Patent Publication Number: US-2023142442-A1

Title: Providing uniform background image illumination with zero-order light from a phase light modulator to a spatial light modulator

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to U.S. Provisional Patent Application No. 63/277,739, which was filed Nov. 10, 2021, is titled “Optical Method For Utilizing Zero Order Light In A Phase Light Modulator (PLM) To Spatial Light Modulator Illuminator (SLM),” and is hereby incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     Projection-based displays project images onto surfaces, such as onto a wall or a screen, to present video or still pictures. Such displays can include cathode-ray tube (CRT) displays, liquid crystal displays (LCDs), and spatial light modulator (SLM) displays, etc. 
     SUMMARY 
     In accordance with at least one example of the disclosure, an apparatus includes a phase light modulator (PLM) configured to produce background image illumination including background image light and zero-order light, a first lens array including first lenses optically coupled to the PLM and configured to project the background image light, a second lens array optically coupled to the first lens array and including second lenses configured to project the background image light projected from the first lens array, an optical tunnel extending between the first lens array and the second lens array, where the optical tunnel is optically coupled to the PLM and configured to project the zero-order light, an embedded lens in the second lens array optically coupled to the optical tunnel and configured to focus the zero-order light projected by the optical tunnel, and focusing optics optically coupled to the second lens array and to the embedded lens and configured to focus the background image light and the zero-order light onto a background image plane of an SLM. 
     In accordance with at least one example of the disclosure, a device includes one or more light sources, a PLM optically coupled to the one or more light sources, a first lens array optically coupled to the PLM and comprising first lenses, a second lens array optically coupled to the first lens array and comprising second lenses, an optical tunnel extending between the first lens array and the second lens array, an embedded lens in the second lens array optically coupled to the optical tunnel, focusing optics optically coupled to the second lens array and to the embedded lens and comprising one or more focusing lenses, and a SLM optically coupled to the focusing optics. 
     In accordance with at least one example of the disclosure, a method includes modulating, by a PLM, incident light to produce background light illumination comprising background image light and zero-order light to a first lens array, projecting, by the first lens array, the background image light towards a second lens array, projecting, by an optical tunnel extending between the first lens array and the second lens array, the zero-order light towards an embedded lens in the second lens array, projecting, by the second lens array, the background image light towards focusing optics, projecting, by the embedded lens, the zero-order light towards the focusing optics, focusing, by the focusing optics, light comprising the background image light and the zero-order light towards a SLM, and modulating, by the SLM, the focused light to project an image. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a diagram of a display system, in accordance with various examples. 
         FIG.  2    is a diagram of an apparatus of a display device of the display system of  FIG.  1   , in accordance with various examples. 
         FIG.  3    is a diagram of an apparatus of a display device of the display system of  FIG.  1   , in accordance with various examples. 
         FIG.  4    is a diagram of optical elements in the display device of  FIG.  2   , in accordance with various examples. 
         FIG.  5    is a diagram of optics in the optical elements of  FIG.  4    for projecting a zero-order light with a background image light and provide uniform zero-order light illumination for a projected image, in accordance with various examples. 
         FIG.  6    is a diagram of an optical tunnel in the optical elements of  FIG.  4    for projecting a zero-order light, in accordance with various examples. 
         FIG.  7    shows PLM background images for an image projected by the optical elements of  FIG.  4   , in accordance with various examples. 
         FIG.  8    is a flow diagram of a method for projecting a zero-order light with a background image light from a PLM onto an SLM to provide uniform zero-order light illumination for a projected image, in accordance with various examples. 
     
    
    
     DETAILED DESCRIPTION 
     A projection-based display system can include a SLM device which includes optical elements, such as mirrors or apertures, to generate an image. An SLM modulates the intensity of the light projected on the display by controlling the optical elements to manipulate the light and form the pixels of an image. The SLM may be a digital mirror device (DMD) in which the optical elements are tilting micromirrors. Each micromirror projects a pixel of the image to be displayed. The micromirrors are tilted by applying voltages to the micromirrors to project dark, bright, or shades of light per pixel. Other examples of SLMs include liquid crystal on silicon (LCoS) devices, ferroelectric liquid crystal on silicon (FLCoS) devices, and liquid crystal displays (LCDs). An LCoS device includes an array of liquid crystals on a reflective layer, which form the optical elements or pixels that are controlled to reflect and modulate the intensity of light. The intensity of light is modulated by applying voltage to the liquid crystals, which reorients the crystals in the pixels and accordingly controls the amount of light projected. An FLCoS device includes ferroelectric liquid crystals which have faster voltage than conventional liquid crystals. This causes faster light modulation in the FLCoS devices in comparison to LCoS devices. The optical elements or pixels of an LCD are formed of a transmissive array of liquid crystals that can be controlled, by voltage, to modulate light transmitted through the LCD. AN A projection-based display system may also include multiple light sources, such as laser light sources, of different wavelengths to provide color modes rather than a single lamp or light bulb. The light sources can be operated by simultaneously projecting color modes on the SLM surface to form the image. 
     The projection-based display can also include a PLM positioned between the light sources and the SLM. A PLM may be a micro-electromechanical system (MEMS) device including micromirrors that have adjustable heights with respect to the PLM surface. The heights of the micromirrors can be adjusted by applying voltages. The micromirrors may be controlled with different voltages to form a diffraction surface on the PLM. A controller can control, by applying voltage, the micromirrors individually or in group of adjacent micromirrors to form the diffraction surface. For example, each micromirror can be coupled to respective electrodes for applying a voltage and controlling the micromirror independently from the other micromirrors of the PLM. The diffraction surface is a phase altering reflective surface to light incident from the light sources. The phase altering reflective surface forms a hologram for projecting illumination patterns of light that form an image onto an image projection surface for viewing the image. The holograms are formed by adjusting the heights of the micromirrors to form the diffraction surface of the PLM. The micromirrors of the PLM may be controlled by changing the voltages applied to the micromirrors to modify the diffraction surface and accordingly the hologram. This also changes the angle by which the incident light on the surface of the PLM is reflected with respect to the surface. 
     The PLM can be controlled to reflect and project the incident light from the light sources onto the surface of the SLM through focusing and projection optics. The reflected light from the PLM provides a backlight to the SLM according to high dynamic range (HDR) modulation technique that increase image brightness. According to the HDR modulation technique, the light distribution on the SLM is modulated by the PLM to cause pixel areas in the image to receive more light intensity causing brighter areas in the image. 
