromirror control scheme as that implemented in the U.S. Pat. No. 5,214,420 and by most of the conventional display system imposes a limitation on the quality of the display. Specifically, when applying conventional configuration of control circuit has a limitation that the gray scale of conventional system (PWM between ON and OFF states) is limited by the LSB (least significant bit, or the least pulse width). Due to the On-Off states implemented in the conventional systems, there is no way to provide shorter pulse width than LSB. The least brightness, which determines gray scale, is the light reflected during the least pulse width. The limited gray scales lead to degradations of image display.
Specifically, in
The dual-state switching as illustrated by the control circuit controls the micromirrors to position either at an ON of an OFF angular orientation as that shown in
When adjacent image pixels are shown with great degree of different gray scales due to a very coarse scale of controllable gray scale, artifacts are shown between these adjacent image pixels. That leads to image degradations. The image degradations are specially pronounced in bright areas of display when there are “bigger gaps” of gray scales between adjacent image pixels. It was observed in an image of a female model that there were artifacts shown on the forehead, the sides of the nose and the upper arm. The artifacts are generated due to a technical limitation that the digital controlled display does not provide sufficient gray scales. At the bright spots of display, e.g., the forehead, the sides of the nose and the upper arm, the adjacent pixels are displayed with visible gaps of light intensities.
As the micromirrors are controlled to have a fully on and fully off position, the light intensity is determined by the length of time the micromirror is at the fully on position. In order to increase the number of gray scales of display, the speed of the micromirror must be increased such that the digital control signals can be increased to a higher number of bits. However, when the speed of the micromirrors is increased, a strong hinge is necessary for the micromirror to sustain a required number of operational cycles for a designated lifetime of operation, In order to drive the micromirrors supported on a further strengthened hinge, a higher voltage is required. The higher voltage may exceed twenty volts and may even be as high as thirty volts. The micromirrors manufacture by applying the CMOS technologies probably would not be suitable for operation at such higher range of voltages and therefore the DMOS micromirror devices may be required. In order to achieve higher degree of gray scale control, a more complicate manufacturing process and larger device areas are necessary when DMOS micromirror is implemented. Conventional modes of micromirror control are therefore facing a technical challenge that the gray scale accuracy has to be sacrificed for the benefits of smaller and more cost effective micromirror display due to the operational voltage limitations.
There are many patents related to light intensity control. These patents include U.S. Pat. Nos. 5,589,852, 6,232,963, 6,592,227, 6,648,476, and 6,819,064. There are further patents and patent applications related to different shapes of light sources. These patents includes U.S. Pat. Nos. 5,442,414 and 6,036,318 and Application 20030147052. The U.S. Pat. No. 6,746,123 discloses special polarized light sources for preventing light loss. However, these patents and patent application do not provide an effective solution to overcome the limitations caused by insufficient gray scales in the digitally controlled image display systems.
Furthermore, there are many patents related to spatial light modulation that includes U.S. Pat. Nos. 2,025,143, 2,682,010, 2,681,423, 4,087,810, 4,292,732, 4,405,209, 4,454,541, 4,592,628, 4,767,192, 4,842,396, 4,907,862, 5,214,420, 5,287,096, 5,506,597, and 5,489,952. However, these inventions have not addressed or provided direct resolutions for a person of ordinary skill in the art to overcome the above-discussed limitations and difficulties.
Therefore, a need still exists in the art of image display systems applying digital control of a micromirror array as a spatial light modulator to provide new and improved systems such that the above-discussed difficulties can be resolved.
Incidentally, an address electrode for driving a mirror is placed under the mirror. The reason is that the mirror and address electrode need to be placed as closely to each other as possible in order to effectively generate a sufficient magnitude of a coulomb force for driving the mirror because the coulomb force for driving it is inversely proportional to the second power of the distance between the electrode and mirror. Further, the coulomb force is also dependent on the area size of the address electrode, that is, the coulomb force increases with the area size of the address electrode. That is, the address electrode with a sufficient area size needs to be placed under the mirror.
A miniaturization of a mirror device is naturally accompanied by a reduction in the space for placing an address electrode. In addition, a stopper member is placed under the mirror separately from the address electrode for regulating the deflection angle of a mirror by letting it abut the stopper member. In a situation in which the mirror device is miniaturized, causing the space used for placing the address electrode to become tight, the configuring of the address electrode and a stopper for determining the deflection angle of the mirror as practiced in the conventional technique will be faced with a technical problem that a space for placing the address electrode is further reduced, making it very difficult to obtain a sufficient magnitude of the coulomb force.
The purpose of the present invention is to provide both a mirror device attaining a higher grade of gradation without requiring an excessive drive voltage therefor and a projection apparatus comprising such a mirror device.
A first aspect of the present invention is a mirror device, comprising a plurality of mirror elements, wherein each of the mirror elements comprises a deflectable mirror, and an elastic member for deflectably supporting the mirror, wherein the mirror allows to be controlled under a first deflection control state in which incident light is reflected toward a first direction, a second deflection control state in which the incident light is reflected toward a second direction, and a third deflection control state in which the mirror oscillates between the first deflection control state and second deflection control state, wherein the mirror device reproduces gradations by combining the first through third deflection control states, and the natural oscillation cycle T of the oscillation system constituted by the mirror and elastic member satisfies:
110 [μsec]>T=2π*√(I/K)>2 [μsec],
where “I” is the moment of rotation of the oscillation system and “K” is the spring constant of the elastic member.
A second aspect of the present invention is the mirror device according to the first aspect, wherein the elastic member is made of any of materials such as silicon, metal, ceramics and glass, or of a composite structure containing any of the aforementioned materials.
A third aspect of the present invention is the mirror device according to the first aspect, wherein the elastic member is structured within a plane vertical to the mirror surface.
A fourth aspect of the present invention is the mirror device according to the first aspect, wherein the mirror element further comprises an electrode for generating electrostatic force, wherein
the mirror is driven to the first and second deflection control states by the electrostatic force and is controlled under the third deflection control state by removing the electrostatic force.
A fifth aspect of the present invention is the mirror device according to the fourth aspect, wherein the maximum amplitude of the mirror is determined by a physical placement of the electrode that generates the electrostatic force.
A sixth aspect of the present invention is the mirror device according to the fourth aspect, wherein each of the mirror elements comprises at least one of the electrodes mutually independently on either side of the deflection axis of the mirror.
A seventh aspect of the present invention is the mirror device according to the fourth aspect, wherein each of the mirrors is driven by a singularity of the electrodes.
An eighth aspect of the present invention is the mirror device according to the first aspect, wherein the mirror is formed as a square, wherein
the deflection axis of the mirror is approximately on the diagonal line of the present mirror.
A ninth aspect of the present invention is a projection apparatus comprising: a light source; a light source control circuit for controlling the light source; an illumination optical system for condensing the light emitted from the light source into a mirror device; and a control circuit for controlling the mirror device on the basis of an input signal, wherein the mirror device comprises a plurality of deflectable mirrors, and an elastic member for deflectably supporting the mirror, wherein the mirror allows to be controlled under a first deflection control state in which incident light is reflected toward a first direction, a second deflection control state in which the incident light is reflected toward a second direction, and a third deflection control state in which the mirror oscillates between the first deflection control state and second deflection control state, wherein the mirror device reproduces gradations by combining the first through third deflection control states, and the natural oscillation cycle T of the oscillation system constituted by the mirror and elastic member satisfies:
110 [μsec]>T=2π*√(I/K)>2 [μsec],
where “I” is the moment of rotation of the oscillation system and “K” is the spring constant of the elastic member.
A tenth aspect of the present invention is the projection apparatus according to the ninth aspect, wherein the light source control circuit controls the light source so as to stop emitting light other than during a period in which a control signal generated by the control circuit drives the mirror element of the mirror device.
An eleventh aspect of the present invention is the projection apparatus according to the ninth aspect, wherein the emission cycle of the light source is shorter than the natural oscillation cycle.
A twelfth aspect of the present invention is the projection apparatus according to the ninth aspect, wherein the emission cycle of the light source is 1/n times the natural oscillation cycle, where “n” is an integer.
A thirteenth aspect of the present invention is the projection apparatus according to the twelfth aspect, wherein the “n” is variably regulated.
A fourteenth aspect of the present invention is the projection apparatus according to the ninth aspect, wherein the emission frequency of the light source is no less than 10 times a natural oscillation frequency corresponding to the natural oscillation cycle, or no more than 1/10 of the natural oscillation frequency.
A fifteenth aspect of the present invention is the projection apparatus according to the ninth aspect, wherein the maximum amplitude of the mirror in the third deflection control state does not depend upon a physical placement of the electrode but depends upon the timing of the control circuit applying a voltage to the electrode.
A sixteenth aspect of the present invention is the projection apparatus according to the ninth aspect, wherein the light source is a laser light source or a light emitting diode (LED) light source.
The present invention is described in detail below with reference to the following Figures.
First is a description of an outline of an example of a mirror device according to a preferred embodiment of the present invention.
[Outline of the Device]
The first is a description of a mirror device.
Projection apparatuses each generally using a spatial light modulator, such as a transmissive liquid crystal, a reflective liquid crystal, a mirror array and the like, are widely known.
