Systems and methods for digital laser projection with increased contrast using fourier filter

Abstract

To provide a projection system achieving a static contrast ratio of at least 30000 to 1.SOLUTION: An optical filter to increase contrast of an image generated with a spatial light modulator includes a lens for spatially Fourier transforming modulated light from the spatial light modulator, and an optical filter mask positioned at a Fourier plane of the lens to filter the modulated light. The modulated light has a plurality of diffraction orders, and the optical filter mask transmits at least one of the diffraction orders of the modulated light and blocks a remaining portion of the modulated light. A method that improves contrast of an image generated with a spatial light modulator includes steps of: spatially Fourier transforming modulated light from the spatial light modulator onto a Fourier plane; and filtering the modulated light by transmitting at least one diffraction order of the modulated light at the Fourier plane and blocking a remaining portion of the modulated light at the Fourier plane.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] [Cross - Reference to Related Applications] This application claims priority based on U.S. Provisional Application No. 62 / 651,657, filed on April 2, 2018, and European Patent Application No. EP18180390.9, filed on June 28, 2018, and incorporates the entire disclosure content of both applications herein by reference.

Background Art

[0002] The contrast of a projector represents the brightest output of the projector relative to the darkest output of the projector. The contrast ratio is a quantifiable measure of contrast and is defined as the ratio of the luminance of the brightest output of the projector to the luminance of the darkest output of the projector. The contrast ratio according to this definition is also referred to as the "static" or "native" contrast ratio.

[0003] Due to the visual adaptation of the human visual system, the range of luminance detectable by a viewer corresponds to a contrast ratio of approximately 1,000,000,000:1. However, at any given moment, the detectable range of luminance corresponds to a lower contrast ratio than this value. For example, in scotopic vision through only the rod cells in the human eye, the detectable contrast ratio at any given moment can be as high as 1,000,000:1 for some viewers, depending on the scene being viewed, the viewer's adaptation state, and biological factors.

[0004] Viewers in a cinema environment can be in different adaptation states at any given time, so they can view the same scene at different contrast ratios. The differences in adaptation states among viewers can be due to different seating positions relative to the screen, where each viewer is fixating on the screen, and how often and for how long each viewer closes their eyes. Since a cinema is used by multiple viewers, an ideal projector should have a contrast ratio high enough to accurately reproduce the image for all viewers.

SUMMARY OF THE INVENTION

PROBLEMS TO BE SOLVED BY THE INVENTION

[0005] Among projectors compliant with Digital Cinema Initiatives (DCI) specifications, there are those with a contrast ratio of 2,000:1 or less. For these digital projectors, dark and / or black areas of the image may be projected with a luminance high enough to appear brighter than the intended brightness.

MEANS FOR SOLVING THE PROBLEMS

[0006] In a first aspect, an optical filter for increasing the contrast of an image generated using a spatial light modulator includes a lens configured to spatially Fourier transform the modulated light from the spatial light modulator, the modulated light having a plurality of diffraction orders. The optical filter also includes an optical filter mask located on the Fourier plane of the lens. The optical filter mask is configured to filter the modulated light by transmitting at least one diffraction order of the modulated light spatially Fourier transformed by the lens and blocking the remaining portion of the modulated light.

[0007] In an embodiment of the first aspect, the at least one diffraction order is the zero-order diffraction order.

[0008] In an embodiment of the first aspect, the optical filter mask has a transmission region configured to transmit the zero-order diffraction order of the modulated light.

[0009] In an embodiment of the first aspect, the at least one diffraction order includes the zero-order diffraction order and one or more first-order diffraction orders.

[0010] In an embodiment of the first aspect, the optical filter mask has a transmission region configured to transmit two of the zero-order diffraction order and the first-order diffraction orders.

[0011] In an embodiment of the first aspect, the modulated light is one of red light, green light, and blue light.

[0012] In the second aspect, a modulator system for generating an image with increased contrast includes the optical filter of the first aspect and a digital micromirror device implementing a spatial light modulator.

[0013] In the third aspect, a modulator system for generating an image with increased contrast includes the optical filter of the first aspect and a collimating lens located at a position for collimating at least one diffraction order of the modulated light transmitted through the optical filter mask.

[0014] In the fourth aspect, a modulator system for generating an image with increased contrast includes the first, second, and third spatial light modulators. The first, second, and third spatial light modulators are configured to modulate the first, second, and third lights respectively according to an image to generate the first, second, and third modulated lights respectively. The modulator system also includes three optical filters of the first aspect, and the three optical filters of the first aspect form the first, second, and third optical filters respectively. The first, second, and third optical filters are each configured to transmit at least one diffraction order of the first, second, and third modulated lights respectively to generate the first, second, and third filtered lights respectively, and block the remaining portions of the first, second, and third modulated lights respectively. The modulator system also includes a beam combiner configured to combine the first, second, and third filtered lights into output light.

[0015] In an embodiment of the fourth aspect, each of the first, second, and third optical filter masks corresponding to the first, second, and third optical filters respectively has at least one transmission region configured to transmit the zero-order diffraction order and one or more first-order diffraction orders of the corresponding modulated light among the first, second, and third modulated lights.

[0016] In an embodiment of the fourth aspect, each of the first, second, and third spatial light modulators is a digital micromirror device.

[0017] In an embodiment of the fourth aspect, the first, second, and third lights are red, green, and blue respectively.

[0018] In an embodiment of the fourth aspect, the modulator system further includes first, second, and third output lenses located at positions where the first, second, and third filtered lights are collimated respectively before being combined by the beam combiner.

[0019] In an embodiment of the fourth aspect, the modulator system further includes a projector lens configured to project the output light onto a screen.

[0020] In a fifth aspect, a time-division modulator system for generating an image with increased contrast includes a spatial light modulator configured to modulate time-division multiplexed light according to an image to form a repetitive sequence of first, second, and third modulated lights as time-division multiplexed modulated light. The time-division modulator system also includes a lens configured to spatially Fourier transform the time-division multiplexed modulated light onto a Fourier plane, and a filter wheel located on the Fourier plane and including a plurality of optical filter masks, each of the optical filter masks being configured to transmit at least one diffraction order of a corresponding one of the first, second, and third modulated lights spatially Fourier transformed by the lens and block the remaining portions of the corresponding one of the first, second, and third modulated lights, thereby filtering the corresponding one of the first, second, and third modulated lights. The filter wheel is configured to rotate in synchronization with the time-division multiplexed modulated light such that each of the optical filter masks is positioned within the time-division multiplexed modulated light on the Fourier plane when the time-division multiplexed modulated light is the corresponding one of the first, second, and third modulated lights.

[0021] In an embodiment of the fifth aspect, the spatial light modulator is a digital micromirror device.

[0022] In an embodiment of the fifth aspect, the plurality of optical filter masks are three sets of optical filter masks, each set including n optical filter masks per set, where n is a positive integer, and each of the three sets is configured to filter a corresponding one of the first, second, and third modulated lights.

[0023] In an embodiment of the fifth aspect, the plurality of optical filter masks include first, second, and third optical filter masks configured to filter the first, second, and third modulated lights, respectively.

[0024] In an embodiment of the fifth aspect, the first optical filter mask has a transmission region configured to transmit the zero-order diffraction order and one or more first-order diffraction orders of the first modulated light, the second optical filter mask has a transmission region configured to transmit the zero-order diffraction order and one or more first-order diffraction orders of the second modulated light, and the third optical filter mask has a transmission region configured to transmit the zero-order diffraction order and one or more first-order diffraction orders of the third modulated light.

[0025] In an embodiment of the fifth aspect, the first, second, and third modulated lights are red, green, and blue, respectively.

[0026] In an embodiment of the fifth aspect, the filter wheel rotates unevenly and is further configured to stop when each of the first, second, and third optical filter masks is positioned within the time-division multiplexed modulated light.

[0027] In an embodiment of the fifth aspect, the time-division multiplexer system includes a projection lens configured to project at least one diffraction order of the corresponding modulated light among the first, second, and third modulated lights transmitted through the filter wheel onto a screen.

[0028] In the sixth aspect, a method for improving the contrast of an image generated using a spatial light modulator includes spatially Fourier-transforming the modulated light from the spatial light modulator onto a Fourier plane. The modulated light has a plurality of diffraction orders. The method also includes filtering the modulated light by transmitting at least one diffraction order of the modulated light at the Fourier plane and blocking the remaining portion of the modulated light at the Fourier plane. including filtering the modulated light by transmitting at least one diffraction order of the modulated light at the Fourier plane and blocking the remaining portion of the modulated light at the Fourier plane.

[0029] In an embodiment of the sixth aspect, the at least one diffraction order is the zero-order diffraction order.

[0030] In an embodiment of the sixth aspect, the step of transmitting includes transmitting the zero-order diffraction order through a transmission region of an optical filter mask.

[0031] In an embodiment of the sixth aspect, at least one diffraction order includes a zero-order diffraction order and one or more first-order diffraction orders.

[0032] In an embodiment of the sixth aspect, the step of transmitting includes transmitting the zero-order diffraction order and one or more first-order diffraction orders through a transmissive region of an optical filter mask.

[0033] In an embodiment of the sixth aspect, the modulated light is one of red light, green light, and blue light.

[0034] In an embodiment of the sixth aspect, the method further includes operating a plurality of micromirrors of the spatial light modulator to generate modulated light.

[0035] In an embodiment of the sixth aspect, the method includes collimating at least one diffraction order after the step of transmitting.

[0036] In a seventh aspect, a method for projecting a color image with increased contrast includes spatially modulating first, second, and third input lights according to a color image to respectively generate first, second, and third modulated lights. Each of the first, second, and third modulated lights has a plurality of diffraction orders. The method also includes filtering the first, second, and third modulated lights to respectively generate first, second, and third filtered lights by (i) transmitting at least one of the diffraction orders of each of the first, second, and third modulated lights, (ii) blocking the remaining portions of the first, second, and third modulated lights, and (iii) combining the first, second, and third filtered lights into output light.

[0037] In an embodiment of the seventh aspect, the first, second, and third input lights are red, green, and blue, respectively.

[0038] In an embodiment of the seventh aspect, the method further includes the step of projecting the output light onto a screen.

[0039] In the eighth aspect, a time - multiplexing method for generating and projecting an image with increased contrast includes generating time - multiplexed modulated light that forms a repeating sequence of first, second, and third modulated lights by modulating time - multiplexed light using a spatial light modulator according to an image. The method also includes spatially Fourier - transforming the time - multiplexed modulated light using a lens and filtering the time - multiplexed modulated light by rotating a filter wheel in synchronization with the time - multiplexed modulated light. The filter wheel includes a plurality of optical filter masks. Each of the optical filter masks is configured to filter a corresponding one of the first, second, and third modulated lights spatially Fourier - transformed by the lens. The step of rotating includes positioning each of the optical filter masks into the time - multiplexed modulated light at the Fourier plane of the lens when the time - multiplexed modulated light is a corresponding one of the first, second, and third modulated lights. configured. The step of rotating includes positioning each of the optical filter masks into the time - multiplexed modulated light at the Fourier plane of the lens when the time - multiplexed modulated light is a corresponding one of the first, second, and third modulated lights.

[0040] In an embodiment of the eighth aspect, the spatial light modulator is a digital micromirror device.

[0041] In an embodiment of the eighth aspect, the plurality of optical filter masks are three sets of optical filter masks, each set including n optical filter masks per set, where n is a positive integer, and each of the three sets is configured to filter a corresponding one of the first, second, and third modulated lights.

[0042] In an embodiment of the eighth aspect, the plurality of optical filter masks are first, second, and third optical filter masks configured to filter the first, second, and third modulated lights, respectively.

[0043] In an embodiment of the eighth aspect, the filtering includes transmitting the zero-order diffraction order and one or more first-order diffraction orders of the first modulated light through the transmissive region of the first optical filter mask, transmitting the zero-order diffraction order and one or more first-order diffraction orders of the second modulated light through the transmissive region of the second optical filter mask, and transmitting the zero-order diffraction order and one or more first-order diffraction orders of the third modulated light through the transmissive region of the third optical filter mask.

[0044] In an embodiment of the eighth aspect, the first, second, and third modulated lights are red, green, and blue, respectively.

[0045] In an embodiment of the eighth aspect, the rotating step further includes rotating non-uniformly and stopping when each of the first, second, and third optical filter masks is positioned within the time-division multiplexed modulated light.