     The diffraction surface formed by the PLM to modulate and reflect the incident light from the light sources can also split the incident light into multiple light beams, also referred to herein as diffraction orders, that are reflected by the PLM. The diffraction surface includes a structure of repeated surface patterns formed by the micromirrors, also referred to herein as a diffraction grating. The surface patterns are repeated periodically in a direction across the surface and cause the splitting of an incident light beam into the diffraction orders. The incident light beam is formed of an electromagnetic (light) wave having a phase that is altered by the diffraction surface, which splits the light wave into multiple light waves with different phases. The light waves having different phases are reflected by the diffraction surface in different directions and form the diffraction orders. Accordingly, the diffraction orders are reflected away from the surface at different reflection angles, also referred to herein as diffraction angles. The directions or diffraction angles of the diffraction orders depend on the incident angle of the incident light beam, the period of the repeated surface patterns of the diffraction surface, and the wavelength of the incident light. The diffraction orders may also have different intensities. The diffraction surface can also cause the PLM to reflect, such as because of inefficiencies or manufacturing errors in the PLM, a smaller portion of the incident light into a light beam in a center position between the diffraction orders, also referred to herein as a zero-order light. For example, the zero-order light can be approximately 10 percent (%) of the reflected light from the PLM, and the diffraction orders can be approximately 90% of the reflected light. If projected onto the SLM, the zero-order light may illuminate the SLM surface in a nonuniform manner which can cause a variation of illumination on the SLM surface and accordingly nonuniform brightness across the image projected from the SLM. If the zero-order light is blocked instead from reaching the SLM, the overall illumination of the SLM surface is reduced which can reduce brightness in the projected image. 
     This description includes various examples of a display device configured for projecting zero-order light from a PLM onto a SLM for projecting an image to provide uniform illumination of the image without reducing zero-order light intensity. Uniform illumination refers to distributing light evenly across the entire image to illuminate the image without excluding parts of the image. Uniform illumination of the image is provided without blocking the zero-order light which increases light energy efficiency of the device. The zero-order light is projected onto a SLM with other reflected light that form the background image for the SLM, also referred to herein as background image light, such as in HDR image projection. The background image light from the PLM includes the diffraction orders formed by the diffraction surface of the PLM. The same background image can be projected simultaneously on multiple diffraction orders by the PLM. PLM background images projected by the diffraction orders are combined into a single projected image on an image projection surface. Combining multiple instances of a background image that are projected simultaneously by the PLM can provide a more uniform background image for the SLM. For example, the brightness and accordingly the illumination across the combined background image can be more uniform than the illumination across the respective background images. While the individual projected background images can have more illumination on different parts of the image, the illumination in the combined background image can be more even across the image. A background image with more uniform illumination increases the quality of the image projected by the SLM, as perceived by the human visual system (HVS). An apparatus of the display device includes an optical tunnel and optics that are configured to collect the zero-order light from the PLM, direct the zero-order light onto the SLM, and project the zero-order light to provide uniform illumination on the SLM surface. Accordingly, the zero-order light is projected with the background image light, including the diffraction orders, to increase the brightness of the projected image and provide uniform brightness across the projected image. 
       FIG.  1    shows a display system  100 , in accordance with various examples. The display system  100  may be a projection-based display system for projecting images or video, such as according to HDR image projection. The display system  100  includes a projection-based display device  110  configured to project a modulated light  120  onto an image projection surface  130  for viewing the image. Examples of the image projection surface  130  include a wall or a viewing screen. For example, the viewing screen may be a wall screen, a screen of an augmented reality (AR) or virtual reality (AR) display, a three-dimensional (3D) display, the ground or road for a headlight display, a projection surface in a vehicle such as for a windshield projection display, or other display surfaces for projection-based display systems. 
     The modulated light  120  may be modulated by the display device  110  to project still images or moving images, such as video, onto the image projection surface  130 . The modulated light  120  may be formed as a combination of light with multiple color modes provided by the display device  110 . The display device  110  includes an apparatus  200  having one or more light sources (not shown) for providing the light different wavelengths for the color modes. The light at the different wavelengths is modulated by a PLM  204  in the apparatus  200  to provide background image light and zero-order light to a SLM  205  of the apparatus  200 . The SLM  205  provides, based on the background image light and zero-order light, the modulated light  120  that is projected on the image projection surface  130 . The display device  110  also includes one or more controllers  202  coupled to the apparatus  200  for controlling the components of the display device  110  to display the images or video. For example, the controllers  202  can include a first controller for controlling the PLM  204  to modulate light of different wavelengths from respective light sources. The SLM  205  can also be controlled by a second controller  202  to modulate the light from the PLM  204  and provide the modulated light  120 . The controllers  202  may also include a third controller for controlling the light sources. The display device  110  may further include one or more input/output devices (not shown), such as an audio input/output device, a key input device, a display, and the like. 
       FIG.  2    shows an apparatus  201  for projecting images, in accordance with various examples. For example, the apparatus  201  can be part of the display device  110  that projects a modulated light  120 , such as for HDR image projection. The apparatus  201  includes the PLM  204 , the SLM  205 , one or more light sources  207 , and a projection optics  220  that projects the modulated light  120  on the image projection surface  130 . The projection optics  220  can include a single projection lens, as shown in  FIG.  2   , or can include multiple lenses in other examples. The PLM  204 , the one or more light sources  207 , and the SLM  205  are coupled to and controlled by the controllers  202  of the display device  110 . 
     In an example, the controllers  202  may include a first controller  209  for controlling the PLM  204 , a second controller  210  for controlling the SLM  205 , and a third controller  211  for controlling the one or more light sources  207 . The controllers  202  may also include or may be coupled to a processor  212  configured to coordinate between the controllers  202  to control the PLM  204 , the one or more light sources  207 , and the SLM  205 , and accordingly modulate the modulated light  120  to provide the image for projection. For example, the first controller  209  may be an analog controller for controlling micromirrors  213  of the PLM  204 . The analog controller can control switching each of the micromirrors  213  of the PLM  204  between multiple discrete and different heights. The second controller  210  of the PLM  204  can include or be coupled to a static random-access memory (SRAM) (not shown) including an array of memory cells each configured to store bits of memory value for adjusting a respective optical element of the PLM  204 . The memory value is useful to switch the optical element to a discrete height. The second controller  210  may be a digital controller for controlling the optical elements of the SLM  205 , such as micromirrors of a DMD or liquid crystals of an LCoS or LCD. The digital controller can control switching each of the optical elements of the SLM  205 , between an on state and an off state. In the case of a DMD, the on state can rotate a micromirror to reflect/project light to provide a bright pixel in the image, and the off state can rotate the optical element to stop reflecting/projecting light to provide a dark pixel in the image. In the case of an LCoS, FLCoS or LCD, the on state can cause transmitting or reflecting light by the liquid crystal, and the off state can cause blocking the light by the liquid crystal. The second controller  210  of the SLM  205  can include or be coupled to a SRAM (not shown) where each memory cell is configured to store one bit of memory value for adjusting a respective optical element of the PLM  204 . The one-bit memory value is useful to switch the optical element between the on state for reflecting/projecting light and the off state to stop reflecting/projecting light. For example, a zero-bit value can switch the optical element to an off state and a one-bit value can switch the optical element to an on state. The third controller  211  can be a digital controller configured to control switching the one or more light sources  207  on and off, or an analog controller that controls and changes the level of light intensity of the one or more light sources  207 . 
     The PLM  204  can be operated according to HDR modulation techniques to increase the brightness and contrast in the image projected by the SLM  205  on the image projection surface  130 . The image brightness provided by the SLM  205  can be reduced in one or more areas on the surface of the SLM  205  which include pixels that are switched to the off state. In such areas, the light is provided by pixels of the SLM  205  that are switched to the on state, and the brightness lost in such areas can depend on the number of pixels that are switched to the off state. According to the HDR modulation technique, light can be projected and spatially modulated by the PLM  204  to distribute light at the surface of the SLM  205  to cause brighter regions and higher contrast in the image. The light projected by the PLM  204  onto the surface of the SLM  205  compensates for the reduced brightness in the areas of the SLM  205  with the switched off pixels. The spatially modulated light by the PLM  204  can also be projected onto the SLM  205  as background image light that illuminates certain regions of the pixels excluding other regions. Restricting the illumination of the SLM  205  to certain regions of the pixels causes the SLM  205  to provide a higher contrast by the modulated light  120 , where the illuminated pixel regions project brighter areas of the image while the remaining areas remain dark. 