A spatial light modulator is formed as a two-dimensional array arranging from tens of thousands to millions of miniature modulation elements, with the individual elements enlarged and displayed, as the individual pixels corresponding to an image to be displayed, onto a screen by way of a projection lens.
The spatial light modulators generally used for projection apparatuses primarily include two types, i.e., a liquid crystal device for modulating the polarizing direction of incident light by sealing a liquid crystal between transparent substrates and providing them with a potential, and a mirror device deflecting miniature micro electro mechanical systems (MEMS) mirrors with electrostatic force and controlling the reflecting direction of illumination light.
One embodiment of the above described mirror device is disclosed in U.S. Pat. No. 4,229,732, in which a drive circuit using MOSFET and deflectable metallic mirrors are formed on a semiconductor wafer substrate. The mirror allows to be deformed by electrostatic force supplied from the drive circuit and is capable of changing the reflecting direction of the incident light.
Meanwhile, U.S. Pat. No. 4,662,746 has disclosed an embodiment in which one or two elastic hinges retain a mirror. If the mirror is retained by one elastic hinge, the elastic hinge functions as bending spring. If the mirror is retained by two elastic hinges, they function as torsion springs to incline the mirror and thereby the reflecting direction of the incident light is deflected.
Next is an outline description of the configuration of the mirror device.
As shown in
The mirror 4003 of one mirror element 4001 is controlled by applying a voltage to the address electrode provided on the device substrate 4004.
The mirror driven by a drive electrode abuts on a landing electrode structured differently from the drive electrode, and thereby a prescribed tilt angle is maintained. A “landing chip”, which possesses a spring property, is formed on the contact part abutting on the landing electrode so that an operation of the mirror deflecting to the reverse direction upon changing over the control is assisted. The part forming the landing chip and the landing electrode are maintained at the same potential so that the contact will not cause a shorting or the like.
[Outlines of Mirror Size and Resolution]
Next is an outline description of the size of a mirror and the resolution.
The size of a mirror for constituting such a mirror device is between 4 μm and 10 μm for each side, and the mirrors are placed on a semiconductor wafer substrate in such a manner as to minimize the gap between adjacent mirrors so that useless reflection light from the gap does not degrade the contrast of a modulated image.
Further specifically, the ratio (noted as “aperture ratio” hereinafter) of the effective reflection surface to the pixel placement region is commonly set at as approximately no less than 80%, with the reflection ratio approximately designated at no lower than 80%. The gap between adjacent mirrors are preferred to be set as a minimum while avoiding the physical interference, which is specifically designated as, for example, between 0.15 μm and 0.55 μm. Such a mirror device with the aperture improved as described above makes it possible to reduce the energy irradiated on the device substrate through the gap between the adjacent mirrors and accordingly alleviate an operational failure, and the like, due to an extraneous heating and a photoelectric effect.
One mirror device is constituted by forming on a substrate an appropriate number of mirror elements, as image display elements, comprising these mirrors. Here, the appropriate number as image display elements are the numbers, for example, in compliance to the resolution of a display specified by the Video Electronics Standards Association (VESA) and to the television forecasting standard. Here, in the case of configuring a mirror device comprising the number of mirror elements, which is compliant to the WXGA (with the resolution of 1280×768) specified by the VESA and in which mirrors are arrayed in intervals (noted as “pitch” hereinafter) of 10 μm, a sufficiently miniature mirror device is configured, with about 15.49 mm (0.61 inches) of the diagonal length of the display area.
[Outline of the Introduction of Laser Light Source]
Next is an outline description of the introduction of a laser light source.
In a projection apparatus using the above described mirror device as a display device, there is a close relationship among the numerical aperture (NA) NA1 of an illumination light path, the numerical aperture NA2 of a projection light path and the tilt angle α of a mirror.
Let it be assumed that the tilt angle α of a mirror 4003 as 12 degrees. When a modulated light reflected by the mirror 4003 and incident to the pupil of the projection light path is set at the perpendicular direction of a device substrate 4004, the illumination light is incident from a direction inclined by 2a, that is, 24 degrees, relative to the perpendicular of the device substrate 4004. For the light beam reflected by the mirror to be most efficiently incident to the pupil of the projection lens, the numerical aperture of the projection light path is desirably equal to the numerical aperture of the illumination light path. If the numerical aperture of the projection light path is smaller than that of the illumination light path, the illumination light cannot be sufficiently imported into the projection light path, while if the numerical aperture of the projection light path is larger that that of the illumination light path, the illumination light can be entirely imported; the projection lens becomes unnecessarily large, bringing about inconvenience in terms of configuring the projection apparatus. Further in this event, the light fluxes of the illumination light and projection light need to be basically placed apart from each other because the optical members of the illumination system and those of the projection system need to be physically placed respectively.
It is possible to reduce an extraneous space between the light flux of the illumination light and that of the projection light by designing a layout so as to cause the aforementioned two light fluxes to be adjacent to each other as exemplified in
From the above considerations, when a mirror device with the tilt angle of a mirror being 12 degrees is used, the numerical aperture (NA) NA1 of the illumination light path and the numerical aperture NA2 of the projection light path are preferred to be set as follows:
NA1=NA2=sin α=sin 12°
Letting the F number of the illumination light path be Fa and the F number of the projection light path be Fb, then the numerical aperture can be converted into an F number as follows:
Fa=Fb=1/(2*NA)=1/(2*sin 12°)=2.4
In order to maximize the importation of illumination light emitted from a light source possessing non-directivity in the emission direction of light, such as a high-pressure mercury lamp and xenon lamp, which are generally used for a projection apparatus, there is a requirement for maximizing the importing angle of light on the illumination light path side. Considering that the numerical aperture of the illumination light path is determined by the specification of the tilt angle of a mirror to be used, it is clear that the tilt angle of the mirror needs to be large for increasing the numerical aperture of the illumination light path.
The increasing of the tilt angle of the mirror, however, ushers in the problem of requiring a higher drive voltage for driving the mirror. Should the tilt angle of the mirror be increased, the distance between the mirror and an electrode for driving the mirror needs to be increased in order to secure a physical space for the mirror to be tilted.
The electrostatic force F generated between the mirror and electrode is given by the following expression:
F=(∈*S*V2)/(2*d2),
where “S” is the area size of the electrode, “V” is a voltage, “d” is the distance between the electrode and mirror and “∈” is the permittivity of vacuum.
The expression makes it comprehensible that the drive force is decreased in proportion to the second power of the distance d between the electrode and mirror. It is conceivable to increase the drive voltage for compensating the decrease in the drive force associated with the increase in the distance; conventionally, however, the drive voltage is about 5 to 10 volts in the drive circuit by means of a CMOS process used for driving a mirror and therefore a relatively special process such as a DMOS process is required if a drive voltage in excess of about 10 volts is needed. That is not preferable in view of the purchase of a mirror device and the cost reduction.
Further, as for a cost reduction of a mirror device, it is desirable to obtain as many mirror devices as possible from a single semiconductor wafer substrate in view of the improvement of productivity. That is, a miniaturization of the pitch between mirror elements reduces the size of the mirror device per se. It is clear that the area size of an electrode is reduced in association with the miniaturization of the mirror, which also leads to less driving power.
Furthermore, in contrast to the requirement for miniaturizing a mirror device, there is a problem in which the larger a mirror device, the brighter is it possible to illuminate as long as a conventional lamp is used because a conventional lamp with a non-directivity in its emission allows the usage efficiency of light to be substantially reduced. This is attributable to a relationship commonly called etendue.
As exemplified in
y*u=y′*u′,
where “y” is the size of the light source, “u” is the import angle of light on the light source side, “y” is the size of a light source image and “u” is the converging angle on the image side.
That is, the smaller the device on which a light source is attempted to be imaged, the smaller the import angle on the light source side becomes. This results in sacrificing the brightness of the projection image.
Accordingly, the embodiment of the present invention is configured to use a laser light source 4200 with strong directivity of the emission light for the light source as exemplified in
[Outline of Resolution Limit]
Next is an outline description of a resolution limit.
An examination of the limit value of the aperture ratio of a projection lens used for a projection apparatus, which displays the display surface of a mirror device in enlargement, in view of the resolution of an image to be projected, leads to the following.
Where “Rp” is the pixel pitch of the mirror device, “NA” is the aperture ratio of a projection lens, “F” is an F number and “λ” is the wavelength of light, the limit “Rp” with which any adjacent pixels on the projection surface are separately observed is given by the following expression:
Rp=0.61*λ/NA=1.22*λ*F
When the pitch between mirror elements is shortened by using a miniaturized mirror, the relationship among the aperture ratio NA, which is theoretically required for resolving individual mirrors, the F number for the projection lens and the corresponding deflection angle of the mirror is given by the following tables for the wavelength of light at λ=400 nm, the green light (at λ=650 nm) and the red light (at λ=800 nm), respectively.
Tables: The NA required for resolving, in the projected image, adjacent mirror elements and the tilt angle of a mirror for separating the illumination light and projection light with the respective NA: at λ=400 nm
Based on the above tables, it is understood that a sufficient F number for a projection lens required for resolving, in the projected image, individual pixels with, for example 10 μm pixel pitch is theoretically F=20.5, an extremely small aperture, when the wavelength of illumination light is λ=400 nm. A sufficient deflection angle of mirror in this case is mere 1.4 degrees, indicating that the drive voltage for the mirror is extremely low.