[0046] In an embodiment of the eighth aspect, the method further includes projecting the filtered time-division multiplexed modulated light onto a screen after the filtering.

Brief Description of the Drawings

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[0048] FIG. 1 is a functional diagram of an optical filter 110 that improves the contrast of an image generated using a spatial light modulator (SLM). FIG. 1 shows the optical filter 110 in one example of use. Here, the optical filter 110 is implemented in a digital projector 100 and increases the contrast of an image projected onto a screen 116 by the digital projector 100. The digital projector 100 includes an SLM 102 that modulates input light 106 according to input data representing an image to be projected by the digital projector 100 to produce modulated light 104.

[0049] The optical filter 110 filters the modulated light 104 by blocking a part 114 of the modulated light 104. A part 114 of the modulated light 104 is projected onto the screen 116 by the digital projector 100 even when the optical filter 110 is not present and the SLM 102 is controlled so as not to output light to the screen 116. The optical filter 110 outputs the transmitted part of the modulated light 104 as filtered light 108. The digital projector 100 includes a projection lens 112 that projects the filtered light 108 onto the screen 116. In the absence of the optical filter 110, the blocked part 114 of the modulated light 104 corresponds to the lower limit of the luminous intensity of the digital projector 100, and thus determines how dark the projected image is. By blocking the blocked part 114 of the modulated light 104, the optical filter 110 lowers the lower limit, thereby increasing the contrast of the digital projector 100.

[0050] As will be described in detail later, the blocked part 114 of the modulated light 104 corresponds to one or more diffraction orders of the modulated light 104 generated when the input light 106 is diffracted by the SLM 102. The SLM 102 can be any type of spatial light modulator that (1) has a periodic structure that functions as a diffraction grating and (2) modulates the optical phase of the input light 106 to control light between two states (e.g., on state and off state). In one example, the SLM 102 is a digital micromirror device (DMD) that deflects light by tilting a plurality of micromirrors and modulates the optical phase of the input light 106. In other examples, the SLM 102 is a reflective liquid crystal on silicon (LCOS) phase modulator or a transmissive liquid crystal (LC) phase modulator, each of which controls light by modulating the refractive index of the liquid crystal.

[0051] Figures 2 and 3 are a front view and a side view, respectively, of a DMD 200 used to generate an image, which is a part of a digital projector (e.g., digital projector 100). The DMD 200 is an example of the SLM 102. In the following description, both Figures 2 and 3 are referred to.

[0052] The DMD200 is a micro-opto-electro-mechanical system (MOEMS) SLM having a plurality of square micro-mirrors 202 arranged in a two-dimensional rectangular array on a substrate 204 in the x-y plane (see the right-handed coordinate system 220). In one embodiment, the DMD200 is a digital light processor manufactured by Texas Instruments. Each of the micro-mirrors 202 corresponds to one pixel of an image and, by electrostatic drive, tilts about a rotation axis 208 oriented in a direction of -45° with respect to the x-axis to redirect the input light 206. For clarity, FIG. 2 shows only representative micro-mirrors 202 at the corners and in the center of the DMD200, and in FIG. 3, all the micro-mirrors 202 are not labeled.

[0053] FIG. 3 shows the micro-mirrors 202 tilted to redirect the input light 206. The micro-mirror 202(1) is driven to the on posture to specularly reflect the input light 206 into an on reflected light 306 parallel to the z-axis (see the coordinate system 220). The micro-mirror 202(2) is driven to the off posture to specularly reflect the input light 206 into an off reflected light 320 that travels in a direction toward a beam dump (not shown) that absorbs the off reflected light 320. The micro-mirror 202(3) is in a state parallel to the flat substrate 204 (e.g., the x-y plane) and is not driven. The front surface 304 of each micro-mirror 202 may be coated with a layer of deposited metal (e.g., aluminum) that functions as a reflective surface for reflecting the input light 206. The gap 310 between adjacent micro-mirrors 202 may be absorptive. That is, the input light 206 incident on the gap 310 is absorbed by the substrate 204. For clarity, the mechanical structure that physically couples the micro-mirrors 202 to the substrate 204 is not shown. Without departing from the scope of the present invention, the DMD200 may be implemented to direct the on reflected light 306 and the off reflected light 320 in respective directions different from those shown in FIG. 3. Also, the DMD200 may be configured such that each micro-mirror 202 is at an arbitrary angle with respect to the substrate 204 when not driven.

[0054] A digital projector using the DMD200 may be designed considering only specular reflection of the input light 206 from the micromirrors 202. However, the micromirrors 202 and the gap 310 cooperate to form a two-dimensional grating that diffracts the input light 206. Thus, the modulated light propagating away from the DMD200 can form multiple diffraction orders that can be observed as Fraunhofer diffraction patterns (see diffraction patterns 700 and 800 shown in FIGS. 7 and 8 respectively) in the far field of the DMD200 or at the focal plane of the lens. Each diffraction order corresponds to one light beam propagating away from the DMD200 in a corresponding unique direction. By design, most of the optical power of the modulated light from the DMD200 is in the zero-order diffraction order corresponding to the specularly reflected on and off reflected lights 306 and 320.

[0055] Diffraction of the input light 206 by the DMD200 can reduce the projector contrast ratio (PCR) of a digital projector using the DMD200 (e.g., the digital projector 100 of FIG. 1 without the optical filter 110). The PCR of the projector is defined herein as the ratio of the on-luminance and off-luminance (or corresponding to the first and second photometric luminances) measured on a projection screen (e.g., screen 116 of FIG. 1) irradiated by the projector. The on-luminance and off-luminance are generated when the projector is controlled to output its brightest output (e.g., white) and darkest output (e.g., black), respectively. When the digital projector uses the DMD200, the on-luminance and off-luminance are generated when all the micromirrors 202 are in the on and off postures, respectively.

[0056] ​How the DMD200 diffracts the input light 206 can be determined by various parameters such as (1) the wavelength of the input light 206, (2) the direction of the input light 206, (3) the pitch 212 of the DMD200, (4) the width 210 of the gap 310 of the DMD200, and (5) the on-tilt angle and off-tilt angle of the micromirror 202. As shown in FIG. 2, in both the x-direction and y-direction of the DMD200, the pitch 212 is equal to the sum of the width 210 and the micromirror edge length 218. The pitch 212 can be 5 to 15 microns. The width 210 can be less than 1 micron. In one example, the pitch 212 is 7 to 8 microns and the width 210 is 0.7 to 0.9 microns.

[0057] FIG. 4 is a side view of an optical filter 400 that spatially filters the modulated light 402 from the DMD 200 to increase the PCR of the digital projector 100. The optical filter 400 is an example of the optical filter 110. In the optical filter 400, the DMD 200 may be replaced by other types of SLMs 102 (e.g., a reflective LCOS modulator or a transmissive LC phase modulator) as long as it does not depart from the scope of the present invention. The optical filter 400 includes a lens 404 that spatially Fourier-transforms the modulated light 402 by focusing the modulated light 402 onto the Fourier plane 408. In FIG. 4, the modulated light 402 is represented by a plurality of arrows. Each arrow corresponds to one diffraction order and indicates the unique direction in which that diffraction order propagates. In one embodiment, the center of the DMD 200 is located on the optical axis 422 defined by the lens 404 as shown in FIG. 4. In other embodiments, the center of the DMD 200 is offset from the optical axis 422. The lens 404 has a focal length 410, and the Fourier plane 408 is located at the focal plane of the lens 404. An optical filter mask 412 located on the Fourier plane 408 spatially filters the modulated light 402 Fourier-transformed by the lens 404. The spatial Fourier transform performed by the lens 404 converts the propagation angle of the modulated light 402 of each diffraction order to the corresponding spatial position on the Fourier plane 408. Thereby, the lens 404 can select the desired diffraction order and not select the unwanted diffraction order by spatial filtering on the Fourier plane 408. The spatial Fourier transform of the modulated light 402 on the Fourier plane 408 corresponds to the Fraunhofer diffraction pattern of the modulated light 402.

[0058] The optical filter mask 412 has at least one transmission region 416 that completely or partially transmits at least one diffraction order of the modulated light 402 as the filtered light 414 through the optical filter mask 412. In certain embodiments, the optical filter mask 412 is opaque at positions where unwanted diffraction orders of the modulated light 402 impinge. In some embodiments, the optical filter mask 412 is opaque at positions where the optical filter mask 412 does not have a transmission region 416. In other embodiments, the optical filter mask 412 is configured to reflect a desired diffraction order, as opposed to transmission, in order to spatially separate the desired diffraction order from unwanted diffraction orders.

[0059] In one embodiment, the optical filter 400 includes a collimating lens 418 that collimates the filtered light 414 into parallel light 420. The collimating lens 418 can facilitate the integration of the optical filter 400 with other optical elements or optical systems. For example, the lens 418 can couple the filtered light 414 to a further optical element (e.g., a projector lens 112, or a beam combiner 1504 described below with reference to FIG. 15) located downstream of the optical filter 400. The collimating lens 418 has a focal length 424 and is positioned such that the focal plane of the collimating lens 418 coincides with the Fourier plane 408. Although the focal lengths 410 and 424 are shown to be equal in FIG. 4, the focal lengths 410 and 424 can be different from each other without departing from the scope of the present invention. In other embodiments, the optical filter 400 includes a lens similar to the collimating lens 418 that optically couples the filtered light 414 to a further optical element (e.g., a projector lens 112) located downstream of the optical filter 400.

[0060] For clarity, FIG. 4 shows only the diffracted beams diffracting in one direction (e.g., the x direction). However, since the DMD 200 diffracts in two dimensions, the modulated light 402 includes diffracted beams diffracted by the DMD 200 in a second dimension (e.g., the y direction) perpendicular to the optical axis 422. Each diffracted beam in the two-dimensional diffraction pattern can be represented by a pair of integers that specify the diffraction order of the diffracted beam for each of the two dimensions. In this specification, "zero order" refers to one diffracted beam having a zero order in both of the two dimensions. Also, without departing from the scope of the present invention, each arrow shown as part of the modulated light 402 in FIG. 4 can indicate a group of diffraction orders in the vicinity of each other, such as a group consisting of a zero-order diffraction order and a plurality of first-order diffraction orders.

[0061] FIGS. 5 and 6 are side views of an exemplary digital projector 500 that includes the DMD 200 and the projection lens 112 but does not include the optical filter 110. FIGS. 5 and 6 show how the diffraction orders of the modulated light 402 from the DMD 200 reduce the PCR of the digital projector 500. In FIG. 5, the digital projector 500 generates an on light intensity by driving all of the micromirrors 202 of the DMD 200 to the on position (see the micromirror 202(1) in the enlarged view 516). In FIG. 6, the digital projector 500 generates an off light intensity by driving all of the micromirrors 202 of the DMD 200 to the off position (see the micromirror 202(1) in the enlarged view 616). In FIGS. 5 and 6, the centers of the DMD 200 and the projection lens 112 are located on the optical axis 422 in the x direction and the y direction (see the coordinate system 220). In the following description, reference will be made to FIGS. 5 and 6.

[0062] In FIG. 5, the DMD 200 diffracts the input light 206 to produce an on-modulated light 502 having a plurality of on-diffraction beams 504. In FIG. 6, the DMD 200 diffracts the input light 206 to produce an off-modulated light 602 having a plurality of on-diffraction beams 604. In the far field of the DMD 200, each on-diffraction beam 504 corresponds to one diffraction order or peak of the Fraunhofer diffraction pattern formed by the on-modulated light 502, and each on-diffraction beam 604 corresponds to one diffraction order or peak of the Fraunhofer diffraction pattern formed by the off-modulated light 602. In the far field of the DMD 200, each of the on- and off-diffraction beams 504, 604 corresponds to a k-vector having one of a plurality of propagation directions 510. In the examples of FIGS. 5 and 6, the propagation directions 510 are represented by dashed lines. Each of the on- and off-diffraction beams 504, 604 is aligned in a straight line with one of the propagation directions 510 and is represented by a solid line arrow having a length corresponding to its power or intensity.

[0063] One aspect of this embodiment is that, for a certain fixed direction of the input light 206, while the power / intensity of the on- and off-diffraction beams 504, 604 changes when the micromirrors 202 of the DMD 200 are switched between the on and off postures, when the micromirrors 202 of the DMD 200 are switched between the on and off postures, the propagation directions 510 of the on- and off-diffraction beams 504, 604 remain the same. This is achieved.

