1. Field of the Invention
The subject matter described herein relates generally to image projectors and hand-held electronic devices and, more specifically but not exclusively, to despeckling laser-image-projection systems.
2. Description of the Related Art
This section introduces aspects that may help facilitate a better understanding of the invention(s). Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.
A projector is a device that integrates a light source, optics, electronics, and a light-modulating element for the purpose of projecting an image or a sequence of images, e.g., from a computer or video input, onto a wall or screen for large-image viewing. There are many projectors available in the market, and they are differentiated by their size, resolution, performance, and other features. Some projectors employ laser light sources because the use of lasers enables creation of vibrant images with extensive color coverage that can be difficult to achieve with other (non-laser) light sources.
A compact image projector, e.g., one that can be incorporated into a cell phone and used to project a relatively large image on a wall or an 8.5″×11″ sheet of paper, is of great interest to electronic-equipment manufacturers. While the compactness of modern hand-held electronic devices is advantageous for portability purposes, their relatively small size, by its very nature, creates a disadvantage with respect to the display of visual information. More specifically, the display screen of a cell phone, personal digital assistant (PDA), or portable media player is typically too small to present most documents in their original full-page format, or graphics and video content at their original resolution. Having a compact image projector instead of or in addition to a regular display screen in a hand-held electronic device would help to solve these problems because it would enable the user to display and view the visual information in its most-appropriate form.
One significant obstacle to laser-image projection is the speckle phenomenon that tends to superimpose a granular structure on the perceived image. Since speckle can both degrade the image sharpness and annoy the viewer, speckle mitigation is highly desirable. However, the small size of a compact image projector makes it relatively difficult to incorporate an adequate despeckling functionality therein.
Disclosed herein are various embodiments of a laser-image-projection system having (i) a fly's eye (FE) integrator including a plurality of lenslet pairs and (ii) a configurable optical diffuser, both located along an optical path between a laser and a spatial light modulator (SLM). In various embodiments, the optical diffuser introduces a temporally varying pattern of angular divergence into a laser beam directed toward the FE integrator for transmission to the SLM. In various embodiments, the FE integrator produces a plurality of illumination patches that are superimposed on the SLM in a manner that is substantially independent of the temporal variations introduced by the optical diffuser. Consequently, the regions of illumination produced by different pairs of opposing lenslets can overlap despite the presence of temporal variations in the angular divergence produced by the optical diffuser. Advantageously, the optical diffuser and FE integrator work together in a synergistic manner to enable the laser-image-projection system to be relatively compact and to provide relatively high illumination homogeneity across the SLM, relatively high temporal/spatial stability of the illumination patch, relatively high optical throughput between the laser and the projection screen, and relatively low speckle noise in the projected image.
According to one embodiment, an optical device for projecting an image has a configurable optical diffuser adapted to produce a diffuse optical beam having a temporally varying pattern of angular divergence. The optical device also has a fly's eye (FE) integrator adapted to shape the diffuse optical beam. The optical device further has a spatial light modulator (SLM) adapted to spatially modulate the shaped optical beam produced by the FE integrator to project the image.
According to another embodiment, an optical device for projecting an image has a pair of crossed cylindrical lenses to provide collimated coherent light. The optical device also has a moveable diffuser to receive the collimated coherent light to produce a diffuse optical beam having a temporally varying pattern of angular divergence. The optical device further has a plurality of lenslet pairs arranged side by side with each other and adapted to shape the diffuse optical beam.
Other aspects, features, and benefits of various embodiments of the invention will become more fully apparent, by way of example, from the following detailed description and the accompanying drawings, in which:
Light source 110 has a set of three lasers 112a, 112b, and 116, each adapted to generate pulsed light of a designated color, e.g., red, green, and blue, respectively. Lasers 112a-b and 116 can be synchronized so that modulator section 160 receives a periodic train of pulses. For example, each illumination period may have three or more sequential pulses of different colors, wherein the pulses appear at a selected repetition rate. Commonly owned U.S. Pat. No. 7,502,160 and U.S. Patent Application Publication No. 2009/0185140 describe various methods of time multiplexing light pulses of various colors suitable for use in light source 110. Said U.S. Patent and U.S. Patent Application Publication are incorporated herein by reference in their entirety.