     The PLM  204  includes the PLM micromirrors  213  as adjustable optical elements which form a grid of pixels on the surface of the PLM  204 . The heights of the PLM micromirrors  213  with respect to the surface can be adjusted by applying voltages to the PLM  204 . The first controller  209  controls the PLM  204  by changing the voltages applied to the PLM  204  to adjust the heights of the PLM micromirrors  213 , which form a diffraction surface. The diffraction surface is formed by providing different heights of the PLM micromirrors  213  across the grid of pixels on the surface. The diffraction surface of the PLM micromirrors  213  modulates and reflects an incident light  216  from the one or more light sources  207 . For example, the incident light  216  includes one or more color modes at respective wavelengths that are directed from the one or more light sources  207  to the PLM  204  through respective lenses  214  and mirrors  215 . In examples, the light sources  207  can be three light sources that provide three color modes at three respective wavelengths, such as for blue, green, and red light. As shown in  FIG.  2   , the mirrors  215  can be dichroic mirrors configured to reflect the light from the respective light sources  207  for the respective color modes to the PLM  204 , and transmit light for the other color modes from the other mirrors  215  on a same optical path to the PLM  204 . In other examples, the mirrors  215  can be reflective mirrors that reflect the light from the respective light sources  207  on separate optical paths to the PLM  204 . The lenses  214  can be similar lenses that determine the diameter and beam profile of the incident light  216  on the surface of the PLM  204 . In other examples, the color modes of the incident light  216  from the one or more light sources  207  to the PLM  204  can be directed by other optics or can be projected in a straight path without optical elements. 
     The light sources  207  can be controlled, by a controller  202  (e.g., third controller  211 ), to project the incident light  216  for each color mode at a time to the PLM  204  in a time multiplexing sequence. Accordingly, each light source  207  is switched on at a time in a certain sequence and rate to project light at a respective color mode from the PLM  204  to the SLM  205 . This causes projecting in the modulated light  120  each color mode at a time at the same rate. The rate can be sufficiently fast to perceive, by the HVS, the time multiplexed color modes in the projected image as a single full color image. For example, the image projection rate can be between 1/30 and 1/60 second. 
     The incident light  216  from the one or more light sources  207  is modulated and reflected by the PLM  204  to provide a background image light  217  which is projected through a background image optical path  218  towards the SLM  205 . The background image light  217  forms the background image on the surface of the SLM  205  and includes diffraction orders provided by modulating and reflecting the incident light  216  by the diffraction surface of the PLM micromirrors  213 . The diffraction surface also provides a zero-order light  219  projected at a center position with respect to the other diffraction orders of the background image light  217  in the background image optical path  218  and onto the center of the background image on the surface of the SLM  205 . The apparatus  201  includes in the background image optical path  218  a first lens arrays  221 , a second lens array  222 , and focusing optics  223  positioned between the PLM  204  and the SLM  205 . 
     For example, the first lens array  221  is a N×N array of lenses  224 , where N is an integer number. The N×N array is an array of adjacent lenses  224  that are arranged across the first lens array  221  and face the PLM  204 . The second lens array  222  is also a N×N array of lenses  225 , where N is the same number of the lenses  224  in the first lens array  221 . The lenses  225  in the second lens array  222  may be similar to and have the same size of the lenses  224 . For example, the first lens array  221  can include four adjacent lenses  224  that are arranged in a 2×2 array (as shown in a front view of first lens array  221  in  FIG.  2   ). In this case, the second lens array  222  also includes four lenses  225  similar to and aligned respectively with the four lenses  224 . Each pair of lenses  224  and respective lens  225  can project a respective image in the diffraction orders of the background image light  217  from the PLM  204  to the focusing optics  223 . The lenses  224  and similarly the lenses  225  may be rectangular or circular lenses. In other examples, the lenses  224  and  225  may have other shapes. The shapes of the lenses  224  and  225  determine the beam profile of the background image light  217  and accordingly the shape of the projected background image. For example, square lenses  224  and  225  can provide a square shaped background image, or rectangular lenses  224  and  225  can provide a rectangular shaped background image. The second lens array  222  also includes an embedded lens  226  positioned at the center of the second lens array  222  between the lenses  225 . The size (e.g., diameter) of the embedded lens  226  may be smaller than the diameter of the lenses  224  in the second lens array  222 . The embedded lens  226  can project the zero-order light  219  projected from the PLM  204  through the first lens array  221  to the focusing optics  223 . 
     An optical tunnel  227  is also positioned between the first lens array  221  and the second lens array  222 . The optical tunnel  227  is configured to transmit the zero-order light  219  from the first lens array  221  to the second lens array  222 . The optical tunnel  227  is an optical waveguide for the zero-order light  219  that extends from the center of and through the first lens array  221  to the center of the second lens array  222 . The embedded lens  226  is configured to focus and project the zero-order light  219  from the optical tunnel  227  onto the focusing optics  223 . 
     The focusing optics  223  can include one or more focusing lenses that are positioned and aligned to focus the background image light  217  onto an intermediate image plane in the background image optical path  218 . The intermediate image plane is at the focus point of the focusing optics  223 . A diffuser  228  can be positioned in the intermediate image plane to reduce speckle that may be caused by wave interference in the background image light  217 , such as in the case of light sources  207  for coherent light (e.g., laser light sources). The apparatus  201  further includes illumination optics  230  including one or more lenses between the intermediate image plane at the diffuser  228  and the SLM  205 . The illumination optics  230  project the background image light  217  and the zero-order light  219  from the intermediate image plane at the diffuser  228  or the focus point of the focusing optics  223  onto the surface of the SLM  205 . For example, as shown in  FIG.  2   , the illumination optics  230  can include a first lens  231  that collimates a spreading or defocused beam of the background image light  217  from the from the diffuser  228 , and a second lens  232  that focuses the collimated beam from the first lens  231  onto a third lens  233  of the illumination optics  230 . The third lens  233  projects and adjusts the profile of the background image light  217  to fit on the surface of the SLM  205 . In other examples, the illumination optics  230  can include fewer or more than three lenses to project and adjust the profile of the background image light  217  from the diffuser  228  on the SLM  205 . The apparatus  201  may include a first prism  235  positioned between the illumination optics  230  and the SLM  205 . The first prism  235  directs the background image light  217  and the zero-order light  219  from the illumination optics  230  onto the SLM  205 . 