[0064] In the example of FIG. 5, the input light 206 is a monochromatic plane wave propagating toward the DMD 200 such that the on-diffraction beam 504(1) propagates along the optical axis 422. The on-diffraction beam 504(1) contains most of the power of the on-modulated light 502. The on-diffraction beam 504(1) may represent the zero-order diffraction order of the on-modulated light 502 or a plurality of diffraction orders in the vicinity of each other (e.g., the zero-order diffraction order and some first-order diffraction orders).

[0065] FIG. 5 also shows an on-diffraction beam 504(2) that propagates along a direction different from that of the on-diffraction beam 504(1) but passes through the effective aperture 508 of the projector lens 112. The power in the on-diffraction beam 504(2) is smaller than the power in the on-diffraction beam 504(1). A plurality of on-diffraction beams 518 including the on-diffraction beams 504(1) and 504(2) pass through the effective aperture 508 of the projector lens 112. The projector lens 112 projects the on-diffraction beam 518 onto a projection screen as on-projected light 514.

[0066] FIG. 5 also shows an on-diffraction beam 504(3) that propagates along a direction away from the effective aperture 508. The projector lens 112 does not project the on-diffraction beam 504(3) onto the projection screen. The power in the on-diffraction beam 504(3) is a small portion of the power of the on-modulated light 502. Therefore, even if the on-diffraction beam 504(3) is excluded from the on-projected light 514, the impact on the optical power efficiency of the digital projector 500 is minimal.

[0067] FIG. 6 shows off-diffraction beams 604(1), 604(2), and 604(3) corresponding to the on-diffraction beams 504(1), 504(2), and 504(3) of FIG. 5, respectively. The off-diffraction beam 604(3) propagates in a direction away from the optical axis 422 and diverges from the effective aperture 508. Most of the power of the off-modulated light 602 is in the off-diffraction beam 604(3), so it is not projected onto the projection screen.

[0068] In FIG. 6, the off-diffraction beams 604(1) and 604(2) pass through the effective aperture 508 and are projected as part of the off-projected light 614. The power in the off-diffraction beams 604(1) and 604(2) is smaller compared to the power in the off-diffraction beam 604(3). However, the power in the off-diffraction beams 604(1) and 604(2) increases the off-luminance of the digital projector 500, thus reducing the PCR of the digital projector 500.

[0069] With most of the optical power of the on-modulated light 502 in the on-diffraction beam 504(1), among the plurality of on-diffraction beams 518 that form the on-projected light 514 through the effective aperture 508, the other on-diffraction beam 504 has relatively low power, so its contribution to the power in the on-projected light 514 is negligibly small. However, the corresponding off-diffraction beam 604 passing through the effective aperture 508 can significantly increase the power in the off-projected light 614 and reduce the PCR of the digital projector 500.

[0070] Another aspect of this embodiment is to filter the diffraction orders corresponding to the on-diffraction beams with low optical power such as the on-diffraction beam 504(2) above while minimizing the reduction of the optical power output and efficiency of the digital projector 500, thereby increasing the PCR. To identify the diffraction orders to be filtered, the diffraction order contrast ratio (DOCR) can be used. In this specification, for each propagation direction 510 passing through the effective aperture 508, the DOCR is defined as the ratio of the optical powers of a corresponding pair of on-diffraction beams and off-diffraction beams with the same diffraction order and the same propagation direction. For example, the diffraction orders corresponding to the on- and off-diffraction beams 504(1) and 604(1) have a high DOCR. The diffraction orders with a high DOCR are beneficial for increasing the PCR and can be preferably selected for projection onto the projection screen. On the other hand, the on- and off-diffraction beams 504(2) and 604(2) correspond to diffraction orders with a low DOCR. The diffraction orders with a low DOCR reduce the PCR and can be preferably removed by filtering to increase the PCR of the digital projector 500. For clarity, FIGS. 5 and 6 show only the diffraction beams 504, 604 that diffract in one dimension (e.g., the x direction). However, the DMD 200 diffracts the input light 206 two-dimensionally such that the modulated lights 502 and 602 also include diffraction beams diffracted by the DMD 200 in a second dimension perpendicular to the optical axis 512 (e.g., the y direction).

[0071] For simplicity, FIGS. 5 and 6 show only the diffraction beams 504, 604 that diffract in one dimension (e.g., the x direction). However, the DMD 200 diffracts the input light 206 two-dimensionally such that the modulated lights 502 and 602 also include diffraction beams diffracted by the DMD 200 in a second dimension perpendicular to the optical axis 512 (e.g., the y direction).

[0072] Figures 7 and 8 are diagrams plotting the intensities of exemplary Fraunhofer diffraction patterns 700 and 800 of the on-modulated light 502 and the off-modulated light 602, respectively. The diffraction patterns 700 and 800 correspond to the Fourier transforms generated by the lens 404 on the Fourier plane 408 when the digital projector 100 operates with the DMD 200 and the optical filter 400. The diffraction patterns 700 and 800 are numerically generated according to procedures that will be described in more detail in the "Numerical Analysis" section later. Each of the diffraction patterns 700, 800 includes a plurality of equally spaced diffraction peaks corresponding to the diffraction beams 504 and 604, respectively. In FIGS. 7 and 8, the horizontal axis 704 and the vertical axis 706 indicate the direction cosines of the diffraction peaks with respect to the x-axis and the y-axis of the coordinate system 220, respectively. In FIGS. 7 and 8, the intensities of the diffraction patterns 700 and 800 are shown according to the intensity scale 708.

[0073] The circles 702 in FIGS. 7 and 8 represent the effective apertures 508 in FIGS. 5 and 6. The diffraction peaks within the circle 702 represent the diffraction beams 518, 618 projected by the projector lens 112 as the on and off projection lights 514, 614, respectively. In FIG. 7, the brightest (e.g., highest intensity) diffraction peak 710 at the center of the circle 702 corresponds to the on-diffraction beam 504(1) in FIG. 5 and / or the zero order of the on-modulated light 502. The diffraction peaks located outside the circle 702 are not projected onto the projection screen.

[0074] In FIG. 8, since the brightest diffraction peak 810 corresponding to the off-diffraction beam 604(3) is located outside the circle 702 at a position with a higher value of the direction cosine, it is not projected onto the projection screen. However, the plurality of low-power diffraction peaks 812 within the circle 702 are projected onto the projection screen as the off projection light 614, increasing the off light intensity and reducing the PCR.

[0075] To increase the PCR, the optical filter 400 can be implemented to reduce the off-photometric intensity by blocking the diffraction orders located within the circle 702 that contribute relatively more to the off-photometric intensity than to the on-photometric intensity. The Fraunhofer diffraction patterns 700 and 800 represent the Fourier transform of the modulated light 402, and show how the transmission region 416 can be configured such that the optical filter mask 412 transmits the desired diffraction orders for projection and blocks all other unwanted diffraction orders that would otherwise be projected. Specifically, using the parameters of the lens 404, the direction cosines corresponding to each desired diffraction peak can be converted to spatial positions on the optical filter mask 412 (as possible positions of the transmission region 416 for the desired diffraction peak to pass through the optical filter mask 412). Similarly, the direction cosines associated with each unwanted diffraction peak can be converted to spatial positions on the optical filter mask 412 that block (e.g., filter) the diffraction peaks that the optical filter mask 412 is not opaque to face.

[0076] In one embodiment, the optical filter mask 412 includes one transmission region 416 having a size, shape, position, and orientation selected to optimize the PCR and / or light power efficiency of the digital projector. In another embodiment, the optical filter mask 412 has a plurality of transmission regions 416, and for each transmission region 416, the size, shape, position, and / or orientation are selected to optimize the PCR and / or light power efficiency of the digital projector.

[0077] Figures 9-14 are front views of an example of the optical filter mask 412 of FIG. 4, showing an exemplary configuration of the transmission region 416. In each of Figures 9-14, a plurality of positions 902 of diffraction orders, such as diffraction orders associated with different pairs of corresponding on and off diffraction beams 504 and 604, are indicated by X forming a two-dimensional grid. For example, in Figure 9, position 902(2) indicates one diffraction order blocked by the optical filter mask 900, and position 902(1) indicates one diffraction order transmitted by the optical filter mask 900.

[0078] Figures 9 and 10 show exemplary optical filter masks 900 and 1000 each having a circular transmission region 904 and 1004, respectively. Each of the circular transmission regions 904 and 1004 can be a hole or a material that is at least partially transmissive to light. The circular transmission regions 904 and 1004 are examples of the transmission region 416. The circular transmission region 904 has a size that allows one diffraction order to pass through the optical filter mask 900. The circular transmission region 1004 has a size that allows a plurality of diffraction orders (e.g., nine diffraction orders forming a 3×3 grid as shown in FIG. 9) to pass through the optical filter mask. Figures 9 and 10 show the circular transmission regions 904 and 1004 located at the centers of the optical filter masks 900 and 1000, respectively (and thus the centers are located on the optical axis 422), but the circular transmission regions 904 and 1004 can be offset without departing from the scope of the present invention.

[0079] Figures 11 and 12 show exemplary optical filter masks 1100 and 1200 each having a square transmission region 1104 and 1204, respectively. Each of the square transmission regions 1104 and 1204 is a square hole or a square material that is at least partially transmissive to light. The square transmission regions 1104 and 1204 are examples of the transmission region 416. The square transmission region 1104 is located at the center of the optical filter mask 1100 and has a size that allows a plurality of diffraction orders (e.g., nine diffraction orders forming a 3×3 grid as shown in FIG. 11) to pass through the optical filter mask 1100. The square transmission region 1204 is offset from the center of the optical filter mask 1200 and has a size that allows a plurality of diffraction orders (such as four diffraction orders forming a 2×2 grid) to pass through the optical filter mask 1200.

[0080] FIG. 13 shows an exemplary optical filter mask 1300 having an irregular polygonal transmission region 1304 configured to transmit three diffraction orders in the vicinity of each other through the optical filter mask 1300. The irregular polygonal transmission region 1304 is an example of the transmission region 416 and can be a hole or a material that is at least partially transmissive to light.

[0081] FIG. 14 shows an exemplary optical filter mask 1400 having a plurality of circular transmission regions 1404, for example four transmission regions 1404. Each circular transmission region 1404 has a position and size such that one diffraction order is transmitted through the optical filter mask 1400. The circular transmission regions 1404 are an example of the plurality of transmission regions 416.

[0082] The transmission region 416 can have a different shape, size, and / or position different from the examples of FIGS. 9 - 14 without departing from the scope of the present invention. In one class of implementation examples, each of the examples of the transmission region 416 shown in FIGS. 9 - 14 is a hole formed in the optical filter mask 412 (e.g., by drilling, milling, or etching). In another class of implementation examples, each of the examples of the transmission region 416 shown in FIGS. 9 - 14 is an optically transparent window, an optically translucent window, or a color filter (e.g., a dichroic filter or a thin film filter) physically bonded to the optical filter mask 412 or embedded within the optical filter mask 412. In the examples of FIGS. 9 - 14, the optical filter mask (e.g., the optical filter mask 900) is circular, but each of these optical filter masks can have a different shape (e.g., square or rectangular) without departing from the scope of the present invention. In some of the examples of FIGS. 9 - 14 (e.g., the optical filter masks 900 and 1000), the optical filter mask is configured such that its center is disposed on the optical axis 422, but each of these optical filter masks can be configured such that its center is offset from the optical axis 422 without departing from the scope of the present invention.

[0083] The optical filter mask 412 can be formed from a metal such as aluminum or stainless steel. This metal can be anodized or blackened to enhance the absorption of light blocked by the optical filter mask 412. Alternatively, the optical filter mask 412 can be formed from a semiconductor substrate such as silicon, in which a transmission region 416 is provided by etching or grinding. In another embodiment, the optical filter mask 412 is formed from an optically transparent substrate (e.g., glass) coated with a light-absorbing material (e.g., black paint) to block light in regions that do not coincide with the transmission region 416. In another embodiment, the optical filter mask 412 is an active optical filter mask having a dynamically configurable transmission region 416 such as an array of electronically controlled mirrors.