However, the using of an illumination lens matching such a projection lens and of a conventional lamp with no directivity makes it impossible to secure a sufficient level of brightness in the image. Accordingly, the use of a laser light source, avoiding the above described problem attributable to the etendue, makes it possible to increase the F number for the illumination and projection optical systems to the number indicated in the table and also reduce the deflection angle of a mirror element as a result, thus enabling configuring a compact mirror device with a low drive voltage.
Further, the introducing of a laser light source as in the present embodiment provides the benefit as follows. That is, the lowering of a drive voltage by introducing the laser light source makes it possible to further reduce the thickness of the circuit wiring pattern of a control circuit provided for controlling a mirror. Here, setting the deflection angle of the mirror at a minimum for each frequency of light as the target of modulation, it is possible to further reduce the power consumption. That is, the deflection angle of the mirror can be smaller for a mirror device for modulating, for example, the blue light than the deflection angle of a mirror for the mirror device for modulating the red light. This fact means a possibility that, when, for example, single color laser light sources are used for light sources, the respective illumination light paths are individually provided and the optimal NAs are set for the respective illumination light paths, and thereby a projection apparatus can be configured without requiring increases in the sizes of optical components used in the apparatus.
Further, it is possible to cause the laser light source to perform pulse emission by comprising a circuit that emits the pulse emission of ON and OFF lights alternately for a predetermined period. Controlling the pulse emission of the light source makes it possible to adjust intensity in accordance with the image signal (that is, in accordance with the brightness and hue of the entire projection image) and to finely express the gradations of the display image. Further, lowering the output of the laser light makes it possible to make the dynamic range of an image variable and darken the entire screen in response to a dark image.
Furthermore, performing a pulse control makes it possible to turn OFF a laser light source as appropriate during a non-image display period or during a period for changing over the colors of a display image in one frame. As a result, a temperature rise due to the irradiation with extraneous light onto a mirror device can be alleviated.
Next is a description, in detail, of a first preferred embodiment of the present invention with the configuration of the above described mirror device kept in mind.
The following is a description, in detail, of a mirror device according to the present embodiment with reference to the accompanying drawings.
In the mirror element shown in
The mirror 4003 is formed as an approximate square, with the length of a side, for example, between 4 μm and 10 μm. Further, the mirror pitch is, for example, between 4 μm and 10 μm. The deflection axis 4005 of the mirror 4003 is on the diagonal line thereof.
The light emitted from a light source possessing a coherent characteristic is incident to the mirror 4003 from a direction of the orthogonal or diagonal relative to the deflection axis 4005. A light source possessing a coherent characteristic is, for example, a laser light source.
The following is a description of the reason for placing the deflection axis of the mirror 4003 on the diagonal line thereof.
As shown in the figures, the diffracted light is generated as a result of irradiating light onto a mirror, and the diffracted light 4110 spreads, that is, the primary diffracted light 4111, the secondary diffracted light 4112, the tertiary diffracted light 4113, and so on, in directions perpendicular to the four sides of the mirror 4003 shown at the center. In this event, the light intensity decreases gradually with the primary diffracted light 4111, secondary diffracted light 4112, tertiary diffracted light 4113, and so on. In the case of using a laser light source, the coherence is improved by the uniformity of the wavelength of a laser light, distinguishing the diffracted light 4110. Note that the diffracted light 4110 also possesses an expansion to the depth direction of the mirror 4003 in three dimensions.
The mirror device 4000 shown in
As a result, the diffracted light 4110 does not enter the projection optical system and thereby it is possible to improve the contrast of an image to be projected. Further, it is also possible to enhance the contrast by set the deflecting angle of the mirror 4003 at a large angle relative to the incidence pupil of the projection lens and also maintain the numerical aperture of the illumination light at a small value, and thereby the OFF light is separated from the incidence pupil of the projection lens by a long distance. Such is the reason for placing the deflection axis of the mirror 4003 on the diagonal line thereof.
The lower end of the elastic hinge 4007 is connected to the device substrate 4004 that includes a circuit for driving the mirror 4003. The upper end of the elastic hinge 4007 is connected to the bottom surface of the mirror 4003. For example, an electrode for securing an electrical continuity and an intermediate member for improving the strength of a member and improving the strength of connection may be placed between the elastic hinge 4007 and the device substrate 4004, or between the elastic hinge 4007 and mirror 4003.
Further, a hinge electrode 4009 may be equipped between the elastic hinge 4007 and device substrate 4004 as exemplified in
Note that a plurality of elastic hinges (refer to 4007a and 4007b) may be placed along the deflection axis 4005 of the mirror 4003 as shown in
As exemplified in
The mirror element 4001 further comprises two memory cells, i.e., a first memory cell 4010a and a second memory cell 4010b, for applying voltages to the address electrodes 4008a and 4008b.
The first and second memory cells 4010a and 4010b each has a dynamic random access memory (DRAM) structure comprising field effect transistors (FETs) and a capacitance in this configuration. The structures of the individual memory cells 4010a and 4010b are not limited as such and may instead be, for example, a static random access memory (SRAM) structure or the like.
Further, the individual memory cells 4010a and 4010b are connected to the respective address electrodes 4008a and 4008b, a COLUMN line 1, a COLUMN line 2 and a ROW line.
In the first memory cell 4010a, an FET-1 is connected to the address electrode 4008a, COLUMN line 1 and ROW line, respectively, and a capacitance Cap-1 is connected between the address electrode 4008a and GND (i.e., the ground). Likewise in the second memory cell 4010b, an FET-2 is connected to the address electrode 4008b, COLUMN line 2 and ROW line, respectively, and a capacitance Cap-2 is connected between the address electrode 4008b and GND.
Controlling the signals on the COLUMN line 1 and ROW line applies a predetermined voltage to the address electrode 4008a, thereby making it possible to tilt the mirror 4003 toward the address electrode 4008a. Likewise, controlling the signals on the COLUMN line 2 and ROW line applies a predetermined voltage to the address electrode 4008b, thereby making it possible to also tilt the mirror 4003 toward the address electrode 4008b.
Note that a drive circuit for each of the memory cells 4010a and 4010b is commonly equipped inside of the device substrate 4004. The controlling of the respective memory cells 4010a and 4010b in accordance with the signal of image data enables control of the deflection angle of the mirror 4003 and carry out the modulation and reflection of the incident light.
Next is a description of the address electrode comprised in a mirror element according to the present embodiment.
In the present embodiment, the address electrode also fills the role of a stopper for determining the deflection angle of a mirror. The deflection angle of a mirror is an angle determined by the aperture ratio of a projection lens that satisfies a theoretical resolution determined from the pitch of adjacent mirrors on the basis of the above described expression:
d=0.61*λ/NA=1.22*λ*F
Alternatively, the deflection angle of a mirror may be set at no lower angle than the determined angle. Since a laser light possesses a uniform phase, there is more volume of diffracted light than in the light emitted from a mercury lamp. Therefore, an adverse influence of diffracted light can be prevented by setting the deflection angle of mirror at a larger angle than an appropriate angle approximated from the numerical aperture NA of the light flux of a laser light source and the F number for a projection lens, and thereby the diffracted light is difficult to be reflected toward the projection lens. The deflection angle of mirror is, for example, 10 to 14 degrees, or 2 to 10 degrees, relative to the horizontal state of the mirror 4003. Alternatively, a configuration in which the address electrode also fills the role of a stopper enables a maximization of a space for placing the electrode when a mirror element is miniaturized as compared to a conventional case in which an address electrode and a stopper are provided separately.
Here, what is well known is a phenomenon called “stiction”, that is, a mirror 4003 sticks to the contact surface between the mirror 4003 and address electrode (i.e., also a stopper) due to a surface tension or intermolecular force when the mirror is deflected. Accordingly, a partial form of the address electrode is configured as a circular are as shown in
Further, a part of the address electrode including at least the part contacting with the mirror 4003 may be provided with a passivation layer, such as halide, in order to reduce the occurrence of a stiction phenomenon between the mirror 4003 and address electrode.
Moreover, an elastic member integrally formed with an address electrode may be used as stopper.
The form of the address electrode is configured to be a trapezoid constituted by the top side and bottom side, which are approximately parallel to the deflection axis 4005, and by sloped sides approximately parallel to the contour line of the mirror 4003 of the mirror device in which the deflection axis 4005 of the mirror 4003 is matched with the diagonal line thereof as shown in
Meanwhile, as a configuration for preventing an incidence of undesirable reflection light into the projection light path, at least a part of the electrode may be covered with a low reflectance material or a thin film layer having the film thickness equivalent to ¼ of wavelength λ of the visible light.
A difference in potentials needs to be generated between the mirror and electrode for driving the mirror by electrostatic force. The present embodiment using the electrode also as stopper is configured to provide the surface of the electrode or/and the rear surface of the mirror with an insulation layer(s) in order to prevent an electrical shorting at the mirror contacting with the electrode. Further, in the case of providing the surface of the electrode with an insulation layer, the configuration may also be such that the insulation layer is provided to only a part including the contact part with the mirror.
Next is a description of one example related to the size and form of an address electrode.
Referring to
The example shown in
d1>(L1*d2)/L2
This configuration provides a good usage efficiency of the space under the mirror and maintains a stable deflection angle of the mirror.
