The following relates to the optical arts. It finds particular application in projection display devices, and will find more specific application in conjunction with illustrative projection devices such as televisions, computer displays, overhead projector displays, theatre projectors, and so forth.
Existing television products employ cathode ray tube (CRT) units, liquid crystal display (LCD) units, or plasma display units. CRT-based displays are rapidly losing favor as the display modality of choice due to factors such as high weight, bulkiness, and fragility of the CRT units. LCD and plasma technologies are presently the dominant technology, especially for high definition television (HDTV).
A more recently developed display technology is Digital Light Processing (DLP) technology, which is based on the digital micromirror device (DMD) developed by Texas Instruments, Inc. A DMD device is a microelectromechanical system (MEMS) including an array of micromirrors that are adjustable between two discrete angular positions by suitable MEMS actuators. Each micromirror corresponds to a pixel of the display. One of the discrete angular positions of the micromirror corresponds to the pixel being “on” such that it reflects light into the optical projection system to illuminate the corresponding pixel of the projector display. The other of the discrete angular positions of the micromirror corresponds to the pixel being “off” such that it reflects light elsewhere—usually to an optical sink that is also thermally heatsinking to dissipate heat from the unprojected portion of the light. DLP technology is in use in commercial HDTV products, theatre projectors, and so forth.
DLP technology is touted by its proponents as providing more colors, larger screen sizes, and faster raster updates compared with competing technologies such as LCD and plasma displays. The projection-based DLP technology is readily scalable to large screen sizes using high quality projection optics. Raster updates are fast in part because the updates occur on a small-sized MEMS chip rather than across an entire large-area display as in non-projection-based LCD and plasma technologies. Grayscale levels are provided by fast binary switching of the micromirror between the “on” and “off” states, with the grayscale level being controlled by the duty cycle of the “on” state. Existing DLP displays can readily provide 1024 shades of gray, which in a color display translates into millions of attainable colors.
In spite of these advantages, DLP technology has disadvantages, some of which are a consequence of the complexity of the underlying DMD device. Texas Instruments initially developed the DMD device in 1987. Even driven by the powerful motivator of the lucrative consumer electronics market, it took about a decade of further research and development before DLP projection displays became commercially available at the consumer level. Projection television development has continued to focus on the DLP technology to the present time, and DLP projection televisions are now available from most major brand television manufacturers.
One complexity of the DMD device is the precision with which the angular position of the movable micromirror must be set. The angle of the “on” position of each micromirror must be precisely controlled to ensure that each micromirror properly illuminates its corresponding pixel in the projected display. Because the micromirrors operate independently, such precise angular positional control must be maintained across the entire two-dimensional array of micromirrors. To accomplish this, in some DMD devices each micromirror is configured to come into contact with a landing in the “on” and “off” positions. However, this can lead to a failure mode known as “sticking” in which an individual micromirror adheres or coheres to the landing surface of the MEMS device such that it no longer switches properly.
In accordance with certain illustrative embodiments shown and described as examples herein, a projection display device is disclosed, comprising: an optical interface; a microelectromechanical system including an array of movable elements corresponding to image pixels, each movable element being movable toward the optical interface to locally frustrate total internal reflection at the optical interface and movable away from the optical interface to not locally frustrate total internal reflection at the optical interface; a light source arranged to illuminate the optical interface at an angle effective for producing total internal reflection at the optical interface; and an optical projector system arranged to project one but not both of (i) light from the light source that undergoes total internal reflection at the optical interface, and (ii) light from the light source that is frustrated by the movable elements from undergoing total internal reflection at the optical interface.
In accordance with certain illustrative embodiments shown and described as examples herein, an image formation device is disclosed, comprising: an optical interface; and a microelectromechanical system including an array of movable elements corresponding to image pixels, each movable element being movable toward the optical interface to locally frustrate total internal reflection at the optical interface and movable away from the optical interface to not locally frustrate total internal reflection at the optical interface.
