Laser illuminated displays with enhanced uniformity and / or eye safety

Abstract

A display for displaying an image to a viewer includes an image generator having an illumination subsystem that generates illumination of at least a first color, the image generator utilizing the illumination to generate an image. A projection optical system projects the illumination from the image for display to the viewer. The illumination subsystem includes a first laser that generates a first laser beam of the first color having a first polarization and a second laser that generates a second laser beam of the first color having a second polarization. The first polarization and the second polarization are orthogonal at at least one location within the projection optical system, thereby projecting a quasi-unpolarized image.

Description

[Technical field]

[0001] The present invention relates to displays for presenting images to a viewer, and in particular to displays illuminated by laser light sources.

[0002] Certain displays, such as near-eye and head-up displays, need to be highly bright so that they are adequately viewable when used in bright conditions, such as outdoor use during the day. This is especially true for augmented reality displays, where visual elements are overlaid on top of a bright outdoor scene.

[0003] Laser-based illumination systems can achieve this high average brightness when used to illuminate a spatial light modulator (SLM), or when used alone by scanning a very bright spot across the projected field. The scanning is performed by two vertical actuators moving a single mirror or two separate mirrors. For color displays, the bright spot is generated by three separate lasers (red, green, and blue).

[0004] The use of lasers offers the advantages of brightness and energy efficiency, but can also pose image non-uniformity and, in some cases, eye safety challenges. Summary of the Invention

[0005] The present invention is a display.

[0006] In accordance with the teachings of one embodiment of the present invention, there is provided a display for displaying an image to a viewer, comprising: (a) an image generator having an illumination subsystem that generates illumination of at least a first color, the image generator utilizing the illumination to generate an image; and (b) a projection optical system for projecting illumination from the image for display to the viewer, wherein the illumination subsystem comprises a first laser that generates a first laser beam of the first color having a first polarization, and a second laser that generates a second laser beam of the first color having a second polarization, wherein the first polarization and the second polarization are orthogonal at at least one location within the projection optical system.

[0007] According to a further feature of an embodiment of the present invention, the first laser and the second laser are two of a set of at least three lasers in the illumination subsystem, the at least three lasers having mutually orthogonal polarizations at at least one position within the projection optical system.

[0008] According to a further feature of an embodiment of the invention, the first laser beam and the second laser beam are arranged to generate side-by-side beams having mutually orthogonal polarizations.

[0009] According to a further feature of an embodiment of the invention, the projection optical system comprises a collimating optical system for generating a collimated image, and a light-guiding optical element having a pair of parallel outer major surfaces for guiding the collimated image within the light-guiding optical element by internal reflection, the light-guiding optical element having a coupling-out arrangement for redirecting at least a portion of the collimated image to a viewer.

[0010] According to a further feature of an embodiment of the present invention, the first laser beam and the second laser beam are combined into a single beam by a polarizing beam combiner of the illumination subsystem to form a single beam containing two orthogonal polarizations.

[0011] According to a further feature of an embodiment of the invention, the projection optical system comprises a collimating optical system for generating a collimated image, and a light-guiding optical element having a pair of parallel outer major surfaces for guiding the collimated image within the light-guiding optical element by internal reflection, the light-guiding optical element having a coupling-out arrangement for redirecting at least a portion of the collimated image to a viewer.

[0012] According to a further feature of an embodiment of the invention, the first and second laser beams have a first and second wavelength, respectively, a difference between the first and second wavelengths being less than 5%, and the projection optics includes a length of birefringent material configured to differentially rotate the first and second polarizations from a parallel state to a substantially orthogonal state.

[0013] According to a further feature of an embodiment of the invention, the image generator comprises a polarization-modulating spatial light modulator illuminated by the first laser beam and the second laser beam.

[0014] According to a further feature of an embodiment of the invention, the image generator further comprises a scanning optical arrangement comprising at least one scanning mirror, the scanning arrangement being arranged to generate a scanning movement of the first laser beam and the second laser beam in at least one direction so as to sequentially illuminate different areas of the spatial light modulator.

[0015] According to a further feature of an embodiment of the present invention, the projection optical system comprises a collimating optical system for generating a collimated image and a light-guiding optical element for transmitting the collimated image to a viewer, and the birefringent material is disposed in an optical path from the collimating optical system to the light-guiding optical element.

[0016] According to a further feature of an embodiment of the invention, the collimating optics comprises at least one polarizing beam splitter.

[0017] According to a further feature of an embodiment of the invention, the birefringent material is a block of quartz.

[0018] According to a further feature of an embodiment of the invention, the image generator further comprises a scanning optical arrangement comprising at least one scanning mirror, the scanning arrangement being arranged to generate a scanning movement of the first laser beam and the second laser beam in at least one direction.

[0019] According to a further feature of an embodiment of the invention, the illumination subsystem further comprises a third laser and a fourth laser generating a second color laser beam having an orthogonal polarization at the image, and a fifth laser and a sixth laser generating a third color laser beam having an orthogonal polarization at the image.