Note that, while the example shown in
Meanwhile, when considering an occurrence of stiction between the address electrode and mirror, it is possible to state that the closer the contact point to the deflection axis, the more advantageous it is because the moment impeding the motion of the mirror due to the stiction is smaller.
Further, in the case of configuring the electrode to constitute d1=d2, the point on the electrode determining the deflection angle of the mirror is P2 and the configuration is determined so as to satisfy the following expression:
cot θ=d2/L2
Next is an outline description of the circuit comprisal of the mirror device according to the present embodiment.
As exemplified in
The external interface unit 5140 comprises a timing controller 5141 and a selector 5142. The timing controller 5141 controls the row line decoder 5130 on the basis of a timing signal from an SLM controller (no shown in a drawing here). The selector 5142 supplies the column driver 5120 with digital signal incoming from the SLM controller.
In the mirror element array 5110, a plurality of mirror elements 4001 is arrayed at the positions where individual bit lines, which are vertically extended respectively from the column drivers 5120, crosses individual word lines which are horizontally extended respectively from the row decoders 5130.
In each mirror element 4001, electrical potentials are applied to the address electrodes 4008 (i.e., the address electrodes 4008a and 4008b) by way of the memory cells (i.e., the first memory cell 4010a and the second memory cell 4010b), which are exemplified in
Incidentally, as another method of a mirror drive for displaying an image in higher grade of gradations with a reduced drive voltage, there is a technique disclosed in US Patent Application 20050190429. In this disclosure, a mirror is put to a free oscillation in the inherent oscillation frequency, and thereby the intensity of light that is about 25% to 37% of the emission light intensity produced when a mirror is controlled under a constant ON can be obtained during the oscillation period of the mirror. According to such a control, there is no particular need to drive the mirror in high speed, making it possible to obtain a high level of gradations with a low spring constant of a spring member supporting the mirror, and therefore enabling a reduction in the drive voltage. Furthermore, a combination with the method of decreasing the drive voltage by decreasing the deflection angle of a mirror as described above brings forth a greater deal of effect.
According to the present embodiment, the use of a laser light source makes it possible to decrease the deflection angle of a mirror and also miniaturize the mirror device without ushering in a degradation of brightness, and further, the use of the above described oscillation control enables a higher level of gradations without causing an increase in the drive voltage.
Giving a signal (0, 1) to the memory cells 4010a and 4010b (which are not shown here) described in
Note that the present specification document defines the deflection angles of the mirror 4003 as “+” (positive) for clockwise (CW) direction and “−” (negative) for counterclockwise (CCW) direction, with “0” degrees as the initial state of the mirror 4003. Further, an insulation layer 4006 is provided on the device substrate 4004 and a hinge electrode 4009 connected to the elastic hinge 4007 is grounded through the insulation layer 4006.
In either of the states shown in
The total intensity of light reflected by means of the free oscillation toward the projection optical system is certainly lower than the intensity that is produced when the mirror 4003 is continuously in the ON state and higher than the intensity that is when it is continuously in the OFF state. That is, it is possible to make an intermediate intensity between those of the ON state and OFF state. Therefore, a higher gradation image can be projected than with the conventional technique by finely adjusting the intensity as described above.
Although not shown in the drawing, an alternative configuration may be such that only a portion of light is made to enter the projection optical system by reflecting an incident light in the initial state of a mirror 4003. Configuring as such makes a reflection light enter the projection optical system in higher intensity than that produced when the mirror 4003 is continuously in the OFF light state and lower intensity than that produced when the mirror 4003 is continuously in the ON light state (which is called an intermediate state).
Next is an outline description of the inherent frequency of the oscillation system of a mirror device according to the present embodiment.
The fact that a drive voltage can be lowered by obtaining a fine gradation by means of a free oscillation of a mirror is already described above. Now, if an LSB light intensity by way of a common PWM drive is intended to be obtained by an oscillation, the natural oscillation cycle of an oscillation system that includes an elastic hinge is designated as follows:
The natural oscillation cycle T of an oscillation system=2*πT*√(I/K)=LSB time/X [%];
where:
I: the moment of rotation of an oscillation system,
K: the spring constant of an elastic hinge,
LSB time: the LSB cycle at displaying n bits, and
X [%]: the ratio of the light intensity obtained by one oscillation cycle to the Full-ON light intensity of the same cycle
Note that:
“I” is determined by the weight of a mirror and the distance between the center of gravity and the center of rotation;
“K” is determined on the basis of the thickness, width, length and cross-sectional shape of an elastic hinge;
“LSB time” is determined from on the basis of frame time, or one frame time and the number of reproduction bits in the case of a single-panel projection method;
“X” is determined particularly on the basis of the F number of a projection lens and the intensity distribution of an illumination light.
As an example, when a single-panel color sequential method is employed, the ratio of emission intensity by one oscillation is assumed to be 32% and the minimum emission intensity in a 10-bit grayscale is desired to be obtained by an oscillation, then “I” and “K” are designed so as to have a natural oscillation cycle as follows:
T=1/(60*3*210*0.32)≈17.0 μsec.
In contrast, when a conventional PWM control is employed to make the changeover transition time tM of a mirror approximately equal to the natural oscillation frequency of the oscillation system of the mirror and also the LSB is regulated so that a shortage of the light intensity in the interim can be sufficiently ignored, the gray scale reproducible with the above described hinge is about 8-bit even if the LSB is set at five times the changeover transition time tM. That is, it is comprehensible that a 10-bit grayscale can be reproduced by using the elastic hinge that would have made it possible to reproduce about an 8-bit grayscale according to the conventional control.
In a single-panel projection apparatus, an example configuration attempting to obtain, for example, 13-bit grayscale is as follows:
LSB time=(1/60)*(1/3)*(1/213)=0.68 μsec
If a configuration is such that the light intensity obtained in one cycle for the optical comprisal is 38% of the intensity of the case controlling a mirror under a constant ON for the same cycle, the oscillation cycle T is as follows:
T=0.68/0.38%=1.8 μsec
In contrast, when an 8-bit grayscale is attempted to be obtained in the multi-panel projection apparatus, an example comprisal is as follows:
LSB time=(1/60)*(1/3)*(1/28)=21.7 μsec
If a configuration is such that the light intensity obtained in one cycle for the optical comprisal is 20% of the case controlling a mirror under a constant ON for the same cycle, the oscillation cycle T is as follows:
T=65.1/20%=108.5 μsec.
As described above, the present embodiment is configured to set the natural oscillation cycle of the oscillation system, which includes an elastic hinge, between about 1.8 μsec and 110 μsec; and to use three deflection state, i.e., a first deflection state (ON state), in which the light modulated by the mirror element is headed to the projection light path, a second deflection state (OFF state), in which the light is headed to elsewhere other than the projection light path, and a third deflection state (oscillation state), in which the mirror oscillates between the first and second deflection states, thereby enabling the display of a high gradation image without requiring an increase in the drive voltage of the mirror element.
The mirror element 4011 shown in
Here, the mirror 4003 is deflected by the electrical control of one address electrode 4013 and drive circuit. Further, the deflected mirror 4003 is retained at a specific deflection angle by contacting with stopper 4012a or 4012b, which are equipped in the vicinity of the exposed parts on the left and right sides of the address electrode 4013.
Note that, here, an alternative configuration may be such as to eliminate the stopper for securing the area for the electrode as described above and cause the mirror to abut the address electrode directly.
Further, a hinge electrode 4009 connected to the elastic hinge 4007 is grounded through the insulation layer 4006. Such is the comprisal of the mirror element 4011.
Incidentally, the present specification document calls the part, which is exposed above the device substrate 4004, of the address electrode 4013 of
As such, the applying of a voltage by configuring the address electrode 4013 to be asymmetrical, that is, the left side is different from the right side, e.g., the area sizes, in relation to the elastic hinge 4007 or the deflection axis of mirror 4003 generates the difference in coulomb force between (a) and (b), where (a): a coulomb force generated between the first electrode part and mirror 4003, and (b): a coulomb force generated between the second electrode part and mirror 4003. The mirror 4003 can be deflected by differentiating the coulomb force between the left and right sides of the deflection axis of the elastic hinge 4007 or mirror 4003.
Meanwhile,
Requiring only one address electrode 4013 makes it possible to reduce the two memory cells 4010a and 4010b, which correspond to the two address electrodes 4008a and 4008b in the configuration of
Other comprisals are similar to the configuration described for
Next is a description, in detail, of a single address electrode 4013 controlling the deflection of a mirror with reference to
Mirror elements 4011a and 4011b respectively shown in
In contrast,
Next is a description of the control for the deflecting operation of a mirror in the mirror element 4011a or 4011b, each respectively shown in
Referring to
The following description refers to the control of only the mirror element 4011a shown in
Next, the vertical axis of the “address voltage” of
The vertical axis of the “mirror angle” of
Note that the deflection angle is designated at 13 degrees as an example here; the deflection angle is not limited as such.
Further, the horizontal axis of
When the deflecting operation of the mirror 4003 is performed in the configuration of
Referring to
At the time t1, a voltage of 4 volts is applied to the address electrode 4013a, causing the mirror 4003 to be attracted by a coulomb force generated between the mirror 4003 and address electrode 4013a toward the first electrode part having a large area size so that the mirror 4003 shifts from the 0-degree deflection angle at the time t1 to a −13-degree deflection angle at the time t2.