[0084] In some embodiments, the optical filter mask 412 includes a "gradual" or "soft" edge between the transmission region 416 (e.g., transmission regions 904, 1004, 1104, 1204, 1304, and / or 1404) of the optical filter mask 412 (e.g., optical filter masks 900, 1000, 1100, 1200, 1300, and 1400) and the surrounding opaque region. The gradual edge has an optical density that increases monotonically in space from a relatively low value (e.g., 0) at a point close to the transmission region 416 to a relatively high value (e.g., 10 or more) at a point close to the surrounding opaque region, and can be formed from a material (e.g., glass, plastic). Using a gradual edge, the transmission region 416 does not have a sharp edge where the transmissivity drops abruptly (i.e., a step change in the spatial transmissivity). The gradual edge has the advantage of reducing the degree to which the optical filter mask 412 should be accurately positioned on the Fourier plane 408. This is particularly suitable when the input light 106 is emitted from a laser with a small etendue of the point spread function. In comparison, a high etendue laser has a broad point spread function that blurs the light source, thereby relaxing the accuracy required for aligning the optical filter mask 412. In embodiments where the transmission region 416 is a circle of diameter d (e.g., transmission regions 904 and 1004), the gradual edge can be a ring centered on the circle, with an inner diameter of d and an outer diameter of, for example, 1.1d to 1.2d. Any portion of the optical filter mask 412 located farther from the center of the circle than the outer diameter can be opaque.

[0085] FIG. 15 is a functional diagram of an exemplary multi-color digital projector 1500 that achieves increased PCR through optical filtering of each color channel in a spatial multiplexing manner. The multi-color digital projector 1500 has a plurality of optical filters 400 and a corresponding number of DMDs 200. Each optical filter 400 is paired with a corresponding DMD 200 and handles a different primary color corresponding to each. In the digital projector 1500, each DMD 200 may be replaced by another type of SLM 102 (e.g., a reflective LCOS modulator or a transmissive LC phase modulator) without departing from the scope of the present invention. . FIG. 15 shows a multi-color digital projector 1500 having three color channels. The following description relates to these three color channels, but it should be understood that the multi-color digital projector 1500 may have only two color channels or more than three color channels.

[0086] The DMDs 200(1), 200(2), and 200(3) modulate the input lights 206(1), 206(2), and 206(3) respectively to become modulated lights 402(1), 402(2), and 403(3). The modulated lights 402(1), 402(2), and 403(3) are optically filtered by the optical filters 400(1), 400(2), and 400(3) respectively to become filtered lights 414(1), 414(2), and 414(3). The multi-color digital projector 1500 further includes a beam combiner 1504 that combines the filtered lights 414(1), 414(2), and 414(3) to form a multi-color light 1510. The projector lens 112 is configured to project the multi-color light 1510 onto a projection screen (e.g., the screen 116 in FIG. 1). The multi-color digital projector 1500 is an embodiment of the digital projector 100 extended to process three individual color inputs and output multi-color light.

[0087] In one embodiment, the multi - color digital projector 1500 includes collimating lenses 418(1), 418(2), and 418(3) that collimate filtered lights 414(1), 414(2), and 414(3) into parallel lights 420(1), 420(2), and 420(3), respectively. In this embodiment, the beam combiner 1504 combines the parallel lights 420(1), 420(2), and 420(3) as shown in FIG. 15. In an embodiment of the multi - color digital projector 1500 that does not include the collimating lens 418, the beam combiner 1504 combines the un - collimated filtered lights 414(1), 414(2), and 414(3).

[0088] In one embodiment, the multi - color digital projector 1500 includes total internal reflection (TIR) prisms 1502(1), 1502(2), and 1503(3) that reflect input lights 206(1), 206(2), and 206(3) to DMDs 200(1), 200(2), and 200(3), respectively, and transmit modulated lights 402(1), 402(2), and 402(3) through optical filters 400(1), 400(2), and 400(3), respectively. The multi - color digital projector 1500 may include mirrors 1506 and 1508 that redirect parallel lights 420(1) and 420(3) to the beam combiner 1504 as shown in FIG. 15. Although shown as a cross - dichroic (i.e., x - cube) prism in FIG. 15, the beam combiner 1504 can be another type of beam combiner known in the art.

[0089] In one implementation example of the multi - color digital projector 1500, the first, second, and third primary colors are red, green, and blue, respectively. When the input lights 206(1), 206(2), and 206(3) are monochromatic, the wavelengths of the respective input lights 206(1), 206(2), and 206(3) can be selected such that the input lights 206(1), 206(2), and 206(3) represent the red, green, and blue primary colors that are spectrally pure colors, respectively. In one such example, the wavelength of the input light 206(1) representing the red primary color is one of 615 nm, 640 nm, and 655 nm, the wavelength of the input light 206(2) representing the green primary color is one of 525 nm, 530 nm, and 545 nm, and the wavelength of the input light 206(3) representing the blue primary color is one of 445 nm, 450 nm, and 465 nm. Alternatively, the input lights 206(1), 206(2), and 206(3) can be multi - colored such that the red, green, and blue primary colors are not spectrally pure. Without departing from the scope of the present invention, the three primary colors may be a combination of colors different from red, green, and blue.

[0090] The multi - color digital projector 1500 increases the PCR by increasing the PCR of each primary color (e.g., red, green, and blue). Some of the optical processes used by the multi - color digital projector 1500, such as the diffraction of the input light 206 by the DMD 200, the reflection of the modulated light 402 by the TIR prism 1502, and the focusing of the modulated light 402 by the lens 404, are wavelength - dependent. Therefore, the respective Fraunhofer diffraction patterns of the modulated lights 402(1), 402(2), and 402(3) are wavelength - dependent. In one embodiment, the optical filter masks 412(1), 412(2), and 412(3) are individually configured based on the wavelengths of the input lights 206(1), 206(2), and 206(3), respectively, to increase the PCR of the first, second, and third primary colors.

[0091] FIG. 16 is a functional diagram showing an exemplary multi-color digital projector 1600 that achieves increased PCR through time-multiplexed optical filtering of different color channels. The digital projector 1600 includes one DMD 200 and one optical filter 1610 having a filter wheel 1612. FIG. 17 is a plot of optical power versus time for the time-multiplexed light 1601 used as input light to the multi-color digital projector 1600. FIGS. 18 and 19 show examples of the filter wheel 1612. In the following description, reference is made to FIGS. 16-19.

[0092] The time-multiplexed light 1601 includes a repetitive sequence 1702 of input light 206 that is temporally separated. The multi-color digital projector 1600 can be configured to receive and output any number of input lights of different colors, but FIGS. 17-19 and the following description relate to a three-color embodiment of the multi-color digital projector 1600. In this embodiment, the time-multiplexed light 1601 includes input lights 206(1), 206(2), and 206(3) that are temporally separated. FIG. 17 shows an example of the time-multiplexed light 1601. Here, the sequence 1702 includes the input light 206(1) of the first pulse, the input light 206(2) of the second pulse, and the input light 206(3) of the third pulse. The input lights 206(1), 206(2), and 206(3) can represent, for example, the primary colors of red, green, and blue. The pulses of the input lights 206(1), 206(2), and 206(3) are spatially overlapped for using the same DMD 200, optical filter 1610, and projector lens 112. In FIG. 17, the pulses of the input lights 206(1), 206(2), and 206(3) are shown to have similar power (e.g., pulse height), duration (e.g., pulse width), and "off" time (e.g., pulse interval) between the pulses. The multi-color digital projector 1600 can receive the input light 206 characterized by other configurations of power, duration, and "off" time without departing from the scope of the present invention. For example, one of the selected first, second, and third pulses of the input lights 206(1), 206(2), and 206(3) may have a higher power to compensate for the lower diffraction efficiency of the DMD 200 at the wavelength of the input light corresponding to the selected pulse.

[0093] The DMD200 is configured to synchronously modulate the input lights 206(1), 206(2), and 206(3) of the time-multiplexed light 1601 according to an image to obtain a time-multiplexed modulated light 1602. In other words, the micromirror 202 of the DMD200 has a first configuration when the time-multiplexed modulated light 1602 is the first input light 206(1), has a second configuration when the time-multiplexed modulated light 1602 is the second input light 206(2), and is operated to have a third configuration when the time-multiplexed modulated light 1602 is the third input light 206(3). The first, second, and third configurations may be different. In the digital projector 1600, the DMD200 may be replaced by another type of SLM 102 (e.g., a reflective LCOS modulator or a transmissive LC phase modulator) without departing from the scope of the present invention.

[0094] The optical filter 1610 is similar to the optical filter 400 in FIG. 4 except that the optical filter mask 412 is replaced on the filter wheel 1612. The filter wheel 1612 includes a plurality of optical filter masks 412 configured to synchronously filter the input lights 206(1), 206(2), and 206(3) of the time-multiplexed modulated light 1602. For example, in an embodiment where the filter wheel 1612 includes first, second, and third optical filter masks corresponding to the first, second, and third input lights 206(1), 206(2), and 206(3), the motor 1614 rotates the filter wheel 1612 so that when the time-multiplexed modulated light 1602 is the first input light 206(1), the first optical filter mask 412 intercepts the time-multiplexed modulated light 1602 at the Fourier plane 408, filters it, when the time-multiplexed modulated light 1602 is the second input light 206(2), the second optical filter mask 412 intercepts the time-multiplexed modulated light 1602 at the Fourier plane 408, filters it, and when the time-multiplexed modulated light 1602 is the third input light 206(3), the third optical filter mask 412 intercepts the time-multiplexed modulated light 1602 at the Fourier plane 408 and filters it.

[0095] In one embodiment of the multi - color digital projector 1600, the motor 1614 rotates the filter wheel 1612 step - by - step, switching different optical filter masks 412 in synchronization with the pulse trains of the input lights 206(1), 206(2), and 206(3) while maintaining the filter wheel 1612 at a stationary position while each of the pulses of the input lights 206(1), 206(2), and 206(3) propagates through the Fourier plane 408. In this embodiment, the motor 1614 operates as follows. Before the pulses of the input lights 206(1), 206(2), and 206(3) reach the Fourier plane 408, the motor 1614 rotates the filter wheel 1612 to position the corresponding optical filter mask 412 in the path of the time - multiplexed modulated light 1602 at the Fourier plane 408. When the propagation of the corresponding pulse of the filtered light through the optical filter mask 412 is completed, then the motor 1614 rotates the filter wheel 1612 to place the next optical filter mask 412 in the path of the time - multiplexed modulated light 1602 at the Fourier plane 408.

[0096] In some embodiments, the lens 404 implemented within the optical filter 1610 to condense the time - multiplexed modulated light 1602 can be configured to reduce the chromatic aberration that causes the focal length of the lens 404 to vary with wavelength. In one such embodiment, the lens 404 is an achromatic lens designed to obtain similar foci at the wavelengths of the input lights 206(1), 206(2), 206(3) so that the Fourier planes corresponding to each of the three wavelengths are determined to be in similar positions. In another such embodiment, the lens 404 is an apochromatic lens, a super - achromatic lens, an objective lens, a cemented lens comprising a plurality of lens elements, a combination of multiple lenses, and / or other optical elements, or another type of lens known in the art. The lens 404 may have one or more anti - reflection coatings that enhance the transmission of the time - multiplexed modulated light 1602 through the lens 404 at the wavelengths of the input lights 206(1), 206(2), 206(3).

[0097] In one embodiment, the multi - color digital projector 1600 includes a collimating lens 1618 that collimates the time - multiplexed light filtered by passing through the filter wheel 1612 into parallel time - multiplexed light 1606. The parallel time - multiplexed light 1606 is projected onto a screen by the projector lens 112. In another embodiment, the collimating lens 1618 is not included in the multi - color digital projector 1600, and the projector lens 112 is configured to receive the non - collimated time - multiplexed light.

[0098] FIG. 18 is a front view of an exemplary filter wheel 1800 having three sectors 1802, each including one optical filter mask. The filter wheel 1800 is an example of the filter wheel 1612. The motor 1614 rotates the filter wheel 1800 around an axis 1804. Each rotation of the filter wheel 1800 corresponds to one sequence 1702 of the time - multiplexed light 1602. In some embodiments, the motor 1614 rotates the filter wheel 1800 step - by - step as described above. In the example of FIG. 18, the first optical filter mask of the first sector 1802(1) is shown as the optical filter mask 900 of FIG. 9, the second optical filter mask of the second sector 1802(2) is shown as the optical filter mask 1300 of FIG. 13, and the third optical filter mask of the third sector 1802(3) is shown as the optical filter mask 1400 of FIG. 14. However, the optical filter masks of the sectors 1802 may have transmission regions (e.g., transmission region 416) with other shapes, sizes, and positions different from FIG. 18 without departing from the scope of the present invention.