[0020] Also provided in accordance with the teachings of one embodiment of the present invention is a display for displaying an image to a viewer, the display comprising: (a) a set of a red light source generating a red light beam, a green light source generating a green light beam, and a blue light source generating a blue light beam, the red light beam, the green light beam, and the blue light beam being aligned; (b) a scanning optical arrangement comprising at least one scanning mirror, the scanning arrangement being arranged to generate a scanning motion of the red light beam, the green light beam, and the blue light beam in a two-dimensional scan pattern; (c) a display controller comprising at least one processor, the display controller being associated with the set of laser light sources and the scanning arrangement, the display controller being configured to modulate an intensity of each of the laser light sources in synchronization with the scanning motion to generate an image in an image plane; and (d) a collimating optical arrangement for collimating light from the image at the image plane for display to a viewer, the display controller being further configured to (i) determine a current region of interest of the image; and (ii) modify pixel color data for modulation of the laser light sources to reduce saturation of at least a portion of the image outside the current region of interest.

[0021] According to a further feature of an embodiment of the invention there is also provided an eye tracking arrangement arranged to track a current gaze of an eye of a user, the display controller determining a current region of interest based on input received from the eye tracking arrangement.

[0022] According to a further feature of an embodiment of the invention, the image is a video image and the display controller processes content of the video image to derive the current region of interest.

[0023] According to a further feature of an embodiment of the invention, the image is a video image and the display controller receives a data stream indicative of a current region of interest associated with the video image.

[0024] According to a further feature of an embodiment of the present invention, the configuration of the red, green, and blue light sources and the two-dimensional scanning pattern are such that the paths taken by the red, green, and blue light beams do not overlap for the most part.

[0025] Also provided in accordance with the teachings of one embodiment of the present invention is a display for displaying an image to a viewer, the display comprising: (a) a set of laser light sources comprising a red light source generating a red light beam, a green light source generating a green light beam, and a blue light source generating a blue light beam; (b) a scanning optical configuration comprising at least one scanning mirror, the scanning configuration being arranged to generate a scanning motion of the red light beam, the green light beam, and the blue light beam in a two-dimensional scan pattern; (c) a display controller comprising at least one processor, the display controller being associated with the set of laser light sources and the scanning configuration and configured to modulate the intensity of each of the laser light sources in synchronization with the scanning motion to generate an image in the image plane; and (d) a collimating optical configuration for collimating light from the image at the image plane for display to a viewer, the blue light source and / or the scanning optical configuration being configured such that a point spread function of the blue light beam at the image plane is at least twice as wide as a point spread function of the green light beam.

[0026] According to a further feature of an embodiment of the invention, the point spread function of the blue light beam at the image plane is at least three times wider than the point spread function of the green light beam.

[0027] According to a further feature of an embodiment of the invention, the display controller modulates an intensity of the green light source in accordance with pixel data corresponding to an image of a first resolution, and the display controller modulates an intensity of the blue light source in accordance with pixel data corresponding to an image of a second resolution, the second resolution being lower than the first resolution. [Brief description of the drawings]