Then the mirror 4003 is retained onto the stopper 4012a of the first electrode part side or onto the first electrode part.
The distance G1 between the mirror 4003 and the first electrode part and the distance G2 between the mirror 4003 and the second electrode part, both when the mirror 4003 is in the initial state, are the same, and the first electrode part has a larger area than the second electrode part does, and therefore the first electrode part can retain a larger amount of charge. As a result, a larger coulomb force is generated for the first electrode part.
The mirror 4003 is retained onto the stopper 4012a of the first electrode part side or onto the first electrode part as a result of continuously applying a voltage of 4 volts to the address electrode 4013a in accordance with a period on the basis of a data input between the time t2 and time t3.
Then, at the time t3, stopping the data input applies a voltage of “0” volts to the address electrode 4013a. As a result, the coulomb force generated between the address electrode 4013a and mirror 4003 is cancelled. This causes the mirror 4003 retained on the first electrode part side to be shifted to a free oscillation due to the restoring force of the elastic hinge 4007.
Further, the deflection angle of the mirror 4003 becomes θ>0 degrees, and when a voltage of 4 volts is applied to the address electrode 4013a at the time t4 when a coulomb force F1, which is generated between the mirror 4003 and first electrode part, and a coulomb force F2, which is generated between the mirror 4003 and second electrode part, constitutes the relationship of F1<F2, and thereby the mirror 4003 is attracted to the second electrode part.
Then at the time t5, the mirror 4003 is retained onto the stopper 4012b of the second electrode part or onto the second electrode part.
The reason is that the second power of a distance has a larger effect on a coulomb force F than the difference in electrical potentials does, according to the expression.
Therefore, with an appropriate adjustment of the area sizes of the first and second electrode parts, a coulomb force F acts more strongly to the smaller distance G2 of the distance between the address electrode 4013a and mirror 4003, despite that the area S2 of the second electrode part is smaller than the area S1 of the first electrode part. As a result, the mirror 4003 can be deflected to the second electrode part.
Note that the transition time of the mirror 4003 between the time t3 and t4 is preferred to be performed in about 4.5 μsec in order to obtain a high grade of gradation. Further, a control can possibly be performed in such a manner to turn off the illumination light synchronously with a transition of the mirror 4003 so as to not let the illumination light be reflected and incident to the projection light path during a data rewrite, that is, during the transition of the mirror 4003, between the time t3 and t4.
Between the time t5 and t6, the mirror 4003 is continuously retained onto the stopper voltage to the address electrode 4013a.
In this event, no data is input and therefore no image is projected.
Then, at the time t6, a new data input is carried out. The voltage of 4 volts, which has been applied to the address electrode 4013a, is changed over to “0” volts at the time t6 in accordance with the data input. This operation cancels the coulomb force generated between the mirror 4003 retained onto the second electrode part and the address electrode 4013a likewise the case of the time t3 so that the mirror 4003 shifts to a free oscillation state due to the restoring force of the elastic hinge 4007.
Further, a voltage of 4 volts is again applied to the address electrode 4013a at the time t7 when a coulomb force F1, which is generated between the mirror 4003 and first electrode part, and a coulomb force F2, which is generated between the mirror 4003 and second electrode part, constitutes the relationship of F1>F2 when the deflection angle of the mirror 4003 becomes θ<0 degrees, and thereby the mirror 4003 is attracted to the first electrode part, and then the mirror 4003 is retained onto the first electrode part at the time t8.
This principle is understood from the description of the action of a coulomb force between the above described time t3 and t5. Also in this event, the transition time of the mirror 4003 between the time t6 and t7 is preferred to be performed in about 4.5 μsec, and the control is performed in such a manner to turn off the illumination light synchronously with a transition of the mirror 4003 so as to not let the illumination light be reflected and incident to the projection light path during the transition of the mirror 4003.
Then, between the time t8 and t9, the mirror 4003 is continuously retained onto the stopper 4012a of the first electrode part or onto the first electrode part by keeping applying a voltage of 4 volts to the address electrode 4013a.
In this event, data is continuously input and images are projected.
Then, the voltage applied to the address electrode 4013a is changed from 4 volts to “0” volts as the data input is stopped at the time t9. This operation puts the mirror 4003 into the free oscillation state.
Then, applying a voltage to the address electrode 4013a at the time t10 makes it possible to retain the mirror 4003 onto the stopper 4012b of the second electrode part or onto the second electrode part at the time t11 on the same principle as that applied between the time t3 and t5 and between the time t6 and t8.
A repetition of the similar operation enables the control for deflecting the mirror 4003.
Next is a description of a control for returning the mirror 4003, which is retained onto the stopper 4012a of the first electrode part or onto the first electrode part, or onto the stopper 4012b of the second electrode part or onto the second electrode part, back to the initial state.
In order to return the mirror 4003 from the state, in which a voltage is applied to the address electrode 4013a and so the mirror 4003 is retained onto the stopper 4012a of the first electrode part side or onto the first electrode part, or onto the stopper 4012b of the second electrode part or onto the second electrode part, back to the initial state, an appropriate pulse voltage is to be applied.
For example, in a state in which the mirror 4003 is retained onto the stopper 4012a on the first electrode part side or onto the first electrode part, or retained onto the stopper 4012b on the second electrode part side or onto the second electrode part, the changing of the voltages applied to the address electrode 4013a to “0” volts causes the mirror 4003 to perform a free oscillation. When the distance between the address electrode 4013a and mirror 4003 becomes appropriate in the midst of the mirror 4003 heading from the first electrode part side to the second electrode part side, or vice versa, in the state in which the mirror 4003 is performing the free oscillation, a temporary application of an appropriate voltage to the address electrode 4013a generates a coulomb force F that pulls the mirror 4003 back to the first electrode part or second electrode part, to which the mirror has been retained, that is, generates acceleration in a direction opposite to the direction of the mirror 4003 heading, and thereby the mirror 4003 can be returned to the initial state.
The application of a pulse voltage to one address electrode 4013a as described above makes it possible to carry out a control for returning the mirror 4003 from a state in which it is retained onto the stopper 4012a on the first electrode part side or onto the first electrode part, or retained onto the stopper 4012b on the second electrode part side or onto the second electrode part, to the initial state.
Considering the principle of the coulomb force between the mirror and address electrode 4013a as described above, the applying of a voltage to the address electrode 4013a at an appropriate distance between the mirror 4003 and address electrode 4013a also makes it possible to retain the mirror 4003 at the deflection angle of the OFF state by returning the mirror 4003 from the ON state, or at the deflection angle of the ON state by returning the mirror 4003 from the OFF state.
The above description is the same in the case of the address electrode 4013b of the mirror element 4011b shown in
Note that the control of the mirror 4003 of the mirror elements 4011a and 4011b shown in
As described above, the mirror can be deflected to the deflection angle of the ON state or OFF state, or put in the free oscillation state, with a single address electrode of a mirror element.
That is, the configuration of
In the case of the mirror element 4011c as shown in
Note that the deflection angle of the mirror 4003 is retained by using the stoppers 4012a and 4012b in
Further, while the present embodiment is configured to set the control voltages at 4-volt and 0-volt applied to the address electrode 4013a, 4013b or 4013c, such control voltages, however, are arbitrary and other appropriate voltages may be used to control the mirror 4003.
Furthermore, the mirror can be controlled with multi-step voltages to be applied to the address electrode 4013a, 4013b or 4013c. As an example, if the distance between the mirror 4003 and address electrode 4013a, 4013b or 4013c is short, increasing a coulomb force, the mirror 4003 can be controlled with a lower voltage than that when the mirror 4003 is in the initial state.
As described above, even with the configuration in which each mirror element comprises only one address electrode, the using of three deflection states, i.e. the first deflection state (i.e., the ON state) in which the light modulated by the mirror element is headed toward a projection light path, the second deflection state (i.e., the OFF state) in which the deflected light is headed elsewhere other than the projection light path and the third deflection state (i.e., the oscillation state) in which the mirror element oscillates between the first and second deflection states, makes it possible to display an image with a high grade of gradations without requiring an increase in the drive voltage for the mirror element.
As such, each mirror element of the mirror device according to the present embodiment is configured to change the deflection states of the mirror in accordance with the voltage applied to the electrode, deflecting the light incident to the mirror 4003 to specific directions as shown in
Note that
Next is an outline description of a package used for a mirror device according to the present embodiment.
Further, an intermediate member 2009 for joining the individual constituent members comprises a support part 2007 for determining the height of the cover glass 2010, and a joinder member made of fritted glass, solder, epoxy resin, or the like.
The cover glass may further be provided with a light shield layer 2006 for shielding extraneous light and an anti-reflection (AR) coating 2011 for preventing an extraneous reflection of the incident light. The anti-reflection coating 2011 is a coating made of magnesium fluoride or a nanostructure forming of no more than the wavelength applied to a glass surface. The light shield layer 2006 is constituted by a black thin film layer containing carbon, or a multilayer structure consisting of a black thin film layer and a metallic layer.