     The background image light  217  is projected by the lenses  224  in the first lens array  221 , and by the lenses  225  in the second lens array  222 , onto the surface of the SLM  205  through the focusing optics  223 , the illumination optics  230 , and the first prism  235 . For example, a 2×2 array of lenses  224  projects the diffraction orders of the background image light  217  to form a background image at the SLM  205 . Accordingly, the PLM  204  projects, on the diffraction orders in the background image light  217 , four PLM background images to a PLM image plane  240  in front of the first lens arrays  221 . Each pair of lenses  224  and  225  in the first and second lens array  221  and  222 , respectively, projects one of the four images. The PLM background images are projected, combined, and imaged, by the optics in the background image optical path  218  and the illumination optics  230 , into a background image at the SLM  205 . Projecting and combining multiple background images can provide more uniform illumination across the background image at the SLM  205 . The number of lenses  224  in the first lens array  221 , and similarly of the lenses  225  in the second lens array  222 , matches the number of projected background images, where each pair of lenses  224  and  225  is aligned and configured to project one of the background images in the optical path  218 . In other examples, fewer or more than four images can be projected by the PLM  204  on the diffraction orders of the background image light  217  to form a single combined background image at the SLM  205 . For example, the number of background images may be a multiple of two, such as two, four, or eight background images projected simultaneously by the PLM  204 . Increasing the number of images can increase the uniform illumination across the background image at the SLM  20 , and also increase the number of lenses in the first and second lens arrays  221  and  222 . In the example of  FIG.  2   , four background images are provided by the PLM  204  to increase the uniform illumination across the background image at the SLM  205  and limit of the number of lenses  224  and  225  in the first and second lens arrays  221  and  222 , respectively, to four lenses. 
     In examples, the SLM  205  can be a DMD. The DMD includes DMD micromirrors as adjustable optical elements which form a grid of pixels on the surface of the DMD. The tilt of the DMD micromirrors with respect to the surface can be adjusted by applying voltage to the DMD. The second controller  210  can control the SLM  205  by changing the voltages applied to the DMD to adjust the tilt of the respective DMD micromirrors. Controlling the DMD by tilting the DMD micromirrors modulates and reflects the background image light  217  and the zero-order light  219  to provide the modulated light  120  from the DMD to the projection optics  220 . The background image light  217  combined with the zero-order light  219  increase illumination at the surface of the SLM  205  and accordingly the illumination in the modulated light  120  projected through the projection optics  220 . The increased illumination in the modulated light  120  increases the brightness in the projected image on the image projection surface  130 . The background image light  217  and the zero-order light  219  can also compensate for loss of illumination in the modulated light  120  by the SLM  205  if a DMD micromirrors is switched to an off state to provide a dark pixel in the image. The loss of illumination can increase if more DMD micromirrors of the SLM  205  are switched off. 
     The apparatus  201  may also include a second prism  245  positioned between the SLM  205  and the projection optics  220  to direct the modulated light  120  from the SLM  205  to the projection optics  220 . The modulated light  120  provides the image projected on the image projection surface  130  which includes illumination from the zero-order light  219  as reflected by the SLMs  205 . 
     In other examples, the SLM  205  can be a SLM device other than a DMD with adjustable optical elements other than micromirrors. For example, the SLM  205  can be an LCoS or FLCoS with adjustable reflective liquid crystals that form a grid of pixels on the surface of the LCoS or FLCoS. In this case, the LCoS or FLCoS can also be arranged similarly to a DMD, as shown in  FIG.  2   , where the first prism  235  is positioned between the illumination optics  230  and the LCoS/FLCoS, and the second prism  245  is positioned between the LCoS/FLCoS and the projection optics  220 . In another example, the SLM  205  can be an LCD including an array of adjustable transmissive liquid crystals, which form a grid of pixels of the LCD. In this case, the LCD can be aligned with projection optics (e.g., similarly to the projection optics  220 ) to face illumination optics (e.g., similarly to the illumination optics  230 ) in a straight optical path. The LCD is placed between the illumination optics and the projection optics on this straight optical path. The transparency or opacity of the liquid crystals in such devices can be adjusted by applying voltage. The second controller  210  can control the SLM  205  by changing the voltages applied to the liquid crystals to adjust the orientation of the liquid crystals in the pixels, an accordingly, the optical properties, such as the refractive index, of the liquid crystals. Light can be modulated by changing the amplitude, phase, or polarization of light waves based on the optical properties of the liquid crystals. The liquid crystals are controlled to reflect or transmits the background image light  217  and the zero-order light  219  to provide the modulated light  120  from the DMD to the projection optics  220 . 
     In other examples, the display device  110  may include multiple pairs of PLMs and respective SLMs, each pair corresponding to a color mode from a respective light source. In this case, each pair of PLM and SLM can modulate a color mode separately which increases the diffraction efficiency and the projected intensity of each color mode and accordingly increases image quality and power efficiency of the display device  110 .  FIG.  3    shows an apparatus  300  of the display device  110 , in accordance with various examples. The apparatus  300  includes three PLMs  301 , three respective light sources  302 , three respective SLMs  303 , and a projection optics  304  that projects the modulated light  120  on the image projection surface  130 . The projection optics  304  can include a single projection lens or can include multiple lenses in other examples. The PLMs  301 , the light sources  302 , and the SLMs  303  are coupled to and controlled by the controllers  202  of the display device  110 , which can include. For example, the controllers  202  can include a first controller  202  for controlling the PLMs  301  to modulate light of different wavelengths from respective light sources  302 . The SLMs  303  can also be controlled by a second controller  202  to modulate the light from the respective PLMs  301  and provide the modulated light  120 . The first and second controllers  202  can be digital controllers that switch the optical elements of the PLMs  301  and SLMs  303 , respectively, between on and off states. The controllers  202  can also include an analog controller  202  configured to process the image data to provide digital signals to the first and second controllers  202 . 
     The light sources  302  provide three color modes of light, respectively. For example, the color modes include blue light, green light, and red light. The light modes can be directed through respective optical fibers  307  to the respective PLMs  301 . For each color mode, an incident light  309  is projected from a light source  302  to a respective PLM  301  through a respective lens  308 . The PLMs  301  include respective PLM micromirrors  310  with adjustable heights that are controlled by the controller  202  to modulate and reflect the incident light  309 . The incident light  309  from each light source  302  is modulated and reflected by the respective PLM  301  to provide a respective background image light  311 . The light sources  302  can be switched on and off, by a controller  202 , to project the incident light  309  for each color mode at a time to a respective PLM  301  in a time multiplexing sequence at a certain rate. 
     The background image light  311  from each PLM  301  is projected through a background image optical path  312  towards the SLM  303 . According to time multiplexing, each PLM  301  can project the background image light  311  for a respective color mode at a time from a respective light source  302  to a respective SLM  303 . The background image light  311  from each PLM  301  includes diffraction orders provided by modulating and reflecting the incident light  309  by the diffraction surface of the PLM micromirrors  310 . The diffraction surface also provides a zero-order light  313  projected at a center position with respect to the other diffraction orders in the background image optical path  312  and onto the center of the background image on the surface of the SLM  303 . The apparatus  300  includes in the background image optical path  312  a first lens arrays  314 , a second lens array  315 , and focusing optics  316  positioned between the PLMs  301  and the SLMs  303 . 