[0028] The invention is herein described, by way of example only, with reference to the accompanying drawings. [Figure 1A] 1A-1C are schematic side views of displays for displaying images to a viewer, the displays utilizing reflective and diffractive coupling-out configurations, respectively. [Figure 1B]1A-1C are schematic side views of displays for displaying images to a viewer, the displays utilizing reflective and diffractive coupling-out configurations, respectively. [Figure 2A] FIG. 2 shows a schematic diagram of a laser-illuminated image projector for use in the display of FIGS. 1A and 1B. [Figure 2B] 3 shows the laser scan patterns for normal scanning operation, obstruction of the vertical scan mirror, and obstruction of the horizontal scan mirror, respectively. [Figure 2C] 3 shows the laser scan patterns for normal scanning operation, obstruction of the vertical scan mirror, and obstruction of the horizontal scan mirror, respectively. [Figure 2D] 3 shows the laser scan patterns for normal scanning operation, obstruction of the vertical scan mirror, and obstruction of the horizontal scan mirror, respectively. [Figure 3A] 2B is a schematic diagram of forward scan, reverse scan, and dual scan scan patterns for implementation using the image projector of FIG. 2A. [Figure 3B] 2B is a schematic diagram of forward scan, reverse scan, and dual scan scan patterns for implementation using the image projector of FIG. 2A. [Figure 3C] 2B is a schematic diagram of forward scan, reverse scan, and dual scan scan patterns for implementation using the image projector of FIG. 2A. [Figure 3D] 2B is a schematic diagram of a forward scan and a double scan scan pattern for implementation using the image projector of FIG. 2A utilizing two lasers. [Figure 3E] 2B is a schematic diagram of a forward scan and a double scan scan pattern for implementation using the image projector of FIG. 2A utilizing two lasers. [Figure 4A] FIG. 2B is a schematic diagram of an alternative embodiment of the laser illumination portion of the image projector of FIG. 2A, utilizing three aligned lasers. [Figure 4B] FIG. 2 is a schematic diagram of a first option for the relative arrangement of the laser beam with respect to the scan axis. [Figure 4C] 4C show the laser scan patterns when implemented with the laser arrangement of FIG. 4B for normal scanning operation, obstruction of the vertical scanning mirror, and obstruction of the horizontal scanning mirror, respectively. [Figure 4D] 4C show the laser scan patterns when implemented with the laser arrangement of FIG. 4B for normal scanning operation, obstruction of the vertical scanning mirror, and obstruction of the horizontal scanning mirror, respectively. [Figure 4E] 4C show the laser scan patterns when implemented with the laser arrangement of FIG. 4B for normal scanning operation, obstruction of the vertical scanning mirror, and obstruction of the horizontal scanning mirror, respectively. [Figure 5A] FIG. 13 is a schematic diagram of a second option for the relative arrangement of the laser beams, offset with respect to the scanning axis. [Figure 5B] 5A-5C show the laser scan patterns when implemented with the laser arrangement of FIG. 5A for normal scanning operation, obstruction of the vertical scanning mirror, and obstruction of the horizontal scanning mirror, respectively. [Figure 5C] 5A-5C show the laser scan patterns when implemented with the laser arrangement of FIG. 5A for normal scanning operation, obstruction of the vertical scanning mirror, and obstruction of the horizontal scanning mirror, respectively. [Figure 5D] 5A-5C show the laser scan patterns when implemented with the laser arrangement of FIG. 5A for normal scanning operation, obstruction of the vertical scanning mirror, and obstruction of the horizontal scanning mirror, respectively. [Figure 6A] 1 is a schematic diagram of the relative arrangement of six laser beams, offset with respect to the scanning axis. [Figure 6B] FIG. 2B is a schematic diagram of an alternative embodiment of the laser illumination portion of the image projector of FIG. 2A, utilizing six aligned lasers. [Figure 6C] 6B shows the laser scan patterns when implemented with the laser arrangement of FIG. 6A for the cases of obstruction of the horizontal scanning mirror and obstruction of the vertical scanning mirror, respectively. [Figure 6D] 6B shows the laser scan patterns when implemented with the laser arrangement of FIG. 6A for the cases of obstruction of the horizontal scanning mirror and obstruction of the vertical scanning mirror, respectively. [Figure 7A] FIG. 5B is a schematic diagram of the trace that each color follows during scanning of a configuration similar to FIG. 5A. [Figure 7B] 7B is a plot of image intensity as a function of position along line 207 of FIG. 7A, before and after foveal desaturation, in accordance with an embodiment of the present invention. [Figure 7C] 7B is a plot of image intensity as a function of position along line 207 of FIG. 7A, before and after foveal desaturation, in accordance with an embodiment of the present invention. [Figure 7D] FIG. 6B is a schematic diagram of the trace that each color follows during scanning of a configuration similar to FIG. 6A. [Figure 8A] 2B is a schematic diagram of forward scan and dual scan scan patterns for implementation using the image projector of FIG. 2A utilizing two lasers with scan traces spaced apart by a pixel spacing. FIG. [Figure 8B] 2B is a schematic diagram of forward scan and dual scan scan patterns for implementation using the image projector of FIG. 2A utilizing two lasers with scan traces spaced apart by a pixel spacing. FIG. [Figure 9] FIG. 6B is a schematic diagram of the traces that each color follows during scanning in a configuration similar to that of FIG. 6A, but with increased spacing between laser scan traces of like colors. [Figure 10] FIG. 2 is a block diagram of a display controller for implementing aspects of the present invention. [Figure 11A] FIG. 1 is a schematic diagram of a waveguide-based optical aperture expander for delivering an image from an image projector to a viewer's eye. [Figure 11B] 1 shows two intensity patterns produced by polarized light propagating through an optical aperture expander according to a first polarization, a second polarization, and the sum of the two patterns. [Figure 12] FIG. 1 is a schematic diagram of a display utilizing multiple aligned lasers for each color to facilitate depolarization of the projected image. [Figure 13A]FIG. 1 is a schematic diagram of an alternative embodiment of a display utilizing free space optics and with multiple orthogonally polarized laser beams for each color introduced along a common optical path. [Figure 13B] 13B illustrates an alternative configuration of a laser for use in the display of FIG. 13A. [Figure 14] 2 illustrates diagrammatically a further configuration of a display according to an aspect of the present invention; [Figure 15A] FIG. 2 is a schematic diagram of a depolarizer integrated with a waveguide for use in a display according to one embodiment of the present invention. [Figure 15B] FIG. 15B is a schematic diagram of the orientation of the principal axes of the birefringent material utilized in the depolarizer of FIG. 15A. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] The present invention is a display that utilizes laser illumination.

[0030] The principles and operation of a display according to the present invention may be better understood with reference to the drawings and accompanying descriptions.

[0031] By way of preamble, the present invention comprises several separate aspects, each of which addresses problems associated with implementing displays using laser illumination and can be used individually or in synergistic combinations to provide significant advantages.

[0032] More specifically, with reference to the drawings, FIGS. 1A and 1B provide schematic diagrams of an exemplary display in which various aspects of the invention may be implemented, FIGS. 2A-10 relate to aspects of the invention specific to displays based on the scanning motion of a rapidly modulated laser beam, and FIGS. 11A-15B relate to aspects of the invention that address problems arising from the polarization properties of laser illumination.

[0033] Device Overview Referring now to the drawings, FIGS. 1A and 1B show a schematic diagram of the main optical components of the system.

[0034] In general, the invention is illustrated herein with reference to a display 500 for displaying an image to a user's eye 502, the eye being positioned within an eye motion box 504. In a specific non-limiting embodiment, the display includes a light directing optical element (LOE) 506, interchangeably referred to herein as a "substrate" or "waveguide," having a pair of major outer surfaces 508, 510 parallel to one another. An image projector 512, which projects image illumination of a collimated image, is optically coupled to the LOE 506 to introduce the image illumination into the LOE for propagation within the LOE by internal reflection at the major outer surfaces 508, 510. The image projector 512 is interchangeably referred to as a "POD."