In accordance with certain illustrative embodiments shown and described as examples herein, an apparatus is disclosed, comprising: an optical interface arranged to reflect a beam of light by total internal reflection; and a microelectromechanical system configured to spatially modulate the total internal reflection across the optical interface such that the beam reflected by total internal reflection defines an image. In some embodiments of the apparatus, an optical projector system is arranged to project the reflected beam to form a projected image.
In accordance with certain illustrative embodiments shown and described as examples herein, a projection display device is disclosed, comprising: an image forming device including an optical interface and an array of movable elements selectively movable toward or away from the optical interface to selectively locally frustrate total internal reflection at the optical interface such that light totally internally reflected at the optical interface defines an image; and an optical projector system including the image forming device.
Numerous advantages and benefits of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the present specification.
The invention may take form in various components and arrangements of components, and in various process operations and arrangements of process operations. The drawings are only for purposes of illustrating preferred embodiments and are not to be construed as limiting the invention.
With reference to
Optics 14, 16 are arranged to collect the illumination 12 and focus or image it onto an image formation device 20. In the illustrative embodiment of
The image formation device 20 modulates the beam spatially to define an image carried by the modulated illumination. The modulated illumination is received by a projection system 22 that projects the modulated beam onto a screen 24. The projection distance can be substantial for large area projection displays; accordingly, a break B in the projection of the illumination is diagrammatically shown in
The projection system 22 can employ any configuration of optics, such as a plurality of cooperating lenses, parabolic reflectors, or so forth, that project the image defined in the modulated illumination onto the screen 24. For example, if the projection display device is a rear projection television, then the projection system 22 is suitably a rear projector, for example of the type known for use in existing DLP projection televisions, and the screen 24 is suitably a light transmissive diffusing screen, for example of the type known for use in existing DLP rear projection televisions. In this arrangement, light LT transmitted through the screen 24 is viewed by a viewer looking at the light transmissive screen from the side opposite the illumination. In other embodiments, the projection display device may be a theatre projector, in which case the projection system 22 is suitably a ceiling-mounted projection system or the like of the type used in theatres, while the screen 24 is suitably a reflective cinematic screen again of the type used in theatres, and reflected light LR is viewed by a viewer looking at the same side as the illumination.
To enable full color display, an optional color filter wheel 26 is included. The color filter wheel 26 rotates about an axis 28 and includes red, green, and blue filters so that the white light produced by the light source 10 is cycled between red, green, and blue colors (or some other ordering of primary colors). Optionally, the color wheel can include a white region as well to provide a high-quality white light for a white cycle portion. An optical sensor 30 tracks angular position of the rotating color filter wheel 26, for example by detecting passage of an opening across the optical sensor 30, which opening is located at a predetermined angular position on the rotating color filter wheel 26. The optical sensor 30 provides a tracking signal for tracking the cyclic red, green, blue, and optional white illumination. A wired connection 32, or alternatively a wireless connection (not shown) communicates the tracking signal to the image formation device 20 which cyclically forms red, green, blue, and optional white images in coordination with the red, green, blue, and optional white illumination cycling. The red, green, blue, and optional white images are cyclically projected onto the screen 24 at a rate related to the rotation rate of the rotating color filter wheel 26, and the cycling rate is selected to be fast enough for the typical human viewer to see the red, green, blue, and optional white images blended together to form a full color image. For example, the repetition rate for the red/green/blue cycle is preferably at least about 15 Hertz, and more preferably at least about 24-30 Hertz. In some embodiments, the projection rate is greater than 100 Hertz. Substantially faster repetition rates are contemplated so as to provide higher screen refresh rates for applications such as high definition television, video gaming, computer displays, and so forth. Moreover, while the color filter wheel 26 is illustrated as a suitable approach usable in conjunction with a white light source for generating cycling of primary colors and optional white, other approaches are contemplated. For example, the light source 10 can be a light emitting diode (LED) based light source having red, green, blue, and optionally white LED devices that are operated cyclically to generate red, green, blue, and optionally white illumination directly without relying upon color filtering of white light. Other approaches for generating temporal cycling of primary colors synchronized with the image formation device 26 are also contemplated. As used herein, “primary colors” is intended to include white so as to encompass either cycling of red, green, and blue illumination or cycling of red, green, blue, and white illumination.