Note that it is possible to accommodate a plurality of mirror devices and a control circuit 2017 inside of the package shown in
The accommodating of a plurality of devices in one package conceivably includes various benefits in addition to a cost reduction. In a projection apparatus comprising the assembly body 2400, the projecting position of each device is basically adjusted by the positional adjustment of the respective optical elements and so the pixels of the individual mirror devices 2030 and 2040 are overlapped with each other in good accuracy and therefore a reduction in the resolution of the projected image is small. Furthermore, the colors reflected by the respective mirror devices 2030 and 2040 are observed with little blur. Note that
Further, equipping the control circuit 2017 inside of the package as exemplified in
Further, the thermal environment conditions of a plurality of mirror devices placed on the same package substrate become the same, and thereby the shifts in the positions of the mirror elements of the individual mirror devices due to thermal expansion turn to be the same. Therefore the projection states can be made identical. Further, the controls for the individual mirror devices can be handled as the same environment, so an analogous control for the mirrors and the control condition of the voltage value for the memory can be made the same.
Furthermore, a projection apparatus 2500 shown in
Note that the mirror devices 2030 and 2040 and the device substrates 2031 and 2041, which are shown in
As described above, the mirror device according to the present embodiment is configured such that the electrode also fills the role of a stopper for regulating the deflection angle of a mirror and thereby a space utilization efficiency is improved when a mirror element is miniaturized, enabling an increase in the area size of the electrode. Therefore, a parallel application of an oscillation control for a mirror makes it possible to enable both a miniaturization of the mirror device and an enhancement of gradations.
Note that the mirror pitch, mirror gap, deflection angle and drive voltage of the mirror device according to the present embodiment are not limited to the values exemplified in the above description and rather preferably in the following ranges (respective both ends inclusive): the mirror pitch is between 4 μm and 10 μm; the mirror gap is between 0.15 μm and 0.55 μm; the maximum deflection angle of mirror is between 2 degrees and 14 degrees; and the drive voltage of mirror is between 3 volts and 15 volts.
With reference to the accompanying drawings, the following is a description of a projection apparatus according to the present embodiment comprising a mirror device described in detail for the first embodiment.
First is a description of the configuration of a single-panel projection apparatus comprising a single spatial light modulator and displaying a color display with the frequencies of projected light changed in a time series, with reference to
Note that the spatial light modulated used in the present embodiment is actually the mirror device 4000 described in detail for the first embodiment.
A projection apparatus 5010 comprises a single spatial light modulator (SLM) 5100, a control unit 5500, a Total Internal Reflection (TIR) prism 5300, a projection optical system 5400 and a light source optical system 5200 as exemplified in
The projection optical system 5400 is equipped with the spatial light modulator 5100 and TIR prism 5300 in the optical axis of the projection optical system 5400, and the light source optical system 5200 is equipped in such a manner that the optical axis thereof matches that of the projection optical system 5400.
The TIR prism 5300 fills the function of making an illumination light 5600, which is incoming from the light source optical system 5200 placed onto the side, enter the spatial light modulator 5100 at a prescribed inclination angle relative thereto as incident light 5601 and making a reflection light 5602 reflected by the spatial light modulator 5100 transmit so as to reach the projection optical system 5400.
The projection optical system 5400 projects the reflection light 5602 incoming by way of the spatial light modulator 5100 and TIR prism 5300 to a screen 5900 and such, as projection light 5603.
The light source optical system 5200 comprises a variable light source 5210 for generating the illumination light 5600, a condenser lens 5220 for focusing the illumination light 5600, a rod type condenser body 5230 and a condenser lens 5240.
The variable light source 5210, condenser lens 5220, rod type condenser body 5230 and condenser lens 5240 are sequentially placed in the aforementioned order in the optical axis of the illumination light 5600 emitted from the variable light source 5210 and incident to the side face of the TIR prism 5300.
The projection apparatus 5010 employs a single spatial light modulator 5100 for implementing a color display on the screen 5900 by means of a sequential color display method.
That is, the variable light source 5210, comprising a red laser light source 5211, a green laser light source 5212 and a blue laser light source 5213 (which are not shown in a drawing here) that allows independent controls for the light emission states, performs the operation of dividing one frame of display data into a plurality of sub-fields (i.e., three sub-fields, that is, red (R), green (G) and blue (B) in the present case) and making each of the red laser light source 5211, green laser light source 5212 and blue laser light source 5213 emit each respective light in time series at the time band corresponding to the sub-field of each color as described later. Note that the red laser light source 5211, green laser light source 5212 and blue laser light source 5213 may alternatively be replaced with light emitting diodes (LEDs), respectively.
Next is a description of a multi-panel projection apparatus using a plurality of spatial light modulators to continuously modulate the illumination lights with respectively different frequencies using the individual spatial light modulators and carrying out a color display by synthesizing the modulated illumination lights, with reference to
The projection apparatus 5020 comprises a plurality of spatial light modulators 5100, and is equipped with a light separation/synthesis optical system 5310 between the projection optical system 5400 and each of the spatial light modulators 5100.
The light separation/synthesis optical system 5310 comprises a TIR prism 5311, color separation prism 5312 and color separation prism 5313.
The TIR prism 5311 has the function of leading the illumination light 5600 incident from the side of the optical axis of the projection optical system 5400 to the spatial light modulator 5100 as incident light 5601.
The color separation prism 5312 has the functions of separating red (R) light from an incident light 5601 incident by way of the TIR prism 5311 and making the red light incident to the red light-use spatial light modulators 5100, and the function of leading the reflection light 5602R of the red light to the TIR prism 5311.
Likewise, the color separation prism 5313 has the functions of separating blue (B) and green (G) lights from the incident light 5601 incident by way of the TIR prism 5311 and making them incident to the blue color-use spatial light modulators 5100 and green color-use spatial light modulators 5100, and the function of leading the reflection light 5602 of the green light and blue light to the TIR prism 5311.
Therefore, the spatial light modulations of three colors of R, G and B are simultaneously performed at three spatial light modulators 5100, respectively, and the reflection lights resulting the respective modulations are projected onto the screen 5900 as the projection light 5603 by way of the projection optical system 5400, and thus a color display is carried out.
Note that various modifications are conceivable for a light separation/synthesis optical system, in lieu of being limited to the light separation/synthesis optical system 5310.
The TIR prism 5321 has the function of leading an illumination light 5600 incident from the lateral direction of the optical axis of the projection optical system 5400 to the spatial light modulators 5100 as incident light 5601.
The cross dichroic mirror 5322 has the function of separating the lights of red, blue and green colors from the incident light 5601 incoming from the TIR prism 5321, making the incident lights 5601 of the three colors respectively enter the red-use, blue-use and green-use spatial light modulators 5100, and also converging the reflection lights 5602 reflected by the respective color-use spatial light modulators 5100 and leading it to the projection optical system 5400.
This configuration makes it possible to place the plurality of spatial light modulators 5100, for example, by consolidating them into the same packaging unit, such as a package, by saving the space.
The light separation/synthesis optical system 5330 comprises a TIR prism 5331, a prism 5332 and a prism 5333.
The TIR prism 5331 has the function of leading, to spatial light modulators 5100, the illumination light 5600 incident in the lateral direction of the optical axis of the projection optical system 5400 as incident light 5601.
The prism 5332 has the functions of separating a red color light from the incident light 5601 and leading it to the red color-use spatial light modulator 5100, and also capturing the reflection light 5602 and leading it to the projection optical system 5400.
Likewise, the prism 5333 has the functions of separating the incident lights of green and blue colors from the incident light 5601, making them incident to the individual spatial light modulators 5100 equipped correspondently to the respective colors, and capturing the reflection lights 5602 of the respective colors to lead them to the projection optical system 5400.
Note that the multi-panel projection apparatus does not allow an occurrence of a visual problem such as a color break since the individual primary colors are constantly projected, unlike the above described single-panel projection apparatus. Further, a bright image can be obtained because the light emitted from the light source can be effectively utilized. On the other hand, there is a problem such as a complicated adjustment for the positioning of the spatial light modulator corresponding to the light of individual color and an increase in the size of the apparatus.
Such is a description of a three-panel projection apparatus using three spatial light modulators as an example of a multi-panel projection apparatus. The following is a description of a two-panel projection apparatus using two spatial light modulators (i.e., mirror devices) as another example of a multi-panel projection apparatus.
The two-panel projection apparatus 2500 does not project only one color of three colors R, G and B in sequence, nor does it project the R, G and B colors continuously and simultaneously as in the case of a three-panel projection apparatus. A two-panel projection apparatus projects an image by means of a projection method for continuously projecting, for example, a green light source with high visibility and projecting a red light source and a blue light source in sequence.
The two-panel projection apparatus 2500 is capable of changing over colors in high speed by means of pulse emission in 180 kHz to 720 kHz by comprising laser light sources, thereby making it possible to obscure flickers caused by changing over among the light sources of the respective colors.
Further, a projection method for continuously projecting the brightest color and changing over the other colors in sequence on the basis of the image signals can also be adopted. Such projection methods can also be adopted for a configuration making R, G and B lights correspond to the respective mirror devices, as in the three-panel projection method.
The following is a description of the optical comprisal and principle of projection of the two-panel projection apparatus 2500 shown in
The projection apparatus 2500 shown in
The two triangular prisms 2056 and 2059 are joined together to constitute one polarization beam splitter prism 2060. Further, the joined part between the two triangular prisms 2056 and 2059 is provided with a polarization beam splitter film 2055 or coating. The polarization beam splitter prism 2060 primarily fills the role of synthesizing the light reflected by the two mirror devices 2030 and 2040.