[0035] A coupling-out arrangement associated with at least the coupling-out region of the LOE 506 is configured to redirect at least a portion of the image illumination propagating within the LOE to the eye motion box 504 for viewing by the user's eye 502. Exemplary implementations of coupling-out arrangements include, for example, reflective coupling-out arrangements implemented as a plurality of mutually parallel partially reflective surfaces 514 angled relative to the major outer surfaces 508, 510 within the LOE 506, as shown in FIG. 1A. An alternative coupling-out arrangement utilizes one or more diffractive optical elements 516 associated with the LOE 506 and configured to couple out a portion of the image illumination, as shown in FIG. 1B. Implementation details of both reflective and diffractive coupling-out arrangements for light-guiding displays are well known in the art and will not be described in detail here for the sake of brevity.

[0036] Scanning System Considerations FIG. 2A shows a schematic of the optical configuration of an image projector 512 suitable for use in the display 500. The image projector 512 uses a set of lasers 19A. Lasers R, G, and B (representing red, green, and blue, respectively) can be combined by a dichroic combiner 20 (mini lenses not shown) into a single beam with a single polarization. The combined diverging beam is reflected by mirror 22 and refracted by lens 23 onto scanning mirrors 24 and 25, each of which scans about an orthogonal axis. Lens 26 generates the pupil imaging required to couple the scanned beam into the waveguide pupil 34. A microlens array (MLA) or diffuser is preferably placed at the image plane 29 to expand the beam. The embodiment shown here is based on a polarizing beam splitter prism, where a polarizing beam splitter (PBS) 31 diverts the beam to a lens 32, which collimates the beam and exits through a pupil 34, where it enters the LOE (not shown).

[0037] 2B shows a typical two-dimensional scan pattern covering the scanned image field: a combined spot 42 (of all lasers) is scanned along a trajectory 44 while its intensity is modulated according to image data to produce an image field 46.

[0038] Without limiting in any way the utility of the invention, which is established independently of the accuracy of the theoretical analysis below, and for purposes of enhancing understanding of certain aspects of the invention, it is useful to evaluate certain eye safety issues as follows: In the following, round numbers are assumed to provide only a rough estimate of the intensity impinging on the eye.

[0039] Assuming an image of 800x600 pixels, fully refreshed at 60 Hz, the lateral (resonant) mirror needs to oscillate at 60x600 / 2=18KHz (neglecting scanning overhead and assuming illumination in both directions of motion). The time per line is therefore (1 / 18,000) / 2=27 microseconds.

[0040] The time per pixel is 27e-6 / 800=35 nanoseconds.

[0041] The time per frame is 1 / 60 = 16 milliseconds.

[0042] The following calculations assume that the lighting is optimized for a 5000 Nit outdoor daytime white field across the entire image.

[0043] Eye Safety Considerations The following analysis is provided as a rough estimate of parameters and should not be relied upon as the basis for eye safety calculations.

[0044] In a malfunction scenario, one can assume that one or both scanners will fail while the laser continues to operate at a power equivalent to the white screen (nominal maximum power). When both scanners stop vibrating and the spot 42 comes to rest, the intensity is concentrated at one pixel. 5000NITx600x800=2.4xe9NIT

[0045] This is a very bright spot that will cause a person to blink and eventually remove the near-eye display. For comparison, a direct view of the sun at noon is 1.6xe9 NIT. Therefore, this scanner stall situation is uncomfortable for the user and may be dangerous to the eyes.

[0046] 2C shows the situation where scanner 25 is not functioning and only scanner 24 (resonant) is functioning. Since the resonant mirrors move fast, we assume that all the power of the screen is distributed along all lines 50, and therefore the intensity of this line is: Intensity per pixel / pixels per line = 2.4xe9NIT / 800 = 3e6NIT

[0047] This is less damaging than a single spot, but still very expensive and uncomfortable for the user.

[0048] Figure 2D shows the line scan generated when scanner 24 is deactivated and only scanner 25 is active. In this case, because the movement of this scanner is slow (milliseconds per line, but the thermal constant for eye damage is in microseconds), the intensity per pixel can be approximated to be the same as a standing spot (2.4xe9 NIT).

[0049] Scan Pattern Considerations 3A shows a plot 108 in which a scanning laser spot 42 follows a pattern 44. This pattern is produced by a fast lateral scanning resonant mirror 24 and a linear (controllable) mirror 25.

[0050] Inset 45A shows a schematic of pixel placement at the center of the field, where the vertical velocity is set so that pixels generated during movement in both directions (marked as arrows) are adjacent, i.e., a single pixel pitch apart.

[0051] At the edge of the field, some pixels overlap, while in other places there are gaps, as shown in inset 45B.

[0052] In scan 108, the gap between lines 62 in the center of the field is 1 pixel (best adjacent pixel), while the gap at the edge 64 of the pattern approaches 2 pixels (2 pixels overlapping, 1 pixel gap to adjacent pixel). Reverse scan 110 in Figure 3B shows a similar pattern. The scan lines are preferably located at different locations, as shown in the composite trace 112 in Figure 3C. Traces on either side of the field and gaps 70 and 72 appear in sequence, and therefore flicker may occur at the edge of the field.

[0053] It should be noted that human vision is typically particularly sensitive to flicker in peripheral regions of vision and less sensitive to flicker in the center of vision. As a result, the presence of rapidly varying non-uniformities (flicker) on the sides of a scanned image can be very distracting to many viewers.

[0054] Figures 3D and 3E show further options for two-channel interline scanning where all spots represent all overlapping colors. Trace 113A in Figure 3D shows a single frame, while 113B in Figure 3E shows overlaid reverse traces. It is clear that when there is no gap between the channels (marked as hollow and filled circles, shown as 1 pixel distance), there is a 2 pixel gap 73 at the edge of the field, and no gap in the center 75 (adjacent with 1 pixel spacing). Thus, the gap causes flicker at the edges, just like in the single channel case.