The skilled artisan will appreciate that the arrangement of components 10, 14, 16, 22, 24, 26, 28, 30, 32 shown in
Accordingly, the projection display device of
The microelectromechanical system 40 is optionally mounted on a circuit board 50 which optionally also supports a controller 52 embodied in the embodiment illustrated in
With continuing reference to
The microelectromechanical system 40 spatially modulates the TIR to define the image. The microelectromechanical system 40 is suitably manufactured of silicon, silicon carbide, or another material amenable for MEMS fabrication, and includes an array of movable elements 60 corresponding to pixels of the image. The movable elements 60 are movable by microelectromechanical actuators 62 so as to move each movable element 60 toward the optical interface 42 to locally frustrate total internal reflection at the optical interface 42, or away from the optical interface to not locally frustrate total internal reflection at the optical interface. The FTIR effect is based on the presence of an evanescent wave that penetrates beyond the optical interface 42 during TIR. In
On the other hand, when the movable element 60 is moved substantially into the TIR gap GTIR, the total internal reflection is frustrated—that is, FTIR occurs. This is diagrammatically illustrated by the righthand ray of illumination 12 shown in
The distance over which the evanescent wave extends away from the optical interface 42 is dependent upon factors such as the refractive indices n0 and n1, the wavelength of light, and so forth. The size of the TIR gap GTIR depends on the distance over which the evanescent wave extends away from the optical interface 42. The size of the TIR gap GTIR also depends on the optical characteristics of the movable element 60, because it is the change in refractive index induced by the proximate movable element 60 in the region of the evanescent wave that causes the FTIR effect. Typically, movable elements with larger refractive indices tend to more effectively induce the FTIR effect. For illumination 12 in the visible range, n1 of order 1.5-2.0, and movable elements with refractive indices greater than or about 1.5 (this latter condition being satisfied by most solid materials including for example silicon, silicon carbide, and silicon nitride), the TIR gap GTIR is typically of order 10 microns. Moreover, while the TIR gap GTIR is illustrated as a discrete gap, in reality the evanescent wave decays exponentially with increasing distance from the optical interface 42, and so the TIR gap GTIR is not abruptly defined.
In some embodiments, the microelectromechanical system 40 including the movable elements 60 is made of silicon, and the movable elements 60 and associated microactuators 62 take the form of micropistons as shown in
In general, substantially any MEMS fabrication method is suitable for manufacturing the microelectromechanical system 40. Advantageously, the existing mature MEMS fabrication technology used for manufacturing DMD image formation devices such as are used in DLP projection displays is readily adapted to the manufacture of the microelectromechanical system 40.
However, the image formation device 20 has substantial advantages over existing MEMS-based DLP projection display devices that employ DMD image formation devices. In DMD, the reflective micromirrors are moved between “on” and “off” positions, and in the “on” position the micromirror reflects light into the corresponding pixel of the projected image. Accordingly, the micromirror angle must be precisely set in the “on” position in order to precisely reflect light into the corresponding pixel of the projected image. This is typically achieved in DMD by having the micromirror contact a landing.
In contrast, the movable elements 60 do not control the angle of reflected light in the “on” position. Rather, the “on” position corresponds to the movable element 60 being moved away from the optical interface 42 by an amount greater than the TIR gap GTIR. The angle of reflection in this case is determined solely by the optical interface 42, which is a stationary nonmoving interface that can be precisely formed using conventional optical fabrication methods. Further, the angle of reflection across the array can be made highly uniform simply by making the optical interface 42 precisely planar. This is readily accomplished using conventional precision optical element fabrication techniques. Even in the “off” position, precise positioning of the movable elements 60 is not called for. Rather, to turn a pixel “off” it is sufficient for the movable element 60 to penetrate substantially into the TIR gap GTIR—tolerances for this “off” positioning are of order one micron or larger.