Optical beams 114a-b generated by lasers 112a-b are diverging or collimated light beams, each having a generally oval cross-section. The generally oval cross-section is produced because laser 112 emits light having different angular spreads along different axes orthogonal to the emission axis of the laser. For example, laser 112a emits light having different angular spreads along the Y and Z coordinate axes. Similarly, laser 112b emits light having different angular spreads along the X and Z coordinate axes.
Optical beam 118 generated by laser 116 is a diverging or collimated light beam having a generally circular cross-section. This generally circular cross-section is produced because laser 116 emits light having substantially the same angular spreads along different (e.g., Y and Z) axes orthogonal to the emission axis of the laser. An elliptical diffuser 120 located in front of laser 116 transforms optical beam 118 into a diverging light beam 122 having a generally oval cross-section, thereby making light beam 122 qualitatively similar to light beams 114a-b.
A color combiner (also often referred to as an X-cube) 126 (re)directs the light beams received from lasers 112a, 112b, and 116 toward OBS section 130. In one embodiment, emission characteristics of lasers 112a, 112b, and 116, beam-shaping characteristics of elliptical diffuser 120, and relative positions of different optical elements in light source 110 are selected so that, at the output of color combiner 126, the three light beams generated by the lasers overlap spatially and directionally to form a combined diverging light beam 128. In various embodiments, formation of a combined diverging light beam 128 includes maximizing the spatial overlap of two or more of light beams 114a, 114b, and 122. In various embodiments, formation of combined diverging light beam 128 includes optimizing the directionality of one or more of light beams 114a, 114b, and 122.
Although light source 110 is shown in the embodiment of
In one embodiment, each of lasers 112 in light source 110 is implemented as a semiconductor laser diode or a diode-pumped solid-state laser. As known in the art, a typical edge-emitting semiconductor laser diode emits a cone of light having a generally oval cross section. In a representative configuration, the semiconductor laser diodes (lasers 112a-b) are oriented so that the short axis of each respective oval is substantially parallel to the Z coordinate axis while the long axis of the oval is parallel to the XY plane. As a result, optical beam 128 has an anisotropic angular distribution, with the vertical angular spread of the beam being narrower than the horizontal angular spread.
In one configuration, each of lasers 112 and 116 generates S-polarized light, i.e., light whose electric field is substantially parallel to the Z coordinate axis. In addition, light source 110 can have an optional polarizer or other birefringent element (not explicitly shown) that serves to adjust, if necessary, the polarization of optical beam 128 to S polarization. One skilled in the art will understand that light source 110 can alternatively be configured to generate P-polarized light, provided that modulator section 160 is reconfigured accordingly so that a P-polarized input, instead of the S-polarized input, is appropriate for operation of modulator section 160. Such reconfiguration can include, e.g., relocating a spatial light modulator (SLM) 166 and a field lens 164 in modulator section 160 from the shown positions, which are next to a face 163a of a polarization beam splitter (PBS) 162, to similar positions next to a face 163b of the PBS.
In one embodiment, the following commercially available lasers can be used to implement lasers 112 and 116 in light source 110: (1) laser models HL6388MG and HL6385DG manufactured by Opnext, Inc. (Japan) for the generation of red light; (2) laser models NDHB510APA and NDB711E manufactured by Nichia Corporation (Japan) for the generation of blue light; and (3) laser models MiniGreen 200 and MiniGreen 150 manufactured by Snake Creek Lasers LLC (Pennsylvania) for the generation of green light.
OBS section 130 generally serves to (i) produce substantially uniform illumination of SLM 166 in modulator section 160 and (ii) reduce the appearance of speckle in the image projected onto screen 190. More specifically, OBS section 130 transforms optical beam 128 received from light source 110 into optical beam 158 and applies the latter beam to modulator section 160. The transformation of optical beam 128 into optical beam 158 in OBS section 130 is described in more detail below in reference to
SLM 166 is optically coupled in modulator section 160 to PBS 162 as indicated in
In various alternative embodiments, SLM 166 can operate in a transmission mode such that the light transmitted by fly's eye integrator 150 is transmitted sequentially through SLM 166 and PBS 162 toward projection lens 180. In such a transmission mode, a non-beam splitting polarizer can be substituted for the bean-splitting type polarizer illustrated. The term “fly's eye integrator” refers to an arrangement of structures configured to spatially integrate light to improve transmission efficiency and uniformity of the illumination over that provided by simple conventional lenses. For example, a structural arrangement including pairs of lenslets with one lenslet of a pair positioned at about the focal plane of the other lenslet of the pair.