[0055] Color Considerations There are several conflicting considerations regarding the power of the individual lasers. On the one hand, the relative power of the three color sources must be adjusted to provide the desired level of white balance when the colors are mixed. On the other hand, the absolute power must be below the eye safety threshold. The latter requirement is more stringent for blue light than for red and green light. Thus, for safety reasons, blue illumination is most severely limited, and the laser power of the other colors is necessarily kept well below the safety limit in order to maintain white balance. The result is a reduction in the overall brightness of the image due to the safety limit imposed on the blue laser power.

[0056] Polarization Considerations The present invention is primarily concerned with displays that utilize illumination generated by lasers. The term "laser" is used broadly herein for any type of light-generating laser. For compact devices, diode lasers or S-LEDs are particularly preferred. These produce polarized light with a spectral bandwidth typically less than 1 nanometer. When used with a waveguide to relay the image to the eye, polarized image illumination output by a POD produces a non-uniform output image. This is because the polarization of the light changes during total internal reflection (TIR), whereas the output coupling mechanism (diffraction or reflection) is polarization sensitive. The high coherence and fast scanning speed of laser beams make it impractical to depolarize all lasers actively (e.g., by a tunable LCD) or passively (narrow windows of birefringence such as quartz).

[0057] We now consider several approaches, consistent with various aspects of the present invention, for addressing some of the above limitations arising from the use of lasers.

[0058] Strength reduction A group of implementations according to an embodiment of the present invention is shown in FIG. 4 as optical configuration 114. This optical configuration replaces the upper portion of device 100 (FIG. 2A), which differs from that configuration in that lasers R, G, and B are not combined into a single beam using dichroic combiner 20, but are now combined geometrically side-by-side as 19B. As a result, the beams of the three lasers do not overlap. The remainder of the device may be similar to, for example, FIG. 2A and other details of FIG. 1A or FIG. 1B.

[0059] 4B shows one possible arrangement of the orientation of the three laser spots (three circles) 80, 81, and 82 appearing in the image plane. The two orthogonal axes represent the scanning axes of mirrors 24 and 25.

[0060] The images produced by scanning the laser are shown in Figure 4C, where three images are produced with some shift, laser 76 associated with spot 80, 77 associated with 81, and 78 associated with 82. The scanning pattern of the spots is shown as 90 (dashed, solid, and dotted lines). Obviously, the horizontal offset can be digitally corrected and only the area of ​​overlap is used.

[0061] The advantage of 3 spots is that the intensity is distributed and therefore every spot has 1 / 3 the intensity (this is an approximation), so if both scanners fail the intensity per spot will be equal to (color is not taken into account in this calculation for simplicity): 2.4e9NIT / 3=800e6NIT This is lower compared to a single composite spot, which causes a visible reduction in brightness when the spots are spaced at least 0.5 degrees apart.

[0062] The scanning pattern of the three spots of FIG. 4B is shown in FIG. 4C. It is clear that the patterns of all three lasers are nearly overlapping. As a result, the same eye area is illuminated by all three lasers at very short intervals. This aspect of the illumination is highlighted in FIG. 4D, which represents a line 96 produced when the scanner 25 malfunctions and does not move. In this case, the spots 80, 81, and 82 have nearly overlapping paths, which increases the brightness impact. Therefore, the brightness seen by the user can be estimated to be 3e6 NIT, which is equivalent to FIG. 2C. Nevertheless, for spot separations of more than 0.5 degrees, the impact on the eye is lower than a single spot.

[0063] Figure 4E shows the scan pattern when the scanner 24 malfunctions and stops functioning, in which case the line shown in Figure 2D is now split into three lines of 1 / 3 the intensity compared to the composite alternative of Figure 2D.

[0064] A more optimal configuration for reducing the possibility of eye damage is shown in Figures 5A-5D. Figure 5A shows a laser configuration offset at an angle to both scan axes. The scan fields are shown in Figure 5B. The scan fields of each laser 76, 77, and 78 are vertically and horizontally shifted relative to the other scan patterns. Although the spacing is shown here diagrammatically as being similar to or smaller than the pixel pitch, depending on the structural implementation, the angular spacing between the beams is preferably at least 0.5 degrees and may correspond to multiple pixel dimensions, in some cases exceeding 20, or even 50 pixel dimensions. The scan patterns are most preferably completely non-overlapping (ignoring intersections), thus significantly reducing the local intensity of all pixels during normal operation. This is a major advantage for display devices intended to be used continuously.

[0065] 5C and 5D show the line scan patterns produced when scanners 25 or 24 malfunction, respectively. It is clear that the tilted laser orientation avoids scan overlap in both cases, resulting in a factor of 3 reduction in local brightness in both cases.

[0066] FIG. 6A shows a further non-limiting example of an architecture utilizing a combination of six lasers. A partial view of a device implementing such an architecture is shown in FIG. 6B as configuration 132, where the laser assembly 19C is similar to the combination of lasers 19B in FIG. 4A, but with two red lasers (R1, R2), two green lasers (G1, G2), and two blue lasers (B1, B2). The arrangement of the lasers is shown only diagrammatically. FIG. 6A shows diagrammatically the preferred alignment of the laser spots in the image plane in a diagonal configuration. FIG. 6C and FIG. 6D show patterns 136 and 138, respectively, representing line scans that would result from a malfunction of the respective scanning mechanisms, showing non-overlapping line patterns that would be generated by the lasers. As a result, the brightness is reduced by about a factor of six compared to the composite single spot architecture.