The polarization beam splitter film 2055 is a filter for transmitting only an S-polarized light and reflecting P-polarized light.
A slope face of the right-angle triangle cone light guide prism 2064 is adhesively attached to the front surface of the polarization beam splitter prism 2060, with the bottom of the light guide prism 2064 facing upward. The green laser light source 2051, the illumination optical system 2054a corresponding to the green laser light source 2051, the red laser light source 2052, the blue laser light source 2053, and the illumination optical system 2054d corresponding to the red laser light source 2052 and blue laser light source 2053 are equipped beyond the bottom surface of the light guide prism 2064, with the respective optical axes of the green laser light source 2051, red laser light source 2052, blue laser light source 2053 being aligned perpendicularly to the bottom surface of the light guide prism 2064.
Here, the light guide prism 2064 is equipped for causing the respective lights of the green laser light source 2051, red laser light source 2052 and blue laser light source 2053 to enter perpendicularly to the polarization beam splitter prism 2060. Such a light guide prism 2064 makes it possible to reduce the amount of the reflection light caused by the polarization beam splitter prism 2060 when the laser light enters the polarization beam splitter prism 2060.
Further, ¼ wavelength plates 2057a and 2057b are equipped on the bottom surface of the polarization beam splitter prism 2060 on which a light shield layer 2063 is applied in regions other than the areas where the light is irradiated on the individual mirror devices 2030 and 2040. Note that the ¼ wavelength plates 2057a and 2057b may alternatively be equipped on the cover glass of the package.
Furthermore, a light shield layer 2063 is equipped also on the rear surface of the polarization beam splitter prism 2060.
Further, the two mirror devices 2030 and 2040, which are accommodated in a single package, are equipped under the ¼ wavelength plates 2057a and 2057b, and the cover glass of the package is joined to the polarization beam splitter prism 2060 by way of a thermal conduction member 2062. This joinder makes it possible to radiate heat from the cover glass of the package to the polarization beam splitter prism 2060 by way of the thermal conduction member 2062. Further, the circuit boards 2058 comprising a control circuit(s) for controlling the individual mirror devices 2030 and 2040 equipped respectively on both sides of the package.
The mirror devices 2030 and 2040 are respectively placed to form a 45-degree angle relative to the four sides of the outer circumference of the package. That is, the placement is such that the deflecting direction of each mirror element of the mirror devices 2030 and 2040 is approximately orthogonal to the slope face forming the polarization beam splitter prism 2060 and to the plane on which the reflection lights are synthesized. In terms of positioning the mirror devices 2030 and 2040 in relation to the polarization beam splitter prism 2060, a high precision positioning of the two mirror devices 2030 and 2040 within the package by means of the positioning pattern 2016 is very important.
Incidentally, the illumination optical systems 2054a and 2054b each comprises a convex lens, a concave lens and other components, and the projection lens 2070 comprises a plurality of lenses and other components.
The following is the principle of projection of the projection apparatus 2500 shown in
In the projection apparatus 2500, the individual laser lights 2065, 2066 and 2067 are incident from the front direction and are reflected by the two mirror devices 2030 and 2040 toward the rear direction, and then an image is projected by way of the projection lens 2070 located in the rear.
Next is a description of the projection principle starting from the incidence of the individual laser lights 2065, 2066 and 2067 to the reflection of the respective laser lights 2065, 2066 and 2067 at the two mirror devices 2030 and 2040 toward the rear direction, with reference to the front view diagram of the two-panel projection apparatus 2500 shown in
The respective laser lights 2065, 2066 and 2067 from the S-polarized green laser light source 2051, and the P-polarized red laser light source 2052 and blue laser light source 2053 are made to be incident to the polarization beam splitter prism 2060 by way of the illumination optical systems 2054a and 2054b respectively corresponding to the laser lights 2065, and 2066 and 2067, and by way of the light guide prism 2064. Then, having transmitted through the polarization beam splitter prism 2060, the S-polarized green laser light 2065 and the P-polarized red and blue laser lights 2066 and 2067 are incident to the ¼ wavelength plates 2057a and 2057b, which are placed on the bottom surface of the polarization beam splitter prism 2060. Having passing through the ¼ wavelength plates 2057a and 2057b, the individual laser lights 2065, 2066 and 2067 respectively change the polarization by the amount of ¼ wavelengths to become a circular polarized light state.
Then, having passed through the ¼ wavelength plates 2057a and 2057b, the circular polarized green laser light 2065 and the circular polarized red and blue laser lights 2066 and 2067 respectively incident to the two mirror devices 2030 and 2040 that are accommodated in a single package. The individual laser lights 2065, 2066 and 2067 are modulated and reflected by the correspondingly respective mirror devices so that the rotation directions of the circular polarization are reversed.
Next is a description of the projection principle starting from the reflection of individual laser lights 2065, and 2066 and 2067 to the projection of an image with reference to the rear view diagram of the two-panel projection apparatus 2500 shown in
The ON light 2068 of the circular polarized green laser and the mixed ON light 2069 of the circular polarized red and blue lasers, which are reflected by the respective mirror devices 2030 and 2040, passes through the ¼ wavelength plates 2057a and 2057b again and enter the polarization beam splitter prism 2060. In this event, the polarization of the green laser ON light 2068 and that of the mixed red and blue laser ON light 2069 are respectively changed by the amount of ¼ wavelengths to become a linear polarized state with 90-degree different polarization axes. That is, the green laser ON light 2068 is changed to a P-polarized light, while the mixed red and blue laser ON light 2069 is changed to an S-polarized light.
Then, the green laser ON light 2068 and the mixed red and blue laser ON light 2069 are respectively reflected by the outer side surface of the polarization beam splitter prism 2060, and the P-polarized green laser ON light 2068 is reflected again by the polarization beam splitter film 2055. Meanwhile, the S-polarized mixed red and blue laser ON light 2069 passes through the polarization beam splitter film 2055. Then, the green laser ON light 2068 and red and blue laser mixed ON light 2069 are incident to the projection lens 2070, and thereby a color image is projected. Note that the optical axes of the respective lights incident to the projection lens 2070 from the polarization beam splitter prism 2060 are desired to be orthogonal to the ejection surface of the polarization beam splitter prism 2060. Alternatively, there is a viable configuration that does not use the ¼ wavelength plates 2057a and 2057b.
With the configuration and the principle of projection as described above, an image can be projected in the two-panel projection apparatus 2500 comprising the assembly body 2400 that packages the two mirror devices 2030 and 2040, which are accommodated in a single package.
The green laser light 2065 emitted from the green laser light source 2051 orthogonally enters the light guide prism 2064 via the illumination optical system 2054a. In this event, the reflection of the laser light 2065 is minimized by the laser light 2065 orthogonally entering the light guide prism 2064.
Then, having passed through the light guide prism 2064, the laser light 2065 passes through the polarization beam splitter prism 2060 and ¼ wavelength plates 2057a and 2057b, which are joined to the light guide prism 2064, and then, enters the mirror array 2032 of the mirror device 2030.
The mirror array 2032 reflects the laser light 2065 with the deflection angle of a mirror that puts the reflected light in any of the states, i.e., an ON light state in which the entirety of the reflection light is incident to the projection lens 2070, an intermediate light state in which a portion of the reflection light is incident to the projection lens 2070 and an OFF light state in which no portion of the reflection light is incident to the projection lens 2070.
The reflection light of a laser light (i.e., ON light) 2071, from which the ON light state is selected, is reflected by the mirror array 2032 and will be incident to the projection lens 2070.
A portion of the reflection light of a laser light (i.e., intermediate light) 2072, from which the intermediate state is selected, is reflected by the mirror array 2032 and will be incident to the projection lens 2070.
Further, the reflection light of a laser light (i.e., OFF light) 2073, from which the OFF light state is selected, is reflected by the mirror array 2032 toward the light shield layer 2063, in which the reflection light is absorbed.
With this configuration, the laser light enters the projection lens 2070 at the maximum light intensity of the ON light, at an intermediate intensity between the ON light and OFF light of the intermediate light, and at the zero intensity of the OFF light. This configuration makes it possible to project an image in a high level of gradation. Note that the intermediate light state produces a reflection light reflected by a mirror of which the deflection angle is regulated between the ON light state and OFF light state.
Meanwhile, making the mirror perform a free oscillation causes it to cycle three deflection angles producing the ON light, the intermediate light and the OFF light, respectively. Here, the controlling of the number of free oscillations makes it possible to adjust the light intensity and obtain an image in higher level of gradation.
The mirror devices 2030 and 2040 are placed in the package with them respectively forming an approximately 45-degree angle, on the same horizontal plane, in relation to the four sides of the outer circumference of the package as shown in
Further, the heat generated inside of the package is conducted to the polarization beam splitter prism 2060 by way of the thermal conduction member 2062 and is radiated to the outside therefrom. As such, the conduction of the heat generated in the mirror device to the polarization beam splitter prism 2060 improves the radiation efficiency. Further, the heat generated by absorbing light is radiated to the outside instantly because the light shield layer 2063 is exposed to the outside.
When a mirror element reflects the incident light toward a projection lens 2070 at an intermediate light intensity (i.e., an intermediate state) that is the intensity between the ON light and OFF light states, an effective reflection plane needs to be conventionally taken widely in the longitudinal direction of the slope face of a prism.