     The first lens array  314  and the second lens array  315  are N×N array of lenses, where N is an integer. For example, the first lens array  314  is a N×N array of lenses  317  that are arranged across the first lens array  314 . The second lens array  315  is also a N×N array of lenses  318 , where N is the same number of the lenses  224  in the first lens array  221 . The lenses  318  in the second lens array  315  may be similar to and have the same size of the lenses  317 . For example, the first lens array  314  can include four adjacent lenses  317  that are arranged in a 2×2 array (as shown in a front view of first lens array  314  in  FIG.  3   ). In this case, the second lens array  315  also includes four lenses  318  similar to and aligned respectively with the four lenses  317 . Each pair of lenses  317  and respective lens  318  can project a respective image in the diffraction orders of the background image light  311  from the PLMs  301  to the focusing optics  316 . The lenses  317  and similarly the lenses  318  may be rectangular or circular lenses. In other examples, the lenses  317  and  318  may have other shapes. The second lens array  315  also includes an embedded lens  319  positioned at the center of the second lens array  315  between the lenses  318 . The size (e.g., diameter) of the embedded lens  319  may be smaller than the diameter of the lenses  318  in the second lens array  315 . The embedded lens  319  can project the zero-order light  313  projected from the PLMs  301  through the first lens array  314  to the focusing optics  316 . 
     An optical tunnel  320  is also positioned between the first lens array  314  and the second lens array  315 . The optical tunnel  320  is configured to transmit the zero-order light  313  from the first lens array  314  to the second lens array  315 . The optical tunnel  320  is an optical waveguide for the zero-order light  313  that extends from the center of and through the first lens array  314  to the center of the second lens array  315 . The embedded lens  319  is configured to focus and project the zero-order light  313  from the optical tunnel  320  onto the focusing optics  316 . The focusing optics  316  can include one or more focusing lenses that are positioned and aligned to focus the background image light  311  onto an intermediate image plane at the focus point of the focusing optics  316 . A diffuser  321  may be positioned in the intermediate image plane to reduce speckle that may be caused by wave interference in the background image light  311 . 
     The apparatus  300  also includes in background image optical path  312  optics for directing the background image light  311  from each PLM  301  to the first lens array  314 . For example, as shown in  FIG.  3   , a prism cube  322  is positioned on a path of the background image light  311  between one of the PLMs  301  and the first lens array  314 . A mirror  323  is also positioned on the path of the background image light  311  between each of the other two PLMs  301  and the prism cube  322 . The mirrors  323  direct the background image light  311  from the other two PLMs  301  to the prism cube  322 . The apparatus  300  may also include in the background image optical path  312  intermediate lenses  324  to project the background image light  311  from the respective PLMs  301  onto the mirrors  323  and the prism cube  322 . 
     The apparatus  300  further includes illumination optics  330  including one or more lenses between the intermediate image plane at the diffuser  321  and the SLMs  303 . The illumination optics  330  project the background image light  311  and the zero-order light  313  from the intermediate image plane at the diffuser  321  onto the SLMs  303 . For example, as shown in  FIG.  3   , the illumination optics  330  can include a first lens  331  that collimates a spreading or defocused beam of the background image light  311  from the from the diffuser  321 , and a second lens  332  that focuses the collimated beam from the first lens  331  onto a third lens  333  of the illumination optics  330 . The third lens  333  projects and adjusts the profile of the background image light  311  to fit on the surface of the SLMs  303 . In other examples, the illumination optics  330  can include fewer or more than three lenses to project and adjust the profile of the background image light  311  from the diffuser  321  on the SLMs  303 . The apparatus  300  may include a first prism  334  positioned between the illumination optics  330  and the SLMs  303  to direct the background image light  311  and the zero-order light  313  from the illumination optics  330  onto the SLMs  303 . A prism filter  335  is also placed between each SLM  303  and the first prism  334  to filter the respective color mode in the background image light  311  which is received by the respective SLM  303 . Each prism filter  335  also transmits the respective color mode in the modulated light  120  from the respective SLM  303  onto the first prism  334  towards the projection optics  304 . 
     Each prism filter  335  is configured to direct a color mode of the background image light  311  and the zero-order light  313  provided by a respective PLM  301  to a respective SLM  303 , and transmit the other color modes towards the other SLMs  303 . For example, as shown in  FIG.  3   , a first prism filter  335  optically coupled to a first SLM  303  is configured to direct red light in the background image light  311  and the zero-order light  313  from the first prism  334  to the first SLM  303 , and to transmit the remaining light in the background image light  311  and the zero-order light  313  towards a second SLM  303  and a third SLM  303 . A second prism filter  335  is optically coupled to the first prism filter  335  and the second SLM  303 . The second prism filter  335  is configured to direct to the second SLM  303  blue light in the background image light  311  and the zero-order light  313  which is transmitted by the first prism filter  335 , and to transmit the remaining light to the third SLM  303 . The third prism filter  335  is optically coupled to the second prism filter  335  and the third SLM  303 . The third prism filter  335  is configured to transmit to the third SLM  303  green light in the background image light  311  and the zero-order light  313  which is transmitted by the second prism filter  335 . The third prism filter  335  also transmits the green light in the modulated light  120  from the third SLM  303  to the second prism filter  335 . The second prism filter  335  is configured to transmit blue light in the modulated light  120  from the second SLM  303  with the green light from the third prism filter  335  to the first prism filter  335 . The first prism filter  335  transmits red light in the modulated light  120  from the first SLM  303  with the green light and blue light from the second prism filter  335  to the first prism  334  and towards the projection optics  304 . 
     The background image light  311  is projected by the lenses  317  in the first lens array  314 , and similarly the second lens array  315 , onto the SLMs  303  through the focusing optics  316 , the illumination optics  330 , and the first prism  334 . For example, a 2×2 array of lenses  317  projects diffraction orders in the background image light  311 . PLM background images projected by the diffraction orders in the background image light  311  to form the background image at the SLMs  303 . Accordingly, each PLM  301  projects, on the diffraction orders in the background image light  311 , four PLM background images to a PLM image plane  336  in front of the first lens arrays  314 . The PLM background images are projected, combined, and imaged, by the optics in the background image optical path  312  and the illumination optics  330 , into a background image in the background image light  311  at the SLMs  303 . 
     In examples, the SLMs  303  can be a DMDs. The DMDs include respective SLM micromirrors with adjustable tilts that are controlled by one or more controllers  202  to modulate and reflect the background image light  311  and the zero-order light  313  to provide the modulated light  120  from the DMD to the projection optics  220 . In other examples, the SLMs  303  can be SLM devices other than DMDs with adjustable optical elements other than micromirrors. For example, the SLMs  303  can be LCoS or FLCoS devices with adjustable reflective liquid crystals that form a grid of pixels on the surface of the LCoS or FLCoS. The SLMs  303  can also be LCDs with adjustable transmissive liquid crystals. The LCDs can be aligned with and placed between projection optics (e.g., similarly to the projection optics  304 ) and illumination optics (e.g., similarly to the illumination optics  330 ) in a straight optical path. The liquid crystals in the LCoS/FLCoS or LCDs can be controlled by one or more controllers  202  by voltages to reflect or transmit the background image light  311  and the zero-order light  313  to provide the modulated light  120 . 
     The apparatus  300  may also include a second prism  338  positioned between the prism filters  335  and the projection optics  304  to direct the modulated light  120  from the SLMs  303  to the projection optics  304 . The modulated light  120  provides the image with background image light projected on the image projection surface  130  which includes uniform illumination from the zero-order light  313  as reflected by the SLMs  303 . 