[0067] Other laser orientations are possible. In all cases, particularly preferred embodiments avoid positioning two or more lasers along a line parallel to either of the main scan axes.

[0068] Blue laser blurring for eye safety and reduced computational load Note that the human eye's perception of color is more sensitive to resolution at green wavelengths than at blue and red wavelengths of the visual spectrum, and therefore image sharpness, as measured by the optical point spread function (PSF) of the image plane, is less important for blue and red illumination than for green illumination.

[0069] On the other hand, from a laser safety point of view, the maximum permissible exposure (MPE) for direct eye exposure to blue light is about 0.1 mW / cm2 for long-term (e.g., several hours) and about 1.0 mW / cm2 for green light. (These figures are based on Maini, AK (Ed.). (2013). Lasers and Optoelectronics: Fundamentals, Devices and Applications, 599-603. https: / / doi.org / 10.1002 / 9781118688977, Appendix A, Tables A.4 and A.6.)

[0070] To take advantage of these observations, according to one aspect of the invention, a display for displaying an image to a viewer utilizes a set of laser light sources including at least one each of red light sources R1, R2 generating a red light beam, green light sources G1, G2 generating a green light beam, and blue light sources B1, B2 generating a blue light beam. A scanning optical arrangement utilizing at least one scanning mirror 24, 25 generates a scanning motion of the red light beam, green light beam, and blue light beam in a two-dimensional scanning pattern. A display controller including at least one processor is associated with the set of laser light sources and the scanning arrangement and configured to modulate the intensity of each laser light source in synchronization with the scanning motion to generate an image at an image plane. A collimating optical arrangement collimates light from the image at the image plane for display to a viewer.

[0071] A particular feature of certain embodiments of one aspect of the present invention is that the blue light source and / or the scanning optical configuration is configured such that the point spread function of the blue light beam at the image plane is at least twice as wide, and preferably at least three times as wide, as the point spread function of the green light beam.

[0072] The width of the point spread function (PSF) can be intuitively defined by the apparent width of the laser spot in the image plane, but can be more accurately defined by using the full width at half maximum (FWHM), which is related to the spot intensity.

[0073] As a specific example, the power density of the blue light illumination on the image plane can be increased by approximately 10 PSF (diameter) while maintaining the green PSF and preferably the red illumination PSF at their optimal values. ^ A magnification of 0.5 = 3.2 reduces the power by a factor of 10. As a result, the power of the blue laser, and the total power of all three lasers, can be increased by a factor of 10 without exceeding the eye damage threshold, as determined by the MPE.

[0074] It is also possible to reduce the blue resolution provided by the controller 228 (FIG. 10) by widening the blue PSF. As an example, if the green image has an array of 1000x1000 pixels, with a pixel size of 5x5 microns, the blue image after widening the PSF may have an array of 333x333 pixels, and a pixel size of about 15 microns. Reducing the resolution of the blue image not only reduces the size and complexity of the electronics in the controller 228, but also reduces the bandwidth of the required fast pulse modulation in the blue laser driver. If the resolution of the red image is also reduced, a similar reduction in size, complexity, and bandwidth applies to the controller 228 and the red laser driver. Thus, the display controller 228 modulates the intensity of the green light source according to pixel data corresponding to an image of a first resolution, and modulates the intensity of the blue light source, and optionally also the red light source, according to pixel data corresponding to an image of a second resolution, the second resolution being lower than the first resolution.

[0075] The broadening of the blue PSF can be accomplished in a variety of ways, either in the relay section (after MLA 29) or in the laser section (before MLA 29) of the optical layout of Figure 1. For example, chromatic aberration can be introduced in the relay section so that the lens focal lengths corresponding to the red, green, and blue illumination are different. By placing the focal plane of 32 (or 276) at a distance relative to MLA 29 that is suboptimal (defocused) for blue and near optimal for green and red, the blue PSF will be broadened and the blue image will have lower resolution at the image display.

[0076] However, using chromatic aberration for PSF spreading in relay may not be the best approach for two reasons. First, the lack of collimation of the blue optics causes spatial non-uniformity of the blue image intensity and possibly other image artifacts. Furthermore, in some cases, the observer's eye may compensate or "adapt" to the defocus of the blue image and refocus the eye's lens to sharpen the blue laser image. In such cases, the power density at the retina of the eye after adaptation may exceed the MPE.

[0077] Another more preferred approach to PSF broadening, which overcomes the above difficulties, is to defocus the blue laser beam in the laser section before the light reaches the image plane at the MLA 29. This not only broadens the PSF as required, but also partially removes the optical phase information after passing through a diffuser or MLA in the image plane. As a result, the image on the image display cannot be refocused by the observer's eye, and the reduced power density on the retina of the eye is maintained. Furthermore, the blue illumination reaching the entrance pupil 34 is well collimated by the lens 32 (or 276), and no non-uniformity appears in the blue image presented to the observer.

[0078] Note that PSF broadening for blue illumination can also be achieved by shifting the position of the blue laser in Figure 1 along the optical path to a position that is slightly out of focus at the image plane. All of the methods for broadening the PSF can also be applied to red illumination.

[0079] The aforementioned PSF broadening of blue laser illumination can be implemented both when deploying the lasers side-by-side, or when combining the laser beams along a common axis. The side-by-side configuration, as already discussed, offers the added advantage of distributing the light intensity over a wider area, and is therefore usually preferred.