SLM 166 can display a new pattern for each laser pulse. The polarization change induced by SLM 166 causes the light reflected by the SLM (from the pixels that are in the ON state) to be transmitted by PBS 162 towards projection lens 180 and screen 190, without being reflected by the PBS back toward OBS section 130. In effect, projection lens 180 is used to image the reflection pattern displayed by SLM 166 onto screen 190. If the pulse repetition rate is sufficiently high (e.g., greater than the flicker fusion rate), then the images corresponding to the three different colors are fused together in the human eye, thereby creating a perceived color image.
One skilled in the art will understand that light modulation by each pixel can be (i) binary, e.g., as described above (the pixel is either ON or OFF, so that the corresponding spot in the image is either bright or dark), or (ii) on a gray scale, which is achieved by a digital driving scheme at a rate faster than the image refresh rate for SLM 166. An alternative way to implement the gray-scale mode is to drive SLM 166 in an analog manner, wherein a pixel can be fully ON, fully OFF, and anything in between.
In various embodiments, SLM 166 is a liquid-crystal-on-silicon (LCOS) spatial light modulator. A suitable LCOS SLM that can be used as SLM 166 is manufactured by JVC Corporation and is commercially available as part of JVC Projector Model DLA-HD2K. In various alternative embodiments, SLM 166 can be a reflective switching fabric, such as a Micro-Electro-Mechanical Systems (MEMS) switch.
Lens 134a has a shorter focal length than lens 134b because the divergence angle of optical beam 128 in the horizontal (XY) plane is greater than the divergence angle of that beam in the vertical (YZ) plane. Light beam 128 first passes through lens 134a, which collimates that beam in the horizontal plane (see
FE integrator 150 comprises two two-dimensional arrays 250a-b of spherical lenslets 252. Lenslet arrays 250a and 250b are hereafter referred to as the objective lenslet array and the field lenslet array, respectively. Lenslet arrays 250a-b are arranged in a tandem as indicated in
The thickness of FE integrator 150 is selected so that lenslet arrays 250a-b are located in each other's focal planes. As a result, each lenslet 252 of objective lenslet array 250a images the angular distribution of the corresponding portion of an incoming optical beam 140 (see also
In the absence of optical diffuser 138, optical beam 140 is substantially the same as optical beam 136 (see also FIGS. 1 and 2A-B) and is a substantially collimated beam. When objective lenslet array 250a is illuminated with a collimated optical beam, the angular distribution of that beam is very narrow, which produces a plurality of virtual point sources S at a focal plane 254 of the objective lenslet array. Each lenslet 252 of field lenslet array 250b images its opposing lenslet 252 of objective lenslet array 250a at infinity. The effect of condenser lens 154 is to superimpose these images and place them at the front panel of SLM 166 (plane 266 in
With continued reference to
Optical diffuser 138 diffuses the collimated light of optical beam 136 into relatively small random angles as (exaggeratingly) illustrated in
In one embodiment, optical diffuser 138 is a transmissive liquid crystal diffuser configured to produce a dynamically changing light-scattering pattern. Temporal changes in the light-scattering pattern produce temporal and spatial variations in the pattern of angular divergence introduced by optical diffuser 138 into optical beam 140.