       FIG.  4    shows optical elements  400  in the display device  110 , in accordance with various examples. The optical elements  400  may be part of the apparatus  201  in the background image optical path  218  or part of the apparatus  300  in the background image optical path  312 . The optical elements  400  include a first lens array  401 , a second lens array  402 , a focusing lens  403 , and a background image plane  404 . The first lens array  401  and the second lens array  402  are N×N arrays of lenses  405  and  406  that are arranged across the first lens array  401  and second lens array  402 , respectively. The lenses  406  in the second lens array  402  may be similar to and have the same size of the lenses  405  in the first lens array  401 . For example, the first lens array  401  and the second lens array  402  can include four adjacent lenses  405  and four adjacent lenses  406 , respectively, that are arranged in a 2×2 array. Only two of the four lenses  405  and two respective lenses  406  are shown in the cross section view of the optical elements  400  in  FIG.  4   . 
     Each of the four pairs of lenses  405  and  406  are configured and aligned to project a respective PLM background image  407  in a background image light  408  from a PLM image plane  409  in front of the first lens array  401  to the focusing lens  403 .  FIG.  4    also shows a front view of the PLM image plane  409  including four similar PLM background images  407 . The PLM image plane  409 , also referred to herein as a Fourier Transform plane, can be in front of a PLM, such as the PLM  204  in the apparatus  201  or the PLM  301  in the apparatus  300  of the display device  110 . The PLM background images  407  are projected on the diffraction orders  410  that form the background image light  408  from one or more PLMs. For example, each diffraction order in the background image light  407  is useful to project one of the PLM background images  407  from the surface of the PLM through the first lens array  401 , the second lens array  402 , and onto the focusing lens  403 . The lenses  405  in each pair of lenses  405  and  406  can project the respective PLM background image  407  in the background image light  408  at the PLM image plane  409  onto a respective lens  406 , which in turn projects the PLM background image  407  in the background image light  408  onto the focusing lens  403 . 
     For example, the first lens array  401  and second lens array  402  are the first lens array  221  and second lens array  222  of the apparatus  201  which project the background image light  217  from the PLM  204 . The PLM image plane  409  is the PLM image plane  240  and the focusing lens  403  is part of the focusing optics  223  in the apparatus  201 . In this case, the background image plane  404  can be on the surface of the SLM  205 , or may be the intermediate image plane at the diffuser  228  or the focus point of the focusing optics  223 . In another example, the first lens array  401  and second lens array  402  are the first lens array  314  and second lens array  315  of the apparatus  300  which project the background image light  311  from the PLMs  301 . The PLM image plane  409  is the PLM image plane  336  and the focusing lens  403  is part of the focusing optics  316  in the apparatus  300 . In this case, the background image plane  404  can be on the surface of the SLMs  303 , or may be the intermediate image plane at the diffuser  321  or the focus point of the focusing optics  316 . 
     The PLM also projects a zero-order light  411  with the background image light  408 . For example, the zero-order light  411  is the zero-order light  219  projected by the PLM  204  in the apparatus  201  or the zero-order light  313  projected by the PLMs  301  in the apparatus  300 . The zero-order light  411  is projected in a center position between the diffraction orders  410  of the background image light  408 . Accordingly, a zero-order light spot  412  appears as the projection of the zero-order light  411  at the PLM image plane  409  in a center position between the PLM background images  407  (as shown in the front view of the PLM background images  407  in  FIG.  4   ). 
     The focusing lens  403  projects and focuses the background image light  408  and the zero-order light  411  from the second lens array  402  onto the background image plane  404 .  FIG.  4    shows an example of one focusing lens  403  that focuses the background image light  408 . In other examples, the background image light  408  and the zero-order light  411  can be focused by focusing optics including multiple lenses, such as the focusing optics  223  in the apparatus  201  or the focusing optics  316  in the apparatus  300 .  FIG.  4    also shows a front view of the background image plane  404 . The diffraction orders  410  of the focused background image light  408  overlap in the background image plane  404  and accordingly the respective PLM background images  407  in the background image light  408  are combined into an SLM background image  414  on the background image plane  404  (as shown in the front view of the background image plane  404 ). In examples, the SLM background image  414  can have a rectangular image profile as the PLM background images  407 . For example,  FIG.  4    shows an example of square shaped PLM background images  407  that provide a square shaped SLM background image  414 . In other examples, rectangular shaped background images  407  provide a square shaped SLM background image  414 . 
     If the zero-order light  411  is projected directly through the first lens array  401 , the second lens array  402 , and the focusing lens  403  onto the background image plane  404 , the zero-order light  411  may not be focused in a uniform manner on the background image plane  404 , and accordingly may not illuminate the SLM background image  414  in a uniform manner. The nonuniform illumination of the SLM background image  414  by the zero-order light  411  can cause nonuniform brightness and accordingly reduced image quality in the image projected from the SLM. For example, in this case, the zero-order light  411  may appear at the background image plane  404  as a defocused circular light spot that does not illuminate in a uniform manner the rectangular or square shaped SLM background image  414 . The defocusing of the zero-order light  411  at the background image plane  404  can be related to beam spreading in the zero-order light  411  based on the beam profile (e.g., Gaussian beam profile) and the propagation distance between the first lens array  401 , the second lens array  402 , and the focusing lens  403 . 
     To increase uniform illumination of the SLM background image  414  by the zero-order light  411  at the background image plane  404 , the optical elements  400  also include an optical tunnel  415  positioned between the first lens array  401  and the second lens array  402 , and aligned with the center of the first lens array  401  and the second lens array  402  (as shown in the cross section view of the optical elements  400  in  FIG.  4   ). The optical tunnel  415  is configured to project the zero-order light  411  from the first lens array  401  to the second lens array  402 . The optical tunnel  415  is an optical waveguide for the zero-order light  411  that extends from the center of and through the first lens array  401  to the center of the second lens array  402 . As shown in  FIG.  4   , the optical tunnel  415  extends through the first lens array  401  from one end facing the PLM image plane  409  to the other end facing the second lens array  402 . The optical tunnel  415  also extend partially into the second lens array  402 , at a certain depth inside the second lens array  402 , at one end of the second lens array  402  that faces the first lens array  401 . For example, the optical tunnel  415  can extend from a first end of the second lens array  402  which faces the first lens array  401  into approximately a quarter (25%) or half (50%) of the total thickness of the second lens array  402 , in the direction of the optical axis passing through the first lens array  401  and second lens array  402 . In examples, the optical tunnel  415  can extend from the first end of the second lens array  402  into a portion of the total thickness of the second lens array  402  that is determined to provide a certain spread in the projection of the zero-order light  411  on the background image plane  404 . 