[0080] Eliminate flicker by reducing color fidelity In Fig. 7A, the different color spots of Fig. 5A or Fig. 6A are presented with different shapes for clarity. The color laser spots 202 include a circle with a solid line scan as red, a rectangle with a dashed line scan as green, and a square with a dashed line scan as blue.

[0081] The laser is placed in the focal plane with a specific orientation that takes into account the scanning speed so that during scanning the traces (scan lines) do not overlap as shown. The gap between two scans of red 204 contains the blue 206 and green 208 traces.

[0082] Human vision is insensitive to color in the peripheral field, so as long as all colors are illuminated (some deviation from white), the observer perceives no difference between all illuminated traces. Thus, the gap 204 is perceived as uniformly illuminated and there is no gap at the edge of the field of view. Therefore, minimal flicker is observed for images with white or balanced gray levels (including color combinations) at the periphery. However, if the peripheral area of ​​the projected image is dark, this means that one or more other colors will have a lower intensity, leaving an area that is not illuminated, and flicker will occur.

[0083] To take advantage of these observations, according to one aspect of the present invention, a display for displaying an image to a viewer utilizes a set of laser light sources including at least one red light source R1, R2 generating a red light beam, at least one green light source G1, G2 generating a green light beam, and at least one blue light source B1, B2 generating a blue light beam, where the red light beam, the green light beam, and the blue light beam are aligned side by side. A scanning optical arrangement utilizes at least one scanning mirror 24, 25 to generate a scanning motion of the red light beam, the green light beam, and the blue light beam in a two-dimensional scanning pattern. A display controller including at least one processor is associated with the set of laser light sources and the scanning arrangement and modulates the intensity of each laser light source in synchronization with the scanning motion to generate an image at an image plane. A collimating optical arrangement collimates light from the image at an image plane for display to a viewer.

[0084] A particular feature of a particular embodiment of one aspect of the present invention is that the display controller is further configured to (i) determine a current region of interest of the image, and (ii) modify pixel color data for modulation of the laser light source to desaturate at least a portion of the image outside the current region of interest. In other words, after determining which regions are not the current region of interest, the saturation of those regions can be reduced while maintaining their luminance, thereby at least partially equalizing the intensities of the three colors, thereby filling gaps between different color scans that may otherwise cause distracting flicker.

[0085] This approach is illustrated in Figures 7B and 7C. Figure 7B shows the image illuminance along the vertical line 207 of Figure 7A in an area of ​​the image that is only red (two nearby intersections are shown as a single peak 208). The gaps between the scan lines are evident. Figure 7C shows the image illuminance along the same line of the same image, but with suppressed color fidelity (reduced saturation but maintained luminance). In this case, red (solid line) remains the dominant color, but green (dashed line) and blue (dotted line) are also illuminated, resulting in an overall luminance envelope 210 with reduced non-uniformity. Because human peripheral vision is less sensitive to color, this is perceived as uniform illumination, thereby reducing the perceived flicker.

[0086] As mentioned above, the above approach is based on the observation that in the center of the field of vision, the human eye is sensitive to color but not to flicker, while in the peripheral field of vision the opposite is true, i.e. the eye is insensitive to color but more sensitive to flicker. Using these eye properties, the visibility of flicker can be reduced without compromising the perceived color fidelity.

[0087] Various approaches can be used to define the "region of interest" in an image. In certain applications, especially when different parts of the display are used to present different types of information, it may be possible to predefine certain areas of the display as being more important to color fidelity and other areas as being less important. More preferably, the "region of interest" is dynamically defined by one of several approaches. Options include, but are not limited to: 1. Use of an eye tracking arrangement deployed to track the current gaze of a user's eyes, whereupon a display controller determines a current region of interest based on input received from the eye tracking arrangement. 2. If the image to be displayed is a video image, the display controller processes the content of the video image to derive the current region of interest, for example in areas with high motion. 3. Optionally, if the image is a video image, the display controller may receive, along with the image data, an associated data stream indicating a current region of interest associated with the video image.

[0088] In each of these cases, it is assumed that the observer is looking at the region of interest, so the color fidelity within that region is not altered. If flicker is present in this region, it will be unnoticeable because the central field of vision is insensitive to flicker. The color fidelity of the remaining field of vision may be reduced by image processing (according to the distance from the region of interest and any non-uniformities that may occur within that region of the display). In this way, flicker is suppressed while making the color changes unnoticeable. As explained, as the region of interest moves across the field of vision, the region in which color fidelity is maintained also moves.

[0089] FIG. 7D is similar to FIG. 7A, but shows a configuration of six laser spots 212, two of each RGB color, similar to that shown in FIG. 6A above. Other configurations are possible. By configuring the traces to be non-overlapping (by selecting staggered laser orientations, spacing, and scanning speeds), the gaps 214 (between adjacent traces of the red laser in this example) are filled with traces of other colors. By setting the appropriate tilt and vertical scanning speed, a uniform distribution of traces is produced. In this way, the color fidelity management described above with reference to FIG. 7B and FIG. 7C can be implemented very efficiently to achieve flicker elimination.