In contrast, the projection apparatus 2500 is enabled to provide a wide effective reflection plane in the thickness direction of the polarization beam splitter prism 2060 even when the mirror element as described above has the intermediate state. With this configuration, a total reflection condition with which the reflection light from the mirror element is reflected by the slope face of the polarization beam splitter prism 2060 can be alleviated.
Next is a description of a suitable projection lens when the mirror device comprised in the projection apparatus according to the present embodiment is miniaturized.
If a mirror device of which the diagonal size is 0.95 inches is used for a rear projection system with about 65-inch screen size, a required projection magnification ratio is about 68. If a mirror array of which the diagonal size is 0.55 inches is used, a required projection magnification ratio is about 118. As such, the projection magnification increases in association with the miniaturization of the mirror array. This ushers in the problem of color aberration caused by a projection lens.
The focal distance of the lens needs to be shortened to increase the projection magnification. Accordingly, the F number for the projection lens is set at 5 or higher by comprising a laser light source. With this, it is possible to use a projection lens with the F number at 2 times, and the focal distance at a half, as compared to the case in which a mercury lamp is comprised and a focal distance is 15 mm with the F number at about 2.4 for the projection lens. The usage of a projection lens with a large F number makes it possible to reduce the outer size of the projection lens. This in turn reduces the image size with which a light flux passes through the illumination optical system, thereby making it possible to suppress a color aberration caused by the projection lens.
Therefore, in the case of using a laser light source with a mirror device miniaturized, to between 0.4 inches and 0.87 inches, the deflection angle of mirror can be reduced to between ±7 degrees and ±5 degrees and the F number for a projection lens can be increased. Alternatively, the setting of the numerical aperture NA of an illumination light flux between 0.1 and 0.04 with the deflection angle of mirror maintained at ±13 degrees makes it possible to reflect the OFF light to a large distance from the projection lens, improving the contrast of the projection image.
As described above, the projection magnification of a projection lens can be set at 75× to 120× by reducing the numerical aperture NA of the light flux emitted from a laser light source, using a mirror device with which the deflection angle of mirror is reduced to between ±7 degrees and ±5 degrees and in which the mirror array is miniaturized to a diagonal size of 0.4 inches to 0.87 inches, and thereby the F number for a projection lens is increased.
Meanwhile, when a mirror device is moved forward or backward relative to the optical axis of projection, a distance with which an image blur (i.e., out of focus) of a projected image is permissible is called a focal depth. When an image is projected with a permissible blur in a degree of the mirror size by an optical setup of the same focal distance, projection magnification and mirror size, a depth of focus is approximated as follows:
Depth of focus Z=2*(permissible blur)*(F number)
Here, the depth of focus Z is proportional to the F number of a projection lens. That is, the permissible distance of the shift in positions of a placed mirror device, relative to the optical axis of projection, increases with F number. This factor is represented by the relationship between a permissible circle of confusion and a depth of focus. As an example, where the F number of a projection lens is “8” and the permissible blur is equivalent to a 10 μm mirror size in the above described approximation equation, the depth of focus is:
Z=2*10*8=160 [μm]
Further, where a mirror size is 5 μm and an F number is 2.4, the depth of focus is 24 μm. Here, considering the errors of a projection lens and other components of the optical system, the depth of focus is preferred to be no larger than 20 μm or several micrometers or less. With this in mind, when the top or bottom surface of a package substrate is taken as reference, the difference in heights of the reflection on the surface of mirrors placed respectively on both ends of a mirror array is preferred to be no more than 20 μm.
Further, a blurred image of dust, et cetera, perched on the surface of a cover glass can be made invisible by providing a distance between the top surface of a mirror and the bottom surface of the cover glass with a distance of no less than the value of the depth of focus. It is therefore preferred to provide the distance between the top surface of the mirror and the bottom surface of the cover glass with a distance of at least 20 times, or more, of the mirror size.
Next is a description of the embodiment of a control unit used for a projection apparatus according to the present embodiment.
The control unit 5500 comprises a frame memory 5520, an SLM controller 5530, a sequencer 5540, a light source control unit 5560 and a light source drive circuit 5570.
The sequencer 5540, constituted by a microprocessor and the like, controls the operation timing and the like of the entirety of the control unit 5500 and spatial light modulators 5100.
The frame memory 5520 retains the amount of one frame of input digital video data 5700 incoming from an external device (not shown in a drawing herein), which is connected to a video signal input unit 5510. The input digital video data 5700 is updated, moment by moment, every time the display of one frame is completed.
In the case of the single-panel (1×SLM) projection apparatus 5010, one frame (i.e., a frame 6700-1) of the input digital video data 5700 is constituted by a plurality of subfields 6701, 6702 and 6703, in a time sequence, corresponding to the respective colors R, G and B as exemplified in
Note that the mirror control profile 6710 is a mirror control profile consisting of binary data. Here, the binary data means the data in which each bit has different weighting and which includes a pulse width in accordance with the weighting value of each bit. Meanwhile, the mirror control profile 6720 is a mirror control profile consisting of non-binary data. Here, the non-binary data means the data in which each bit has an equal weighting and which includes a pulse width in accordance with the number of continuous bits of “1”.
The mirror control profile generated by the SLM controller 5530 is also input to the sequencer 5540, which in turn transmits a light source profile control signal 5800 to the light source control unit 5560 on the basis of the mirror control profile input from the SLM controller 5530.
The light source control unit 5560 instructs the light source drive circuit 5570 for the emission timing and light intensity of an illumination light 5600 required of the variable light source 5210 corresponding to the driving of the spatial light modulator 5100. The variable light source 5210 performs emission so as to emit the illumination light 5600 at the timing and light intensity driven by the light source drive circuit 5570. With this control, it is possible to change the brightness of a displayed pixel through a continuous adjustment of the emission light intensity of the variable light source 5210 and to control the characteristic of the gradations of the display image in the midst of driving the spatial light modulator 5100, that is, in the midst of displaying an image onto the screen 5900. The emission light intensity of the variable light source 5210 is adjusted by using a mirror control profile used for driving the spatial light modulator 5100, and therefore no extraneous irradiation occurs, making it possible to suppress the heating from the variable light source 5210 and the power consumption thereof.
As such, while the description has been provided by exemplifying the control unit 5500 comprised in the single-panel projection apparatus, in the case of a multi-panel projection apparatus in the meantime, however, a configuration may be such that the SLM controller 5530 and sequencer 5540 control a plurality of spatial light modulators 5100. Another alternative configuration may be in a manner to equip with a plurality of SLM controllers, in place of the SLM controller 5530, so as to control the respective spatial light modulators 5100.
Also in the case of a multi-panel projection apparatus, the structure of the input digital video data 5700 is also different. In the case of, for example, the above described multi-panel (3×SLM) projection apparatuses 5020, 5030 and 5040, the input digital video data 5700 corresponding to one frame (i.e., the frame 6700-1) display period is constituted by a plurality of fields 6700-2 (i.e., which are equivalent to the subfields 6701, 6702 and 6703) corresponding to the respective colors R, G and B, and the fields of the respective colors are output to the plurality of spatial light modulators 5100, respectively, simultaneously in parallel, as exemplified in
Next is a description, in detail, of the embodiment of controlling the variable light source 5210 with the light source profile control signal 5800 corresponding to the mirror control profile.
In this case, one frame of the mirror control profile 6720 is constituted by the combination of a mirror ON/OFF control 6721 on the frame head side and a mirror oscillation control 6722 on the tail end side and is used for controlling the tilting operation of the mirror corresponding to the gray scale of the present frame.
That is, the mirror ON/OFF control 6721 controls the mirror under either of the ON state and OFF state, and the mirror oscillation control 6722 controls the mirror 5112 under an oscillation state in which it oscillates between the ON state and OFF state.
The present embodiment is configured such that the light source control unit 5560 performs a control so as to change the frequencies of the pulse emission of the variable light source 5210 in accord with the signal (i.e., mirror control profile 6720) driving the spatial light modulator 5100. The spatial light modulator 5100 is the above described mirror device 4000 and performs a spatial light modulation of the illumination light 5600 by means of a large number of mirrors corresponding to pixels to be displayed and of the tilting operation of the mirrors.
Note that, for the mirror oscillation control 6722, the pulse emission frequency fp of the variable light source 5210 emitting the illumination light 5600 is preferably either higher (in the case of the light source pulse pattern 6801 shown in
The mirror oscillation control 6722 oscillates at an oscillation cycle tosc (1/fm), and in contrast the light source pulse pattern 6801 perform pulse emission at a pulse emission frequency fp (1/(tp+ti)) with [emission pulse width tp+emission pulse interval ti] as one cycle. In this case, the condition is: fp>(fm*10)
That is, in the example of
As described above, the changing of the frequencies of the pulse emission of the variable light source 5210 makes it possible to adjust the light intensity of the illumination light 5600 emitted from the variable light source 5210.
Note that the present invention may be changed in various manners possible within the scope of the present invention, in lieu of being limited to the configurations exemplified in the above described embodiments.
| Number | Date | Country | |
|---|---|---|---|
| 60967880 | Sep 2007 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 11136041 | May 2005 | US |
| Child | 12231707 | US | |
| Parent | 11183216 | Jul 2005 | US |
| Child | 11136041 | US |