     The optical tunnel  415  is configured to limit or control the beam spread in the zero-order light  411  that propagates between the first lens array  401  and the second lens array  402 . The optical tunnel  415  is also configured to shape a zero-order light spot  413 , which is the projection of the zero-order light  411  on the background image plane  404 . As shown in  FIG.  4   , the zero-order light spot  413  can be projected to cover in a uniform manner and match the shape of the SLM background image  414  at the background image plane  404 . For example, the optical tunnel  415  may be a hollow tunnel, made of a dielectric material such as glass, with reflective inner side walls that direct the light from one end of the optical tunnel  415  to the other end. In other examples, the optical tunnel  415  may be a slab waveguide with a rectangular or square profile that is filled with a dielectric material (e.g., glass). In examples, the optical tunnel  415  may be an optical fiber with a rectangular or square profile. The two ends of the optical tunnel  415  are positioned at the respective centers of the first lens array  401  and the second lens array  402 . The respective centers are central points positioned between the lenses  405  and  406  of the first lens array  401  and the second lens array  402 , respectively. Accordingly, the optical tunnel  415  projects the zero-order light  411  from the first lens array  401  to the second lens arrays  402  without the diffraction orders  410  of the background image light  408 , which are projected by the lenses  405  and  406  around the optical tunnel  415 . 
     The second lens array  402  also includes an embedded lens  416  coupled to and positioned at the center of the second lens array  402 , between the lenses  406  (as shown in the cross section view of the optical elements  400  in  FIG.  4   ). The size (e.g., diameter) of the embedded lens  416  may be smaller than the diameter of the lenses  406  in the second lens array  402 . The embedded lens  416  is configured to project the zero-order light  411  from the optical tunnel  415  to the focusing lens  403 . For example, the embedded lens  416  is the embedded lens  226  in the second lens array  222  of the apparatus  201 , which projects the zero-order light  219  of the PLM  204  from the optical tunnel  227  to the focusing optics  223 . In another example, the embedded lens  416  is the embedded lens  319  in the second lens array  315  of the apparatus  300 , which projects the zero-order light  313  of the PLMs  301  from the optical tunnel  320  to the focusing optics  316 . The embedded lens  416  is also configured to focus and project the zero-order light  411  from the optical tunnel  415  onto the focusing lens  403 , which reduces the spreading and accordingly increases the uniform illumination of the zero-order light spot  413  over the SLM background image  414  on the background image plane  404  (as shown in the front view of the background image plane  404  in  FIG.  4   ). 
       FIG.  5    shows optics  500  in the optical elements  400  for projecting the zero-order light  411  and the background image light  408  and provide uniform zero-order light illumination for a projected image, in accordance with various examples. The optics  500  include the lenses  405  in the first lens array  401  and the lenses  406  in the second lens array  402 . The lenses  406  project the diffraction orders  410  in the background image light  408  from the lenses  405  onto the focusing lens  403  (pictured in  FIG.  4   ). The embedded lens  416  is positioned between the lenses  406  and can be smaller than the lenses  406 . As shown in  FIG.  5   , the size of the embedded lens  416  may match the profile dimension of the optical tunnel  415  which can also be smaller than the size of the lenses  405 . For example, the embedded lens  416  can have approximately the same diameter as the optical tunnel  415  or can have a larger diameter than the optical tunnel  415 . 
       FIG.  5    shows a side view of the first lens array  401  and the second lens array  402  with two lenses  405  and two lenses  406 , respectively, that focus two respective diffraction orders  410  in the background image light  408 . As shown, the optical tunnel  415  can extend entirely through the first lens array  401  and partially into the second lens array  402 . A portion  503  of the optical tunnel  415  may be embedded in the center of the second lens array  402  between the lenses  406 , at the first side of the second lens array  402  which faces the first lens array  401 . For example, the portion  503  can be within 25% to 50% of the total thickness of the second lens array  402  between the lenses  406 . The portion  503  of the optical tunnel  415  at the first side of the second lens array  402  is also aligned with the embedded lens  416  at the second side of the lens array  402  which faces the focusing lens  403 . The diameter of the embedded lens  416  may be equal to or larger than the diameter of the optical tunnel  415  to collect all the zero-order light  411  projected from the optical tunnel  415  onto the embedded lens  416 . The zero-order light  411  is projected from the PLM image plane  409  onto the focusing lens  403  (pictured in  FIG.  4   ) by the optical tunnel  415  and the embedded lens  416  without the lenses  405  and  406 . The diffraction orders  410  in the background image light  408  are projected by the lenses  405  and  406  without the optical tunnel  415  and the embedded lens  416 . 
       FIG.  6    shows the optical tunnel  415  in the optical elements  400  for projecting the zero-order light  411 , in accordance with various examples. The optical tunnel  415  can be a rectangular slab waveguide  601  made of a dielectric material, such as glass, and configured to guide and transfer optical wave modes in the zero-order light  411  from one end of the optical tunnel  415  to the other end. For example, the rectangular slab waveguide  601  can be a hollow tunnel made of the dielectric material, or a can be a tunnel filled with a dielectric material. The propagation of the optical wave modes in the zero-order light  411  is dependent on the profile dimensions of the rectangular slab waveguide  601  including the width (w) and thickness (t), the wavelengths of the zero-order light  411 , and the refractive indices of the dielectric material of the rectangular slab waveguide  601  and of the surround material (e.g., air). In examples, the width and the thickness of the rectangular slab waveguide  601  can be between few millimeters (mm) for an optical tunnel  415  that extends in length to tens of mm. For example, the width and the thickness of the rectangular slab waveguide  601  can be between such as between 0.17 mm and 1.4 mm, or between 1.7 mm and 14 mmm, for a tunnel that has a length within 30 mm. Examples of the dielectric material of the rectangular slab waveguide  601  include SiO 2  and glass. 
     To project the zero-order light  411 , the profile dimensions of the rectangular slab waveguide  601  can be based on the wavelengths of the color modes of the light sources that provide the zero-order light  411 , such as the light sources  207  in the apparatus  201  or the light sources  302  in the apparatus  300 . The profile of the rectangular slab waveguide  601  also determines the shape of a zero-order light spot  413  which is the projection of the zero-order light  411  on the background image plane  404 . The zero-order light spot  413  is formed by relaying the zero-order light spot  412 , which is the projection of the zero-order light  411  at the PLM image plane  409  through the rectangular slab waveguide  601  (the optical tunnel  415 ) between the first and second lens arrays  401  and  402  and through the focusing lens  403 .  FIG.  6    also shows front views of the background image plane  404  and the PLM image plane  409 . The rectangular profile of the rectangular slab waveguide  601  reshapes the circular zero-order light spot  412  on the PLM image plane  409  into a rectangular shaped zero-order light spot  413  on the background image plane  404 . The ratio of the length (l) to height (h) of the rectangular shaped zero-order light spot  413  also matches the ratio of the width (w) to thickness (t) of the rectangular slab waveguide  601 . 
       FIG.  6    shows a rectangular shaped SLM background image  414  on the background image plane  404  that is formed by projecting four rectangular shaped PLM background images  407  of the diffraction orders  410  (pictured in  FIGS.  4  and  5   ) in the background image light  405  through the first and second lens arrays  401  and  402  and focusing the diffraction orders  410  by the focusing lens  403  onto the background image plane  404 . The shape and the ratio of the length (l 1 ) to height (h 1 ) of the rectangular shaped SLM background image  414  on the background image plane  404  are based on the shape and the ratio of the length (l 2 ) to height (h 2 ) of the rectangular shaped PLM background images  407  on the PLM image plane  409 . By determining the shape and the ratio of the width (w) to thickness (t) of the rectangular slab waveguide  601 , the ratio of the length (l) to height (h) of the rectangular shaped zero-order light spot  413  on the background image plane  404  can be matched to the ratio of the length (l 1 ) to height (h 1 ) of the projected and focused rectangular shaped SLM background image  414  to provide uniform illumination of the rectangular shaped SLM background image  414  by the zero-order light  411 . 