[0099] ​In one embodiment, the multi - color digital projector 1600 is configured to display an image without predetermined temporal artifacts, and the period of the sequence 1702 is, for this purpose, shorter than the response time of the human visual system. For example, the multiplexing frequency of the time - multiplexed light 1601 is equal to the reciprocal of the period of the sequence 1702, but higher than the flicker fusion rate that utilizes the persistence of vision. The multiplexing frequency can be 1 kilohertz or more corresponding to a pulse width of less than 1 millisecond for each of the input lights 206(1), 206(2), and 206(3).

[0100] FIG. 19 is a front view of an exemplary filter wheel 1900 having six sectors 1902 each including one optical filter mask. The motor 1614 rotates the filter wheel 1900 around the axle 1804 such that each complete rotation of the filter wheel 1900 corresponds to two consecutive repetitions of the sequence 1702. One advantage of the filter wheel 1900 over the filter wheel 1800 is that since the filter wheel 1900 rotates at half the multiplexing frequency of the time - multiplexed light 1601, the power - consumption requirements and speed requirements of the motor 1614 are reduced. In another embodiment, the filter wheel 1612 has 3×n sectors (n is a positive integer). Each set of 1 set of 3 sectors includes 3 optical filter masks, and each complete rotation of the filter wheel 1900 corresponds to n consecutive repetitions of the sequence 1702, thereby enabling the motor 1614 and the filter wheel 1612 to rotate at 1 / n times the multiplexing frequency of the time - multiplexed light 1601. In one example of use, the motor 1614 rotates the filter wheel 1900 step - by - step such that each optical filter mask of the filter wheel 1900 remains stationary while filtering the corresponding pulse of the input light 206.

[0101] FIG. 20 shows a method 2000 for improving the contrast of an image generated using a spatial light modulator. The method 2000 can be performed by an optical filter 400. The method 2000 includes a step 2002 of spatially Fourier-transforming the modulated light from the spatial light modulator onto a Fourier plane. The modulated light has a plurality of diffraction orders. In one example of step 2002, a lens 404 spatially Fourier-transforms the modulated light 402 onto a Fourier plane 408. The method 2000 also includes a step 2004 of filtering the modulated light Fourier-transformed by step 2002. Step 2004 includes steps 2006 and 2008 that can occur simultaneously. Step 2006 transmits at least one diffraction order of the modulated light on the Fourier plane. Step 2008 blocks the remaining portion of the modulated light on the Fourier plane. In one example of steps 2006 and 2008, an optical filter mask 412 transmits at least one diffraction order of the modulated light 402 through a transmission region 416 on the Fourier plane 408 and blocks the remaining portion of the modulated light 402 on the Fourier plane 408. In another example of steps 2006 and 2008, the optical filter mask 412 transmits the zero-order diffraction order of the modulated light 402 through a transmission region 416 on the Fourier plane 408 and blocks the remaining portion of the modulated light 402 on the Fourier plane 408. In another example of the method 2000, the modulated light 402 is monochromatic light. In another example of the method 2000, the modulated light 402 is one of red light, green light, and blue light. In another example of the method 2000, the modulated light 402 is polychromatic light formed by combining red light, green light, and blue light. In this example, the modulated light 402 may be white light. In one embodiment, the method 2000 further includes a step 2010 of collimating at least one diffraction order of the transmitted modulated light after step 2006. In one example of step 2010, a collimating lens 418 collimates the filtered light 414.

[0102] FIG. 21 shows a method 2100 for projecting a color image with increased contrast through optical filtering of each color channel in a spatial multiplexing scheme. Method 2100 can be performed by a multi-color digital projector 1500. Method 2100 includes step 2102 of spatially modulating first, second, and third input lights according to a color image to generate first, second, and third modulated lights, respectively. The first, second, and third input lights can represent lights for three different respective color channels of the color image as described above with reference to FIG. 15. Each of the first, second, and third modulated lights has a plurality of diffraction orders. In one example of step 2102, DMDs 200(1), 200(2), and 200(3) of FIG. 15 spatially modulate first, second, and third input lights 206(1), 206(2), and 206(3), respectively, to be first, second, and third modulated lights 402(1), 402(2), and 402(3), respectively. Method 2100 also includes step 2104 of filtering the first, second, and third modulated lights (generated in step 2102) to be first, second, and third filtered lights, respectively. In one embodiment, step 2104 performs method 2000 on each of the first, second, and third modulated lights to generate the first, second, and third filtered lights. In one example of such an embodiment of step 2104, optical filter masks 412(1), 412(2), and 412(3) of multi-color digital projector 1500 filter the Fourier-transformed first, second, and third modulated lights 402(1), 402(2), and 402(3), respectively, to be first, second, and third filtered lights 414(1), 414(2), and 414(3), respectively. Step 2104 includes steps 2106 and 2108 that can occur simultaneously. Step 2106 transmits at least one diffraction order of each of the first, second, and third modulated lights. Step 2108 blocks the remaining portions of the first, second, and third modulated lights.In an example of steps 2106 and 2108, the optical filter masks 412(1), 412(2), and 412(3) of the multi-color digital projector 1500 transmit at least one diffraction order of each of the Fourier-transformed first, second, and third modulated lights 402(1), 402(2), and 402(3), and block the remaining portions of the first, second, and third modulated lights 402(1), 402(2), and 402(3). The method 2100 also includes a step 2110 of combining the first, second, and third filtered lights generated in step 2104 to form output light. In an example of step 2110, the beam combiner 1504 combines the first, second, and third filtered lights 414(1), 414(2), and 414(3) into the output light 1510. In an embodiment, the method 2100 further includes a step 2112 of projecting the output light onto a screen. In an example of step 2112, the projector lens 112 projects the output light 1510 onto a screen such as the projection screen 116.

[0103] Without departing from the scope of the present invention, the method 2100 can be extended to process only two color channels or more than three color channels (e.g., four color channels).

[0104] FIG. 22 shows a time multiplexing method 2200 for generating and projecting a color image with increased contrast. The method 2200 is performed by the multi-color digital projector 1600 It can be performed. Method 2200 includes step 2202 of modulating time-multiplexed light using a spatial light modulator according to a color image to be projected to generate time-multiplexed modulated light having a repeating sequence of first, second, and third modulated lights. The first, second, and third modulated lights can represent light for three different color channels of a color image, as described above with reference to FIG. 16. In one example of step 2202, the DMD 200 of the multi-color digital projector 1600 modulates the time-multiplexed light 1601 into time-multiplexed modulated light 1602. Method 2200 also includes step 2204 of spatially Fourier-transforming the time-multiplexed modulated light (generated in step 2202) using a lens. In one example of step 2204, the lens 404 spatially Fourier-transforms the time-multiplexed modulated light 1602. Method 2200 further includes step 2206 of filtering the time-multiplexed modulated light spatially Fourier-transformed in step 2204 by rotating a filter wheel in synchronization with the time-multiplexed modulated light. The filter wheel includes a plurality of optical filter masks. Each optical filter mask is configured to filter a corresponding one of the first, second, and third modulated lights spatially Fourier-transformed by the lens in step 2204. Step 2206 rotates the filter wheel so that each of the optical filter masks is positioned within the spatially Fourier-transformed light when the time-multiplexed modulated light is a corresponding one of the first, second, and third modulated lights. In one example of step 2206, the motor 1614 rotates the filter wheel 1612 in synchronization with the time-multiplexed modulated light 1602, as described above with reference to FIG. 16. In another example of step 2206, the motor 1614 rotates the filter wheel 1612 stepwise so that each optical filter mask remains stationary while filtering its corresponding modulated light. In one embodiment, method 2200 further includes step 2208 of projecting the filtered time-multiplexed modulated light onto a screen.As an example of step 2208, the projector lens 112 projects time-multiplexed light filtered by the optical filter mask 1612 and collimated by the collimating lens 1618 as necessary onto the projector screen.

[0105] Without departing from the scope of the present invention, method 2200 can be extended to process only two color channels or more than three color channels (e.g., four color channels).

[0106] Numerical analysis The following description relates to a numerical analysis for examining how the contrast ratio of a digital projector including a DMD 200 depends on various parameters such as wavelength, on and off tilt angles of the micromirror 202, tolerances of the on and off tilt angles, shape of the transmission region 416 of the optical filter mask 412, angle and spectral diversity of the input light 206, and effective size of the illumination source that generates the input light 206. The digital projectors 100, 500, 1500, and 1600 can be configured according to the parameters examined in these numerical analyses.

[0107] FIG. 23 is a side view of a simulation experiment 2300 in which numerical results are presented in this section. In the simulation experiment 2300, the DMD 200 modulates the input light 206 to obtain modulated light 402 having a plurality of diffraction orders. The Fraunhofer diffraction pattern of the modulated light 402 is calculated, and each diffraction order of the Fraunhofer diffraction pattern is marked as either transmitted or blocked by the spatial filter 2302 depending on the shape and configuration of the spatial filter 2302, thereby modeling the spatial filter 2302. The spatial filter 2302 is an example of the optical filter mask 412. The contrast ratio of the simulation experiment 2300 is the number of times marked as transmitted through the spatial filter 2302 once the micromirror 202 of the DMD 200 is configured in the on position and again when the micromirror 202 of the DMD 200 is configured in the off position. It is obtained by numerically integrating the number of folding times. These two numerical integrations correspond to the on-luminance and off-luminance respectively, and their ratio defines the contrast ratio.

[0108] The Fraunhofer diffraction pattern is calculated for simulation experiment 2300 using the Rayleigh - Sommerfeld form of scalar diffraction theory. The feature of this form is the Rayleigh - Sommerfeld integral that expresses the complex amplitude of the diffracted electric field as an integral (e.g., sum) over spherical waves.

[0109] It should be understood that the numerical analysis presented herein is not limited to DMD200 and can be easily extended to other embodiments of SLM102 such as reflective LCOS phase modulators or transmissive LC phase modulators.

[0110] Figures 24 - 26 are plots of the contrast ratio and optical efficiency versus the half - angle numerically obtained for simulation experiment 2300. To generate the results of Figures 24 - 26, the spatial filter 2302 was modeled as a circular aperture centered on the optical axis 422 and having an aperture diameter 2304. The center of the spatial filter 2302 was placed on the zero - order diffraction order of the modulated light 402 (e.g., the first on and off diffraction beams 504(1) and 604(1)). The circular aperture of the spatial filter 2302 forms the base of a cone whose apex is located at the center of the front surface of the DMD200, and the cone has an axis that coincides with the optical axis 422. The half - angle 2308 is defined herein as half of the apex angle of the cone.

[0111] In Figures 24 - 26, wavelengths of 532 nm, 465 nm, and 617 nm were used for the light in simulation experiment 2300. For the micromirrors 202 of the DMD200, a call - on attitude of + 12° and an off - attitude tilt angle of - 12° were used. For the DMD200, a dimension fill factor of 81% and an area fill factor of 90% were used.

[0112] In FIG. 24, as the half angle 2308 is decreased, the green contrast ratio 2402 increases in a series of “steps” as the diffraction order of the modulated light 402 blocked by the spatial filter 2302 increases. When only the zero - order diffraction of the modulated light 402 is transmitted by the spatial filter 2302, the highest green contrast ratio of 757,000:1 is obtained. As the half angle 2308 increases, the green optical efficiency 2404 increases in a series of “steps” as the diffraction order transmitted by the spatial filter 2302 increases. Since most of the optical power of the green modulated light is at low diffraction orders (e.g., zero - order, first - order, and second - order diffraction orders), the largest step in the green efficiency 2404 occurs at small values of the half angle 2308. At the highest green contrast ratio, the green optical efficiency 2404 is about 80%, that is, 80% of the modulated light 402 is transmitted by the spatial filter 2302.

[0113] In FIG. 25, the blue contrast ratio 2502 and the blue optical efficiency 2504 behave in the same way as the green contrast ratio 2402 and the green optical efficiency 2404, respectively. When only the zero - order diffraction of the modulated light 402 is transmitted by the spatial filter 2302, the highest blue contrast ratio of 850,000:1 is obtained. At the highest blue contrast ratio, the blue optical efficiency 2504 drops steeply from 80% to less than 50%.