In operation, the controller 52 suitably uses a digital control to set the grayscale intensity. This process is closely analogous to the grayscale intensity control used in DMD. Each movable element 60 is movable between a first (e.g., “off”) position at which the movable element 60 locally frustrates total internal reflection at the optical interface 42 and a second (e.g., “on”) position at which the movable element 60 does not locally frustrate total internal reflection at the optical interface 42. Each movable element 60 is moved cyclically toward the optical interface 42 so as to penetrate substantially into the TIR gap GTIR (i.e., into the first or “off” position) and away from the optical interface 42 so as to withdraw from the TIR gap GTIR (i.e., into the second or “on” position). The grayscale level is controlled by the controller 52 by adjusting a characteristic of the cyclical movement of each movable element 60 to control the grayscale level of the corresponding image pixel. For example, a duty cycle of the cyclical movement may be adjusted to control the grayscale level of the corresponding image pixel.
It is also contemplated to configure the controller 52 to adjust the gap between the movable element 60 and the optical interface 42 to adjust the amount of local frustration of total internal reflection at the optical interface 42 so as to set the grayscale level of the corresponding image pixel. This analog approach to grayscale intensity control is possible because of the gradual decay of the evanescent wave with increasing distance away from the optical interface 42.
Yet another advantage of the image formation device 20 over existing MEMS-based projection technologies employing DMD is that the movable elements 60 in some embodiments are arranged to move freely between the first or “off” position and the second or “on” position without contacting a landing in either the “off” or “on” positions. As a result, in these embodiments the “sticking” failure mode sometimes encountered in DMD, in which a micromirror adheres or coheres to a landing and resists breaking contact with the landing, is wholly avoided. Alternatively, landings can be provided in one or both of the first and second positions. In some embodiments the optical interface 42 serves as a landing for the first or “off” position, such that the movable element contacts the optical interface 42 to provide FTIR. The adhesion forces between some optical interface materials such as glass and some MEMS materials such as silicon is relatively small, thus reducing a likelihood of sticking failure.
Still yet another advantage of the image formation device 20 over existing MEMS-based projection technologies employing DMD is higher operating speed. DMD operating speed is limited by the need to precisely position the micromirrors in the “on” positions in order to light corresponding pixels, and potentially by latency introduced by adhesive or cohesive forces between the micromirrors and the associated landings. In contrast, the image formation device 20 can operate with substantially higher tolerances enabling tradeoff between speed and spatial tolerance, and additionally landing contact induced latencies are avoided in embodiments in which the movable elements 60 are arranged to move freely without contacting landings in either the “on” or “off” positions.
With reference to
The projection display device of
With reference to
The skilled artisan will recognize that the full color projection device of
In the illustrated embodiments, the projection system 22 projects light from the light source that undergoes total internal reflection at the optical interface 42. Light which is frustrated by the movable elements 60 is dumped light LD that is discarded and not projected.
However, it is also contemplated to project the light from the light source that is frustrated by the movable elements from undergoing total internal reflection at the optical interface, and to discard the light that undergoes TIR. This latter approach is generally not preferred because it does not attain the advantage that the projected light is reflected from the optical interface 42 which can be precisely constructed. Moreover, the light that is frustrated by the movable elements from undergoing total internal reflection at the optical interface is more likely to have intensity variation due to the precise positioning of the movable elements. Nonetheless, in some embodiments it may be advantageous to project the light from the light source that is frustrated by the movable elements from undergoing total internal reflection. For example, if the microelectromechanical system is optically transparent, it may be possible to have the light from the light source that is frustrated by the movable elements from undergoing total internal reflection pass through the microelectromechanical system, so that the projected light is not reflected at all.
Still further, while projection display devices have been illustrated herein as examples, it is also contemplated to use the image forming device 20, 20′ for other applications, such as for direct viewing applications. Moreover, the term “projection system” as used herein is intended to be broadly construed to encompass any system which processes the image beam to form a viewable image. In a contemplated cellular telephone application, for example, illumination reflected by TIR at the optical interface and spatially modulated by the microelectromechanical system using FTIR is suitably input to a prism that reflects the TIR beam onto a light transmissive screen. In these embodiments, the prism may or may not provide image magnification—in some such embodiments the prism merely redirects the beam onto the light transmissive screen without magnification.
The preferred embodiments have been illustrated and described. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.