       FIG.  7    shows the PLM background images  407  for an image projected by the optical elements  400 , in accordance with various examples. The PLM background images  407  are provided by projecting respective diffraction orders  410  in the background image light  408  on the PLM image plane  409 . The PLM background images  407  of the diffraction orders  410  are similar copies of formed by the diffraction surface of the PLM.  FIG.  6    shows an example of four PLM background images  407  projected by the diffraction orders  410  (pictured in  FIGS.  4  and  5   ) in the background image light  408 . In other examples, the number of PLM background images  407  may be a multiple of two, such as two, four, or eight background images projected simultaneously by the PLM. Each PLM background image  407  may have a different distribution of illumination across the image in the PLM image plane  409 . The illumination distribution is dependent on the illumination of the respective diffraction order  410  in the background image light  408 . The zero-order light spot  412  of the zero-order light  411  (pictured in  FIGS.  4  and  5   ) is also projected on the PLM image plane  409  and illuminates a central point positioned between the four PLM background images  407 . 
       FIG.  7    also shows the SLM background image  414  and the similarly shaped (by the optical tunnel  415 ) rectangular shaped zero-order light spot  413  on the background image plane  404 . The SLM background image  414  is the overlap imaging of the PLM background images  407  through the optical elements  400 . The overlap imaging of the PLM background images  407  on the background image plane  404  causes more uniform illumination of the background image light  408  across the SLM background image  414  in comparison to the PLM background images  407  on the PLM image plane  409 . The rectangular shaped zero-order light spot  413  also uniformly illuminates and covers the SLM background image  414  on the background image plane  404 . The uniform illumination of the SLM background image  414  is further increased by zero-order light spot  413  that is the projection of the zero-order light  411  through the optical tunnel  415  and the embedded lens  416  in the optical elements  400 . 
       FIG.  8    is a flow diagram of a method  800  for projecting a zero-order light with a background image light from a PLM onto an SLM to provide uniform zero-order light illumination for a projected image, in accordance with various examples. For example, the method  800  can be implemented by the optics  500  in the optical elements  400 , or by the optical elements in the apparatus  201  or  300 , of the display device  110 . At step  801 , incident light is modulated by a PLM to produce background image illumination, which contains background image light and zero-order light, to a first lens array. The PLM micromirrors are set to reflect light from a light source and produce the background image illumination for a SLM, including the background image light and zero-order light, to a first lens array. The background image light in the background image illumination is composed of diffraction orders formed by a diffraction surface of the PLM based on setting the PLM micromirrors. The diffraction orders in the background image light produce PLM background images to illuminate the SLM, which projects a projected image accordingly. For example, the PLM micromirrors  213  (or  310 ) of the PLM  204  (or  301 ) in the apparatus  201  (or  300 ) are set based on the controller  202  to reflect light from the light sources  207  (or  302 ) to produce background image illumination containing the background image light  217  (or  313 ) and the zero-order light  219  (or  313 ) for illuminating the SLM  205  (or  303 ), which then projects a projected image. At step  802 , the background image light is projected by the first lens array towards a second lens array. For example, the diffraction orders in the background image light  408  are projected by the lenses  405  of the first lens array  401  to the respective lenses  406  of the second lens array  402 . 
     At step  803 , the zero-order light is projected by an optical tunnel, which extends between the first lens array and the second lens array, towards an embedded lens in the second lens array. The optical tunnel extends through the center of the first lens array and partially through the center of the second lens array. For example, the zero-order light  411  is projected by the optical tunnel  415  through the first lens array  401  and partially through the second lens array  402  at the centers of the first lens array  401  and the second lens array  402 . The zero-order light  411  is projected from the portion  503  of the optical tunnel  415  to the embedded lens  416  at the center of the second lens array  402 . The steps  802  and  803  may be performed simultaneously to project the background image light by the first lens array with the zero-order light by the optical tunnel towards the second lens array. 
     At step  804 , the background image light is projected by the second lens array towards focusing optics. For example, the diffraction orders in the background image light  408  are projected by the respective lenses  406  of the second lens array  402  to the focusing lens  403 . At step  805 , the zero-order light is projected by the embedded lens towards the focusing optics. For example, the zero-order light  411  is projected by the embedded lens  416  at the center of the second lens array  402  onto the focusing lens  403 . The steps  804  and  805  may be performed simultaneously to project the background image light with the zero-order light between the second lens array and the focusing optics. 
     At step  806 , light including the background image light and the zero-order light is focused by the focusing optics towards a SLM. The background image light and the zero-order light can be focused by the focusing optics onto a background image plane on the SLM. The focusing of the zero-order light provides a uniform illumination of the zero-order light across the background image plane. The background image light and the zero-order light can be focused by the focusing optics onto an intermediate image plane at a focus point of the focusing optics. In turn, the intermediate image plane is imaged, also referred to herein as relayed, by illumination optics from the focus point to a background image plane on the surface of the SLM. For example, the focusing optics  223  (or  316 ) focus the background image light  217  (or  313 ) with the zero-order light  219  (or  313 ) onto an intermediate image plane at the focus point of the focusing optics  223  (or  316 ). The intermediate image plane is then relayed by the illumination optics  230  (or  330 ) onto the background image plane on the surface of the SLM  205  (or  303 ). In other examples, focusing and projection optics may project the background image light and the zero-order light from the second lens array onto a background image plane at the surface of the SLM without projecting an intermediate background image plane. At step  807 , the focused light is modulated by the SLM to project an image. The SLM is controlled to modulate and project the background image light and the uniform illumination of the zero-order light to form the projected image. 
     The term “couple” appears throughout the specification. The term may cover connections, communications, or signal paths that enable a functional relationship consistent with this description. For example, if device A provides a signal to control device B to perform an action, in a first example device A is coupled to device B, or in a second example device A is coupled to device B through intervening component C if intervening component C does not substantially alter the functional relationship between device A and device B such that device B is controlled by device A via the control signal provided by device A. 
     A device that is “configured to” perform a task or function may be configured (e.g., programmed and/or hardwired) at a time of manufacturing by a manufacturer to perform the function and/or may be configurable (or reconfigurable) by a user after manufacturing to perform the function and/or other additional or alternative functions. The configuring may be through firmware and/or software programming of the device, through a construction and/or layout of hardware components and interconnections of the device, or a combination thereof. 
     A system or device that is described herein as including certain components may instead be adapted to be coupled to those components to form the described system or device. 
     While certain components may be described herein as being of a particular process technology, these components may be exchanged for components of other process technologies. Systems and devices described herein are reconfigurable to include the replaced components to provide functionality at least partially similar to functionality available prior to the component replacement. 
     Unless otherwise stated, “about,” “approximately,” or “substantially” preceding a value means+/−10 percent of the stated value. Modifications are possible in the described examples, and other examples are possible within the scope of the claims.