[0114] In FIG. 26, the red contrast ratio 2602 and the red optical efficiency 2604 behave in the same way as the green and blue contrast ratios 2402, 2502 and the green and blue optical efficiencies 2404, 2504, respectively. However, the highest red contrast ratio is only 450,000:1. One reason that the highest red contrast ratio is lower than the corresponding highest green and blue contrast ratios is that at the red wavelength of 617 nm, the DMD 200 is irradiated far from the blaze condition. At the highest red contrast ratio, the red optical efficiency 2604 is about 80%.

[0115] FIG. 27 is a Fraunhofer diffraction pattern for the simulation experiment 2300 when the wavelength of light is 532 nm and all the micromirrors 202 of the DMD 200 are in the on position. In FIG. 27, each of the four brightest diffraction orders is surrounded by one of the enclosing frames 2702. The enclosing frame 2702(1) contains the maximum optical power and corresponds to the zero-order diffraction order of the modulated light 402. For each enclosing frame 2702, the enclosing frame 2702 was used as the rectangular aperture (e.g., the transmission region 416) of the spatial filter 2302 to calculate the DOCR. The numerically calculated DOCR was printed within each enclosing frame. For example, in the enclosing frame 2702(1), the DOCR of the zero-order diffraction order of the modulated light 402 is 758,075:1. In one embodiment, the optical filter mask 412 is configured to transmit the zero-order diffraction order of the modulated light 402 and block all other diffraction orders. The optical filter mask 900 is an example of the optical filter mask 412 that can be used with this embodiment. In another embodiment, the optical filter masks 412(1), 412(2), and 412(3) of the multi-color digital projector 1500 can be respectively configured to transmit the zero-order diffraction orders of the modulated lights 402(1), 402(2), and 402(3) and block all other diffraction orders.

[0116] FIG. 28 is a Fraunhofer diffraction pattern for a simulation experiment 2300 when the wavelength of light is 617 nm and all the micromirrors 202 of the DMD 200 are in the on position. In FIG. 28, the four diffraction orders contain most of the optical power of the modulated light 402. Compared with FIG. 27 using a wavelength of 532 nm, the optical power is more evenly distributed among the four diffraction orders. This is because the wavelength of 617 nm is further away from the blaze condition of the DMD 200. A high contrast ratio of 852,000:1 can be obtained by forming a spatial filter 2302 that only transmits the diffraction orders in the enclosure frame 2802(1). However, by blocking the diffraction orders in the enclosure frames 2802(2), 2802(3), and 2802(4), the optical efficiency is significantly degraded.

[0117] As a compromise between the contrast ratio and the optical efficiency, the spatial filter 2302 can be configured to transmit the three diffraction orders with the highest DOCR corresponding to the enclosure frames 2802(1), 2802(2), and 2802(4). In this example of the spatial filter 2302, the positions of the apertures corresponding to the enclosure frames 2802(1), 2802(2), and 2802(4) are not symmetric with respect to the optical axis 422. In one embodiment, the optical filter 400 is configured to transmit three diffraction orders of the modulated light 402 according to FIG. 28. The optical filter mask 1300 is an example of an optical filter mask 412 that can be used with this embodiment. In other embodiments, the optical filter 412 is configured to transmit a non-zero integer number of diffraction orders of the modulated light 402. The maximum number of those diffraction orders is determined by the effective aperture of the lens 404.

[0118] Figure 29 is a plot of the contrast ratio 2902 and the optical efficiency 2904 numerically obtained for the simulation experiment 2300 operated at a wavelength of 617 nm when the on and off tilt angles of the micromirror 202 are +12.1° and -12.1°, respectively. The contrast ratio can be affected by even a small change in the micromirror tilt angle. Compared with Figure 26, when the tilt angle is changed by 0.1°, the highest red contrast ratio increases by more than twice and reaches approximately 1,000,000:1, while the red optical efficiency 2904 remains at about 80%. For comparison, the typical specification of a commercially available DMD is that the tilt angle tolerance is ±0.5°.

[0119] Figures 30 and 31 are plots of the contrast ratio versus the micromirror tilt angle numerically obtained for the simulation experiment 2300. In Figure 30, the off - posture tilt angle is fixed at -12°, and the on - posture tilt angle varies in the range of 11.5° to 12.5°. In Figure 31, the on - posture tilt angle is fixed at +12°, and the off - posture tilt angle varies in the range of -12.5° to -11.5°. In Figure 30, the contrast ratios 3002, 3004, and 3006 correspond to wavelengths of 617 nm, 465 nm, and 532 nm, respectively. In Figure 31, the contrast ratios 3102, 3104, and 3106 correspond to wavelengths of 617 nm, 465 nm, and 532 nm, respectively. In the following description, Figures 30 and 31 are referred to.

[0120] The value of the contrast ratio is generally more susceptible to the amount of change in the off - luminous intensity than in the on - luminous intensity. Therefore, the contrast ratio can depend more strongly on the off - tilt angle than on the on - tilt angle. As shown in Figure 30, the contrast ratios 3002, 3004, and 3006 show little change even when the on - tilt angle changes over a tilt - angle tolerance range of ±0.5°. On the other hand, the contrast ratios 3102, 3104, and 3106 in Figure 31 change more strongly when the off - tilt angle is changed over a similar angle - tolerance range.

[0121] In one embodiment, a digital micromirror device optimized for modulating green light is provided, the digital micromirror device having a call-off attitude tilt angle greater than -12° (e.g., greater than -11.8° or greater than -11.6°). In one example, the call-off attitude tilt angle of the digital micromirror device is -11.5° or greater.

[0122] The digital micromirror device according to this embodiment can improve the on-off contrast ratio when modulating green light.

[0123] In one example, the call-on attitude tilt angle of the digital micromirror device optimized for modulating green light is in the range of +11.5° to +12.5°, for example +12°.

[0124] In one embodiment, a digital micromirror device optimized for modulating red or blue light is provided, the digital micromirror device having a call-off attitude tilt angle less than -12° (e.g., less than -12.2° or less than -12.4°). In one example, the call-off attitude tilt angle of the digital micromirror device is -12.5° or less.

[0125] The digital micromirror device according to this embodiment can improve the on-off contrast ratio when modulating red light or blue light.

[0126] In one example, the call-on attitude tilt angle of the digital micromirror device optimized for modulating red light or blue light is in the range of +11.5° to +12.5°, for example +12°.

[0127] In one embodiment, a modulator system for generating an image is provided, the modulator system comprising a first digital micromirror device configured to modulate red light to generate modulated red light, and A second digital micromirror device configured to modulate green light to generate modulated green light; A third digital micromirror device configured to modulate blue light to generate modulated blue light; Comprising; A modulator system in which the off-state tilt angle of the second digital micromirror device is different from the off-state tilt angles of the first and third digital micromirror devices. For example, the first and third digital micromirror devices are of the first type and have a first off-state tilt angle, and the second digital micromirror device is of the second type and may have a second off-state tilt angle different from the off-state tilt angle of the first type. For example, the first and third digital micromirror devices are digital micromirror devices optimized for modulating the above red or blue light, and the second digital micromirror device is a digital micromirror device optimized for modulating the above green light.

[0128] The modulator system is; First, second, and third optical filters, each optical filter comprising; A lens configured to spatially Fourier transform modulated light having a plurality of diffraction orders from the respective first, second, and third spatial light modulators; An optical filter mask located on the Fourier plane of the lens and configured to transmit at least one diffraction order of the modulated light spatially Fourier transformed by the lens to generate first, second, and third filtered lights respectively, and block the remaining portion of the modulated light to filter the modulated light; and the first, second, and third optical filters; A beam combiner configured to combine the first, second, and third filtered lights into output light; Comprising.

[0129] Furthermore, the modulator system may also include any of the features described herein, such as those related to FIGS. 9-15, as needed.

[0130] FIG. 32 is a plot of the contrast ratio 3202 and the optical efficiency 3204 as a function of the angular diversity of the input light 206, numerically obtained for the simulation experiment 2300 at a wavelength of 532 nm. FIGS. 33 and 34 are Fraunhofer diffraction patterns of the simulation experiment 2300, showing the broadening of the diffraction peak due to the angular diversity of the input light 206. In FIG. 33, the input light 206 is a plane wave having no angular diversity. In FIG. 34, the input light 206 has an angular diversity of 8° half-angle. To obtain the data in FIG. 32, the spatial filter 2302 was provided with a rectangular aperture represented by the enclosure 3302 in FIGS. 33 and 34. In the following description, FIGS. 32-34 are referred to.

[0131] In cinema and other critical viewing environments, digital laser projection of images benefits from the angular diversity and reduced coherence in laser illumination, as it reduces the visibility of dust and other unwanted diffraction artifacts. Also, laser illumination has an increased bandwidth, which has the advantage of reducing the visibility of speckles on the screen.

[0132] Increasing the angular diversity and bandwidth of the laser illumination can degrade the contrast ratio of the optical filtering systems and methods presented herein. In particular, in the Fourier plane, the increased angular diversity and bandwidth broaden the diffraction peak, and the skirts of the peak blur together with the skirts of the peaks in the vicinity of each other. Such broadening of the peak can prevent individual diffraction orders from passing through the spatial filter 2302 without transmitting some of the diffraction orders in the vicinity of each other that should be blocked. As shown in FIG. 32, since the half-angle of the input light 206 is increased to 8°, the contrast ratio is halved from 721,000:1 to 346,000:1. diffraction orders from passing through the spatial filter 2302. As shown in FIG. 32, since the half-angle of the input light 206 is increased to 8°, the contrast ratio is halved from 721,000:1 to 346,000:1.

[0133] Thus, considering the angular diversity and spectral bandwidth, there is a trade-off relationship between (1) the visibility of dust and reduced speckles and (2) the contrast ratio.

[0134] It is understood that the factors causing contrast degradation are factors other than the diffraction of the input light 206 by the DMD 200, for example, scattering of the input light 206 from the surface of the micromirror 202, unwanted stray light and reflection in the cinema room, optical aberration, and / or polarization effects, etc. However, in most digital projectors, diffraction by the DMD 200 is considered to be the main cause, or at least one of the main causes, of contrast degradation. The systems and methods disclosed herein can be easily extended to cases where the contrast degrades due to other factors such as those listed above in addition to diffraction. The systems and methods disclosed herein can enhance the contrast even when other such factors are present.

[0135] Experimental results The above numerical analysis was verified using an experimental setup similar to that of FIG. 4. To demonstrate the highest contrast, the experimental setup was configured to filter the zero-order diffraction order at 532 nm. The optical filter mask 412 had a circular aperture with its center on the optical axis 422. The diameter of the circular aperture and the lens (e.g., lens 404) were selected to form a half-angle of 2° on the Fourier plane. The input light to the DMD 200 was provided by a polarized 532 nm laser. The input light was expanded using a Galilean beam expander formed from two doublets to fill the front surface of the DMD 200. Thereby, diffraction-limited performance was obtained. For simplicity of description, coupling the light to the DMD 200 using a TIR prism was not performed. The DMD 200 was operated at its brightest (e.g., white level) and darkest (e.g., black level) outputs, and the contrast was measured using a spectrometer. 2 <1.1 is given by. The input light was expanded to fill the front surface of the DMD 200 using a Galilean beam expander formed from two doublets. Thereby, diffraction-limited performance was obtained. For simplicity of description, coupling the light to the DMD 200 using a TIR prism was not performed. The DMD 200 was operated at its brightest (e.g., white level) and darkest (e.g., black level) outputs, and the contrast was measured using a spectrometer.

[0136] The contrast ratios of two identical 4K DMDs were measured. At 532 nm and a half-angle of 2°, the contrast ratio predicted by simulation experiment 2300 is about 757,000:1 (see the highest green contrast ratio in Figure 24). Contrast ratios of 254,234:1 and 277,966:1 were measured. These values are about three times lower than the predicted values. The causes of the difference are stray light overflowing from the DMD, stray light emerging from the gaps between the micromirrors of the DMD, and scattering from the surfaces and edges of the micromirrors, etc.