In an alternative embodiment, optical diffuser 138 is a glass-plate diffuser having a fixed light-scattering texture or microstructure. To produce a dynamically changing pattern of angular divergence in optical beam 140, the glass-plate diffuser shakes, vibrates, or moves in an oscillatory manner to move its light-scattering texture/microstructure with respect to FE integrator 150. One skilled in the art will appreciate that various types of periodic or non-periodic motion of optical diffuser 138 can be used. For example, optical diffuser 138 can be configured to move along a planar trajectory that is parallel to the XZ plane (see
Due to the above-described properties of the FE-integrator/condenser-lens combination in OBS section 130, the angular divergence introduced by optical diffuser 138 into optical beam 140 does not noticeably affect the sharpness of the edges of the illumination patch projected onto the active area of SLM 166 and, also, the sharpness of the image formed on screen 190. However, the dynamic variation in the pattern of divergence in optical beam 140 produced by the motion of a glass-plate diffuser causes the corresponding variations in the angular composition of the optical rays received at each point on screen 190. For example, the ray-trace analysis shown in
In laser image projectors, speckle reduction is generally based on averaging two or more independent speckle configurations within the spatial and/or temporal resolution of the detector, such as the human eye. For the human eye, the averaging time can be deduced from a physiological parameter called the flicker fusion threshold or flicker fusion rate. More specifically, light that is pulsating at a rate lower than the flicker fusion rate is perceived by humans as flickering. In contrast, light that is pulsating at a rate higher than the flicker fusion rate is perceived as being constant in time. Flicker fusion rates vary from person to person and also depend on the individual's level of fatigue, the brightness of the light source, and the area of the retina that is being used to observe the light source. Nevertheless, very few people perceive flicker at a rate higher than about 75 Hz. Indeed, in cinema and television, frame delivery rates are between 20 and 60 Hz, and 30 Hz, is normally used. For the overwhelming majority of people, these rates are higher than their flicker fusion rate.
Independent speckle configurations may be produced using diversification of phase, propagation angle, polarization, and/or wavelength of the illuminating laser beam. As clear from the description given above in reference to
For optimal operation of projector 100, the degree of angular divergence introduced by optical diffuser 138 does not preferably exceed a certain threshold value. More specifically, as already indicated above, each lenslet 252 of objective lenslet array 250a images the angular distribution of the corresponding portion of optical beam 140 on the footprint of its opposing lenslet 252 in field lenslet array 250b. This means that each pair of opposing lenslets 252 in FE integrator 150 has a certain degree of directional acceptance. The extent of this directional acceptance depends on the radius of curvature (or focal length) and lateral dimensions of individual lenslets 252. Only light that falls on a lenslet 252 of objective lenslet array 250a within the angular acceptance of the opposing lenslet 252 in field lenslet array 250b is properly relayed to SLM 166 and then to screen 190. Light beyond this directional acceptance produces crosstalk between neighboring pairs of opposing lenslets 252, which manifests itself in form of detrimental ghost illumination patches within the active area of SLM 166.
In various embodiments, different optical elements of OBS section 130 work together in a synergistic manner to enable projector 100 to increase optical throughput between laser source 110 and screen 190 and/or reduce speckle noise in the projected image. For example, the above-described properties of the combination of FE integrator 150 and optical diffuser 138 enable projector 100 to have high optical throughput between laser source 110 and screen 190, high illumination homogeneity across SLM 166 and the screen, and high temporal/spatial stability of the illumination patch despite the angular divergence and temporal variability introduced by configurations of the optical diffuser. At the same time, FE integrator 150 and optical diffuser 138 are able to provide for effective diversification of propagation angle and phase at screen 190, which reduces the speckle noise in the projected image in a very efficient manner.
In contrast, typical prior-art solutions disadvantageously suffer from optical-efficiency losses and/or temporal/spatial instabilities because the use of an optical diffuser makes the light passing therethrough relatively difficult to collect due to the fact that this light is spatially divergent and temporally shifting. In general, the inherent contradiction between (i) the desired homogeneity and stability of the illumination patch for the production eye-pleasing images and (ii) the required high temporal and spatial variability within the illumination patch for speckle-reduction purposes forces prior-art designs to make concessions and/or compromises in either the optical throughput or the attainable level of speckle noise, or both. These problems inherent to prior-art designs can be overcome in projector 100 by the use, in the above-described manner, of FE integrator 150 and optical diffuser 138. This use can advantageously make the above-mentioned concessions/compromises unnecessary and avoidable.
The use of crossed cylindrical lenses 134a-b in OBS section 130 further helps to maximize the above-described advantages of projector 100 over the prior art. More specifically, crossed cylindrical lenses 134a-b enable OBS section 130 to tailor the cross-section of optical beam 136 applied to optical diffuser 138 and thereafter to FE integrator 150 so that (i) the cross-section substantially matches the lateral size/geometry of the FE integrator and (ii) the angular spread in the light received at each particular point on screen 190 is effectively maximized. The former characteristic helps to achieve a high optical throughput despite the anisotropic properties of the laser beams (i.e., elliptical emission cones) produced by lasers 112 in laser source 110. The latter characteristic helps to increase the diversification of angle at screen 190 to a maximum possible degree for the given lateral size of FE integrator 150 (see, e.g.,
Lenslet array 350 can have a dimension ranging, e.g., from about 1 mm to about 20 mm on a side. Lenslet 352 can have an in-plane dimension ranging, e.g., from about 50 μm to about 1 mm. F-numbers for lenslet 352 can range, e.g., from about 0.8 to about 10. These parameters are chosen so that the illumination light patch on SLM 166 can match, to a desired extent, the size and/or geometry of the active area of the SLM.