[0137] Also, as expected considering that the contrast ratio depends on the off-tilt angle, it was observed that the propagation direction of the input light 206 to the DMD 200 affects the contrast ratio. In addition, it was observed that the polarization of the input light 206 affects the black level of the DMD 200, thereby affecting the contrast ratio. Regarding the above experimental results, a waveplate was used to rotate the polarization of the input light to maximize the contrast.

[0138] Considering the sensitivity of the contrast ratio to the micromirror tilt angle and the propagation direction of the input light 206, binning may be used to group DMDs with similar tilt angles. In one embodiment of the three-color digital projector 1500, three binned DMDs with similar tilt angles are used for DMD 200(1), 200(2), and 200(3). In another embodiment, for DMD 200(1), 200(2), and 200(3), three binned DMDs with different tilt angles (e.g., from three different bins) are used, and each of the DMDs has a tilt angle selected to maximize the contrast ratio for a specific wavelength of the input light 206 used with the DMD.

[0139] DLP bit sequence Figure 35 shows a time sequence 3500 of example bit planes 3502 that determine how micromirrors 202 of DMD 200 are controlled to display an image frame. Figure 36 is a reconstructed frame 3600 that shows how an image frame appears when the example bit planes 3502 of Figure 35 control DMD 200 to display an image frame. Reference is made to Figures 35 and 36 in the following discussion.

[0140] Each pixel in a digital video frame is assigned a corresponding pixel level that represents the desired intensity of that pixel. The pixel level may be represented as an n-bit integer, where 0 is the lowest intensity level and 2 n -1 is the highest intensity level. , a frame may be formed as the sum of n bit planes 3502. A white bit in any bit plane 3502 represents an on for the corresponding micromirror 202 of the DMD 200, and a black bit represents an off for the corresponding micromirror 202. The DMD 200 may then select a time interval 2 i × Δt, sequential control 3502(0)、3502(1)と、3502(5)。 English: where Δt is the minimum time interval and i=0,...,n-1 is the bit plane 3502 index. Thus, in the example of FIG. 35 where pixel levels are represented as 6-bit integers, DMD 200 is controlled according to the first bit plane 3502(0) during the first time interval Δt, the second bit plane 3502(1) during the second time interval 2Δt,..., and finally the bit plane 3502(5) during the sixth time interval 32Δt. The bits of the bit planes 3502 (i.e., 0 or 1 for each bit corresponding to on and off, respectively, for the corresponding micromirror 202) are selected such that the time-weighted sum of the bit planes 3502 gives the desired pixel for the frame. The frame is displayed as fast as the human visual system can respond to the time integration of the display sequence 3500 of the bit planes 3502.

[0141] FIGS. 35 and 36 show an example in which the pixel value is represented as a 6-bit integer. However, without departing from the scope of the present invention, the pixel value may be represented by a different number of bits, or there may be the same number of bit planes. FIG. 35 shows a bit plane 3502 having 250 pixels × 250 pixels for clarity, but the bit plane 3502 may be sized to control all the micromirrors 202 of the DMD200 without departing from the scope of the present invention.

[0142] In a conventional digital projection system in which multiple diffraction orders are projected onto a screen, the desired pixel level of a pixel is proportional to the on-time of the micromirror that generates the pixel. However, when blocking the diffraction orders from the DMD200 with an optical filter (e.g., optical filter 412) to increase the contrast, the amount of light passing through the optical filter (e.g., the zero-order diffraction order) also depends on the spatial pattern of the micromirrors 202. The diffraction of the input light from the spatial pattern affects how much power is diffracted into each order and thus how much power passes through the optical filter. In some frames, the combination of the spatial pattern and the optical filter can generate artifacts 3602 in the reconstructed frame 3600. For example, in FIG. 35, the bit plane 3502 controls the DMD200 to form on and off "stripes" with different spatial frequencies, and the phase shift resulting from this spatial pattern changes the amount of light passing through the optical filter, thereby generating artifacts 3602 that appear as vertical "bands". Only three artifacts 3602 are identified in the reconstructed frame 3600, but the reconstructed frame 3600 also includes additional bands of different levels of darkness that are also artifacts. The artifacts 3602 appear as vertical bands in the reconstructed frame 3600. This is because the bit plane 3502 forms vertical on and off stripes with different horizontal spatial frequencies. However, when the bit plane 3502 forms horizontal on and off stripes with different vertical spatial frequencies, the artifacts 3602 become horizontal bands.

[0143] FIG. 37 shows an example of a randomized bit plane 3700 that forms part of a randomized bit plane sequence that can be used with embodiments herein to reduce the presence of artifact 3602. FIG. 38 shows a reconstructed frame 3800 that shows how a video frame appears when the randomized bit plane sequence controls DMD 200 to display one video frame. Compared to reconstructed frame 3600, there is an advantage that the visibility of artifacts in reconstructed frame 3800 is greatly reduced. In the following description, FIGS. 37 and 38 are referred to.

[0144] The randomized bit plane sequence is formed from 2 n −1 randomized bit planes. Randomized bit plane 3700 is an example of one of them. The 2 n −1 randomized bit planes look similar (not necessarily identical), so in FIG. 37, only one is shown for clarity. Similar to bit plane 3502 in FIG. 35, white bits indicate that the corresponding micromirror 202 of DMD 200 is on, and black bits indicate that the corresponding micromirror 202 of DMD 200 is off. Different from bit plane sequence 3500 where the time interval of bit plane 3502 increases by a power of 2, each of the 2 n −1 randomized bit planes has the same time interval Δt.

[0145] By the random assignment of on-bits and off-bits in randomized bit plane 3700, different spatial patterns as shown in FIG. 35 are replaced with randomized spatial patterns, thereby spreading the diffraction effect across all pixels of the frame and reducing the visibility of artifact 3602, which is advantageous.

[0146] In one embodiment, for one frame, the randomized bit plane sequence is 2 n -1 bit plane is generated by initializing all bit planes to turn off all bits. Then, 2 n For each pixel in the -1 bit plane, the sum of 2 the -1 bits for that pixel is not allowed to exceed the corresponding pixel value, while randomly populating (i.e., turning the bits "on"). 2 n Populating the -1 bit plane stops when the sum of the -1 bits for any pixel in the frame equals the corresponding pixel value. 2 n Populating the -1 bit plane stops when the sum of the -1 bits for any pixel in the frame equals the corresponding pixel value. n The above random bit plane sequence has more bit planes than the bit plane sequence 3500 in FIG. 35. However, the two methods may be combined. That is, a hybrid bit plane sequence may be formed using some of the bit planes 3502 (i.e., the bit planes for shorter time intervals) and the random bit planes. However, for each pixel, the weighted sum of all bits in the hybrid sequence is equal to the corresponding pixel level. Additionally, the random bit plane sequence and the hybrid bit plane sequence may be combined with other techniques for adjusting the output power level, such as dithering of the micromirror 202.

[0147] The random bit plane sequence may cause the pixel response to be less linear than the pixel response obtained when not using an optical filter on the DMD200. A non-linear pixel response may be desirable in a display device. This is because human visual perception is non-linear processing, and the non-linearity introduced by the random bit plane sequence more closely matches human visual perception. When a non-linear response is desired, the projection system may be able to display the frame more recognizably, so it may be possible to reduce the number of bits used to represent the pixel values.

[0148] The random bit plane sequence may cause the pixel response to be less linear than the pixel response obtained when not using an optical filter on the DMD200. A non-linear pixel response may be desirable in a display device. This is because human visual perception is non-linear processing, and the non-linearity introduced by the random bit plane sequence more closely matches human visual perception. When a non-linear response is desired, the projection system may be able to display the frame more recognizably, so it may be possible to reduce the number of bits used to represent the pixel values. Since the projection system can display the frame more recognizably, it may be possible to reduce the number of bits used to represent the pixel values.

[0149] The advantages obtained from the randomized or hybrid bit-plane sequences depend on the quality of the input light that irradiates the DMD 200. If the input light is, for example, a monochromatic laser beam with high coherence and low étendue, artifacts 3602 are more visible compared to the case where the input light has high étendue and / or low coherence (e.g., light from a lamp). Therefore, the randomized bit-plane sequences and the hybrid bit-plane sequences are more important for reducing the visibility of artifacts 3602 when the input light is "high quality".

[0150] Advantages The embodiments presented herein have the advantage of increasing the contrast ratio without using an additional DMD. For example, as an alternative to the systems and methods disclosed herein, the contrast ratio may be increased by multi-level modulation, i.e., using two or more DMDs connected in series to block the off-diffraction beam from the first DMD with the second DMD. As a method of increasing the contrast ratio, multi-level modulation has the disadvantage of increasing the cost and complexity of the digital projector due to the second DMD and the corresponding electronic components. Further, some types of digital projectors use three DMDs (one for each of red light, green light, and blue light). In this type of digital projector, using two DMDs for each color increases the total number of DMDs from three to six, further increasing the cost and complexity.

[0151] Another advantage of the embodiments presented herein is that the generation of moiré patterns caused by the interference between the optically filtered projection light and the periodic holes in the screen onto which the projection light is projected can be reduced. In particular, the optical filtering is configured to reduce the high-frequency components of the projection light, thereby "smoothing" the hard edges between pixels when they appear on the screen. This smoothing reduces the beating between the periodic intensity of the projection light and the periodic holes in the screen.

[0152] A further advantage of the optical filtering systems and methods presented herein is that optical filtering can increase the contrast ratio of a digital projector using a Texas Instruments tilt-and-roll pixel (TRP) DLP chip. The micromirrors of the TRP DLP chip do not tilt about an axis oriented at 45° (e.g., the micromirror rotation axis 208 of FIG. 2). As a result, compared to other types of DMD chips, modulated light propagates away from the TRP chip such that the diffraction order of the off-state light (e.g., the off-diffraction beam 604 of FIG. 6) is brighter, thereby increasing the off-luminance and reducing the contrast ratio. By reducing the off-luminance, the optical filtering systems and methods presented herein have the advantage of enabling the TRP chip to be included in projectors for applications that require a high contrast ratio, such as projection conforming to Digital Cinema Initiative (DCI) specifications.

[0153] The above methods and systems may be modified without departing from the scope of the invention. Accordingly, it should be noted that the matters described above and shown in the accompanying drawings should be construed as illustrative and not in a limiting sense. The following claims are intended to cover all the general and specific features described herein and all statements of the scope of the method and system that, as a matter of language, can be said to be included in the following claims.

[0154] Various aspects of the present invention can be understood from the following enumerated example embodiments (EEE). 1. An optical filter for increasing the contrast of an image generated using a spatial light modulator, comprising: a lens configured to spatially Fourier transform modulated light from the spatial light modulator, the modulated light having a plurality of diffraction orders; An optical filter mask located on the Fourier plane of the lens, configured to filter the modulated light by transmitting at least one diffraction order of the modulated light spatially Fourier-transformed by the lens and blocking the remaining portion of the modulated light. An optical filter comprising the same. 2. The optical filter according to EEE1, wherein the at least one diffraction order is the zero order. 3. The optical filter according to EEE2, wherein the optical filter mask has a transmission region configured to transmit the zero order of the modulated light. 4. The optical filter according to EEE1, wherein the at least one diffraction order comprises the zero order and a plurality of first orders. 5. The optical filter according to EEE4, wherein the optical filter mask has a transmission region configured to transmit two of the zero order of the modulated light and the first order of the modulated light. 6. The optical filter according to any one of EEE1 to 5, wherein the modulated light is one of red light, green light, and blue light. 7. A modulator system for generating an image with increased contrast, comprising: The optical filter according to any one of EEE1 to 6; and A digital micromirror device implementing a spatial light modulator. A modulator system comprising the same. 8. A modulator system for generating an image with increased contrast, comprising: The optical filter according to any one of EEE1 to 7; and A collimating lens disposed at a position for collimating at least one diffraction order of the modulated light that has passed through the optical filter mask; A modulator system comprising the same. 9. A modulator system for generating an image with increased contrast, comprising: First, second, and third spatial light modulators configured to modulate first, second, and third lights according to the image to generate first, second, and third modulated lights, respectively; Three optical filters according to any one of EEE1 to 6, each forming a first, second, and third optical filter, and each transmitting at least one diffraction order of the first, second, and third modulated lights to generate first, second, and third filtered lights, respectively, and blocking the remaining portions of the first, second, and third modulated lights; An optical filter configured to combine the first, second, and third filtered lights into output light; A beam combiner configured to combine the first, second, and third filtered lights into output light; A modulator system comprising the same. 10. The modulator system according to EEE9, wherein each of the first, second, and third optical filter masks corresponding to the first, second, and third optical filters has at least one transmission region configured to transmit the zero-order diffraction order and a plurality of first-order diffraction orders of the first, second, and third modulated lights, respectively. 11. The modulator system according to EEE9 or EEE10, wherein each of the first, second, and third spatial light modulators is a digital micromirror device. 12. The modulator system according to any one of EEE9 to 11, wherein the first, second, and third lights are red light, green light, and blue light, respectively. 13. The modulator system according to any one of EEE9 to 12, further comprising first, second, and third output lenses provided at positions for collimating first, second, and third filtered lights, respectively, before being combined by the beam combiner. 14. The modulator system according to any one of EEE9 to 13, further comprising a projector lens configured to project the output light onto a screen. 15. A time-division modulator system for generating an image with increased contrast, a spatial light modulator configured to modulate time-division multiplexed light according to the image to obtain time-division multiplexed modulated light including a repeating sequence of first, second, and third modulated lights; a lens configured to spatially Fourier-transform the time-division multiplexed modulated light onto a Fourier plane; a filter wheel located on the Fourier plane and including a plurality of optical filter masks, each optical filter mask being configured to transmit at least one diffraction order of a corresponding one of the first, second, and third modulated lights spatially Fourier-transformed by the lens and block the remaining portion of the corresponding one of the first, second, and third modulated lights, thereby filtering the corresponding one of the first, second, and third modulated lights, and the filter wheel being configured to rotate in synchronization with the time-division multiplexed modulated light such that each optical filter mask is positioned within the time-division multiplexed modulated light on the Fourier plane when the time-division multiplexed modulated light is the corresponding one of the first, second, and third modulated lights; A time-division modulator system comprising: 16. The time-division modulator system according to EEE15, wherein the spatial light modulator is a digital micromirror device. 17. The time-division modulation system according to EEE15 or EEE16, wherein the plurality of optical filter masks are three optical filter masks each configured to filter the first, second, and third modulated lights respectively. 18. The time-division modulation system according to any one of EEE15 to 17, wherein the plurality of optical filter masks are three sets of optical filter masks, n is a positive integer, and each of the three sets is configured to filter one corresponding to each of the first, second, and third modulated lights. 19. The time-division modulation system according to EEE17, wherein the first optical filter mask has a transmission region configured to transmit a zero-order diffraction order and a plurality of first-order diffraction orders of the first modulated light, the second optical filter mask has a transmission region configured to transmit a zero-order diffraction order and a plurality of first-order diffraction orders of the second modulated light, and the third optical filter mask has a transmission region configured to transmit a zero-order diffraction order and a plurality of first-order diffraction orders of the third modulated light. 20. The time-division modulation system according to EEE19, wherein the first, second, and third modulated lights are red light, green light, and blue light respectively. 21. The time-division modulation system according to EEE20, wherein the filter wheel is further configured to rotate unevenly so as to stop when each optical filter mask is positioned within the time-division modulated light. 22. The time-division modulation system according to EEE21, further comprising a projector lens configured to project at least one diffraction order of each of the first, second, and third modulated lights transmitted through the filter wheel onto a screen. 23. A method for improving the contrast of an image generated using a spatial light modulator, comprising: spatially Fourier-transforming the modulated light from the spatial light modulator on a Fourier plane, the modulated light having a plurality of diffraction orders; filtering the modulated light by transmitting at least one diffraction order of the modulated light on the Fourier plane and blocking the remaining portion of the modulated light on the Fourier plane; A method comprising the above steps. 24. The method according to EEE23, wherein the at least one diffraction order is a zero-order diffraction order. 25. The method according to EEE24, wherein the transmitting step includes transmitting a transmission region of an optical filter mask through the zero-order diffraction order. 26. The method according to EEE23, wherein the at least one diffraction order includes a zero-order diffraction order and a plurality of first orders. 27. The method according to EEE26, wherein the transmitting step includes transmitting a transmission region of an optical filter mask through the zero-order diffraction order and the plurality of first diffraction orders. 28. The method according to any one of EEE23 to 27, wherein the modulated light is one of red light, green light, and blue light. 29. The method according to any one of EEE23 to 28, further comprising operating a plurality of micromirrors of the spatial light modulator to generate the modulated light. 30. The method according to any one of EEE23 to 29, further comprising collimating the at least one diffraction order of the modulated light after the transmitting step. 31. A method for projecting a color image with increased contrast, comprising: spatially modulating first, second, and third input lights according to the image to generate first, second, and third modulated lights respectively, each of the first, second, and third modulated lights having a plurality of diffraction orders; filtering the first, second, and third modulated lights by transmitting at least one diffraction order of each of the first, second, and third modulated lights and blocking the remaining portions of each of the first, second, and third modulated lights to obtain first, second, and third filtered lights respectively; combining the first, second, and third filtered lights to obtain output light; A method comprising the above steps. 32. The method according to EEE31, wherein the first, second, and third input lights are red light, green light, and blue light respectively. 33. The method according to EEE31 or EEE32, further comprising projecting the output light onto a screen. 34. A time-division multiplexing method for generating and projecting an image with increased contrast, comprising: modulating time-division multiplexed light using a spatial light modulator according to the image to generate time-division multiplexed modulated light including a repeating sequence of first, second, and third modulated lights; spatially Fourier-transforming the time-division multiplexed modulated light using a lens; rotating a filter wheel in synchronization with the time-division multiplexed modulated light to perform the following time multiplexing... A step of filtering the re-modulated light, wherein the filter wheel includes a plurality of optical filter masks, and each optical filter mask is configured to filter a corresponding one of the first, second, and third modulated lights spatially Fourier-transformed by the lens, and the rotating step includes positioning each optical filter mask in the time-multiplexed modulated light at the Fourier plane of the lens when the time-multiplexed modulated light is the corresponding one of the first, second, and third modulated lights, the step; A method comprising. 35. The spatial light modulator is a digital micromirror device, and the time-multiplexed modulator system described in EEE34. 36. The plurality of optical filter masks are three optical filter masks each configured to filter the first, second, and third modulated lights, respectively, and the method described in EEE34 or EEE35. 37. The plurality of optical filter masks are three sets of optical filter masks, n is a positive integer, and each of the three sets is configured to filter a corresponding one of the first, second, and third modulated lights, respectively, and the method described in any of EEE34 to EEE36. 38. The filtering step is Transmitting the transmission region of the first optical filter mask through the zero-order diffraction order and a plurality of first-order diffraction orders of the first modulated light; Transmitting the transmission region of the second optical filter mask through the zero-order diffraction order and a plurality of first-order diffraction orders of the second modulated light; Transmitting the transmission region of the third optical filter mask through the zero-order diffraction order and a plurality of first-order diffraction orders of the third modulated light; Including The method described in EEE36. 39. The method according to EEE36 or EEE38, wherein the first, second, and third modulated lights are red light, green light, and blue light, respectively. 40. The method according to EEE36, EEE38, or EEE39, wherein the rotating step further includes a step of rotating non-uniformly and stopping when each optical filter mask is positioned within the time-multiplexed modulated light. 41. The method according to EEE36, EEE38, EEE39, or EEE40, further including a step of projecting the filtered time-multiplexed modulated light onto a screen after the filtering step.

Claims

1. A projection system for projecting an image, comprising: a plurality of laser light sources each generating light of a different primary color; a plurality of digital micromirror devices, each of the digital micromirror devices being configured to receive and modulate light from a different one of the laser light sources, each of the digital micromirror devices including a two-dimensional rectangular array of micromirrors disposed in a plane having a normal, each of the digital micromirror devices being configured to receive light from a corresponding laser light source at an angle other than zero with respect to the normal of the plane, each micromirror being configured to be tilted to an ON angle and an OFF angle, and each micromirror being configured to be tilted between the ON angle and the OFF angle a plurality of times per image frame; a beam combiner configured to combine the modulated light from the plurality of digital micromirror devices; a projection optical system; wherein the projection optical system comprises a Fourier lens configured to spatially Fourier-transform the modulated light from the beam combiner; an optical filter mask located on a Fourier plane of the Fourier lens; and a projection lens configured to project the modulated light from the beam combiner filtered by the optical filter mask onto a diffusive reflective screen such that a plurality of viewers can view the image; wherein the projection system achieves a static contrast ratio of at least 30,000:

1. Projection system.

2. The projection system according to claim 1, wherein the projection system achieves a static contrast ratio of at least 30,000:1 for each of the different primary colors.

3. The projection system according to claim 1, wherein the static contrast ratio is a ratio of a maximum luminance to a minimum luminance of the projected modulated light in a state where each of the laser light sources emits light at a constant luminance.

4. The projection system according to claim 1, wherein the optical filter does not block more than 20% of the optical power of the modulated light.

5. The projection system according to claim 1, wherein the optical filter does not block more than 20% of the optical power of each of the different primary colors of the modulated light from the beam combiner.

6. The laser light source includes a red laser light source, a blue laser light source, and a green laser light source. The digital micromirror device includes a first digital micromirror device that receives and modulates red laser light from the red laser light source, a second digital micromirror device that receives and modulates blue laser light from the blue laser light source, and a third digital micromirror device that receives and modulates green laser light from the green laser light source. The projection system according to claim 1, wherein the optical filter does not block more than 5% of the optical power of the modulated red laser light, does not block more than 5% of the optical power of the modulated blue laser light, and does not block more than 20% of the optical power of the modulated green laser light.

7. The projection system according to claim 1, wherein the projection system achieves a static contrast ratio of at least 60,000:

1.

8. The Fourier lens has a central optical axis. The projection system according to claim 1, wherein the optical filter mask has a transmission region at a position offset from the central optical axis of the Fourier lens on the Fourier plane.

9. The projection system according to claim 1, wherein the optical filter mask has a circular transmission region having a center, and the center of the circular transmission region is offset from the central optical axis of the Fourier lens.

10. The projection system according to claim 1, wherein the optical filter mask includes an optically opaque structure surrounding the transmission region, and the transmission region includes an opening in the optically opaque structure.

11. The projection system according to claim 1, wherein the optical filter mask includes an optically opaque structure surrounding the transmission region, and the transmission region includes a dichroic filter.

12. The projection system according to claim 1, wherein the optical filter mask includes an anodized metal structure surrounding the transmission region, the transmission region includes a circular opening in the anodized metal structure, and the circular opening has a center offset from the central optical axis of the Fourier lens.

13. The projection system according to claim 1, wherein the optical filter mask includes a rectangular transmission region having a center, and the center of the rectangular transmission region is offset from the central optical axis of the Fourier lens.

14. The projection system according to claim 1, wherein the optical filter mask includes an irregular polygonal transmission region having a center, and the center of the irregular polygonal transmission region is offset from the central optical axis of the Fourier lens.

15. The projection system according to claim 1, wherein the optical filter mask includes a transmission region, an intermediate region surrounding the transmission region, and an opaque region surrounding the intermediate region, and the intermediate region has a spatially varying transmittance that decreases as the distance from the transmission region increases.

16. A projection system for projecting an image, a laser light source, a digital micromirror device configured to receive and modulate light from the laser light source, the digital micromirror device including a two-dimensional rectangular array of micromirrors, each micromirror being configured to be tilted to an ON angle and an OFF angle, and each micromirror being configured to tilt a plurality of times between the ON angle and the OFF angle for each image frame, a projection optical system, comprising: The projection optical system includes: a Fourier lens configured to spatially Fourier transform the modulated light from the digital micromirror device; an optical filter mask located on the Fourier plane of the Fourier lens; a projection lens configured to project the modulated light from the digital micromirror device filtered by the optical filter mask onto a diffusely reflective screen so that a plurality of viewers can view the image, comprising: The projection system realizes a static contrast ratio of at least 30,000 to 1. Projection system.

17. The projection system according to claim 16, wherein the static contrast ratio is the ratio of the maximum luminance to the minimum luminance of the projected modulated light when the laser light source emits light at a constant luminance.

18. The projection system according to claim 16, wherein the optical filter does not block more than 5% of the optical power of the modulated light from the digital micromirror device.

19. The projection system according to claim 16, wherein the optical filter does not block more than 20% of the optical power of the modulated light from the digital micromirror device.

20. The projection system according to claim 16, wherein the projection system realizes a static contrast ratio of at least 60,000 to 1.

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