An exemplary lenslet 352 is square or rectangular and the lenslet array is a two dimensional array of lenslets 352, as illustrated in
While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Although speckle reduction in projector 100 has been described in reference to configurable optical diffuser 138, other speckle reduction methods, e.g., those employing polarization and/or wavelength diversities, can additionally be used in that projector. For example, an optional polarization rotator 168 can be configurable and placed (i) between PBS 162 and projection lens 180 or (ii) within the projection lens, or (iii) after the projection lens to impart temporal variations on the state of polarization of beam 170 (see
As used herein, the phrase “superimposed in a manner that is substantially independent of temporal variations” means that the illumination patches (light spots) produced by different pairs of opposing lenslets 252 of FE integrator 150 on the active area of SLM 166 remain overlapped with an overlap area that is, e.g., greater than about 90% of the area of an individual illumination patch despite the presence of temporal variations in the pattern of angular divergence produced by optical diffuser 138.
Embodiments of the invention(s) described above may be embodied in other specific apparatus and/or methods. For example, an image-projection structure disclosed herein can be used for near-eye display applications if projection lens 180 is modified to provide a virtual image of SLM 166 to enable image viewing by looking directly toward projection lens 180 rather than the screen. For such applications, the use of optical diffuser 138 might be optional because the speckle noise may not be strong enough to be sufficiently detrimental to the viewing experience, although inclusion of the diffuser can still assist in enhancing the uniformity of illumination. The described embodiments are to be considered in all respects as only illustrative and not restrictive. In particular, the scope of the invention is indicated by the appended claims rather than by the description and figures herein. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope.
The description and drawings merely illustrate the principles of the invention. It will thus be appreciated that those of ordinary skill in the art will be able to devise various arrangements that, although not explicitly described or shown herein, embody the principles of the invention and are included within its spirit and scope. Furthermore, all examples recited herein are principally intended expressly to be only for pedagogical purposes to aid the reader in understanding the principles of the invention and the concepts contributed by the inventor(s) to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. Moreover, all statements herein reciting principles, aspects, and embodiments of the invention, as well as specific examples thereof, are intended to encompass equivalents thereof.
Unless explicitly stated otherwise, each numerical value and range herein should be interpreted as being approximate as if the word “about” or “approximately” preceded the value of the value or range.
It will be further understood that various changes in the details, materials, and arrangements of the parts which have been described and illustrated in order to explain the nature of this invention may be made by those skilled in the art without departing from the scope of the invention as expressed in the following claims.
It should be understood that the steps of the exemplary methods set forth herein are not necessarily required to be performed in the order described, and the order of the steps of such methods should be understood to be merely exemplary. Likewise, additional steps may be included in such methods, and certain steps may be omitted or combined, in methods consistent with various embodiments of the present invention.
Reference herein to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments necessarily mutually exclusive of other embodiments as some embodiments can be combined with other embodiments to form new embodiments. The same applies to the term “implementation.”
The use of terms such as height, length, width, top, bottom, is strictly to facilitate the description of the invention and is not intended to limit the invention to a specific orientation. For example, height does not imply only a vertical rise limitation, but is used to identify one of the three dimensions of a three dimensional structure as shown in the figures. Such “height” would be vertical where the electrodes are horizontal but would be horizontal where the electrodes are vertical, and so on. Similarly, while all figures show the different layers as horizontal layers such orientation is for descriptive purpose only and not to be construed as a limitation.
The terms “couple,” “coupling,” “coupled,” “connect,” “connecting,” or “connected” refer to any manner in which energy is allowed to be transferred between two or more elements, and includes the indirect transfer of energy such that the interposition of one or more additional elements is contemplated, although not required. Conversely, the terms “directly coupled,” “directly connected,” etc., imply the absence of such additional elements.