[0090] In the examples shown so far, the scan patterns utilizing two channels are shown as being adjacent (FIG. 3D). FIG. 8A shows an alternative configuration in which the laser channels have a gap between them. Placing the laser channels adjacent has the advantage that there is no gap in the center of the field (area 75 in FIG. 3D), and therefore no flicker in the center of the field, but the disadvantage that the gap on the sides of the field is doubled (area 73 in FIG. 3D). In the configuration of FIG. 8A, the two spaced apart channels have a gap between them and are spaced apart as two pixels 216 with a gap of one empty pixel between them. FIG. 8B shows a combined trace that also includes a reverse scan. For this channel configuration, the gap on the sides of the field 218 is only one pixel flicker away from gap 73 in FIG. 3D), except that in this case the center still has a one pixel gap (marked 220 in FIG. 8A), which introduces more flicker than occurs in area 75 in FIG. 3D.

[0091] Figure 9 shows this spaced approach applied to a set of two three-color channels 156. The non-overlapping configuration of traces preferably fills the gaps at the sides of field 222 (similar to region 214 in Figure 7D, but with a narrower gap) and also fills the gap in the center of field 224.

[0092] In this configuration 156, the previously mentioned foveal reduced color fidelity may be advantageously applied across the entire field (not just the edges), so that if the current region of interest is located at the side of the field, color fidelity may be advantageously reduced in the remainder of the image, including the center. Less color fidelity reduction is required because the gaps between traces are smaller.

[0093] It should be noted that here and throughout the drawings, the scanning patterns are shown diagrammatically as if the laser spots were close to each other, one or two pixels apart. However, for eye safety reasons discussed above, and for convenience of manufacture, the physical distance between the laser spots is preferably much larger, preferably corresponding to 0.5 degrees or more for optimal eye safety. However, the traces of the movement of those laser spots are close together (e.g., adjacent pixels, or one pixel apart), as represented in the schematic diagrams. This larger spacing of the laser spots creates correspondingly larger margins on and / or to the sides of the frames described in Figures 4C and 5B. These margins are not shown in most of the drawings here for clarity of presentation in the schematic drawings, which are out of scale.

[0094] FIG. 10 shows a block diagram that illustrates a schematic of a laser driver suitable for implementing this and other aspects of the invention, particularly for the example of a two-channel laser scanning system. The image is received or generated by a CPU processor 228. The CPU can also receive a signal from an eye tracker 226 indicative of the viewer's gaze direction, thereby defining the region of interest. Alternatively, the region of interest is received from recorded data 227 associated with the image to be projected, or is derived directly from the image data. The CPU defines color fidelity modifications as necessary and forwards the image to a pre-processor 230 (which may include two separate pre-processors corresponding to the two laser channels). The pre-processor 230 performs image corrections related to the particular channel. This includes white balance and continuous color fidelity modifications. The hardware 232 adjusts the pulses according to mirror position information from the scanner driver 238, and the laser driver 234 drives the power to the laser 236.

[0095] Depolarization By way of preamble, further aspects of the invention relate to optical assemblies, systems, and methods for reducing non-uniformity in an image coupled out of a light-guiding optical element (LOE or substrate or waveguide) when illumination of the image is obtained from one or more lasers, and when the coupling-out configuration is sensitive to the polarization of the light propagating in the LOE, such as in the case of coupling-out based on obliquely angled partially reflective internal surfaces and diffractive coupling-out configurations. The principles of this aspect of the invention are applicable to a wide range of displays illuminated by light from one or more lasers, including displays based on rapid scanning of a modulated laser beam to illuminate successive pixels one after the other, and displays in which the laser illumination is used to illuminate a spatial light modulator (SLM), such as an LCOS (liquid crystal on silicon) image generator or a micromirror modulator. When illuminating an SLM, the illumination can be a static "flood" illumination, or it can be illumination scanned across the surface of the SLM.

[0096] Injecting unpolarized light into the waveguide typically produces a uniform image. (Various limitations of certain optical designs may result in additional sources of dark fringes in the projected image, but with appropriate optical designs such issues may be avoided and are not addressed here.) According to one aspect of the invention, effectively unpolarized light may be produced by introducing illumination from two uncorrelated lasers of approximately the same wavelength but with orthogonal polarizations. The wavelength deviation between the two lasers (discussed further below) is preferably small enough that the two wavelengths are chromatically perceived as the same color.

[0097] FIG. 11A shows a waveguide configuration 158 with two sets of internal reflectors 250 and 252 to achieve two-dimensional aperture expansion for projecting an image toward the observer's eye. Light from a projector is injected into the waveguide 254 through a prism 258 and guided by total internal reflection (TIR) ​​until it is reflected by reflector 250. The redirected light continues to be guided, and reflector 252 reflects the outgoing light 256 to the observer's eye. Reflectors 250 and 252 are polarization selective. A first polarization orientation of laser light injected into the waveguide 254 produces a non-uniform image, image 260, as shown in FIG. 11B. A second orthogonally polarized laser light injected into the waveguide 254 produces another non-uniform but complementary image 262. By injecting two orthogonally polarized lasers simultaneously, a uniform image is produced, as shown in image 264 in FIG. 11B. A couple of these two orthogonally polarized lasers are preferably implemented for each color (R, G, and B), requiring a total of six lasers.

[0098] Although the examples given herein refer to pairs of two lasers per color, the invention may also be implemented using three or more lasers of the same color. Polarization parameters are commonly described as vectors on a mathematically defined Poincaré sphere. The above definition of two laser beams with orthogonal polarizations can be mathematically generalized as referring to two opposite vectors on the Poincaré sphere, thereby having a resultant Poincaré vector of zero magnitude. This same mathematical description can be used to define the relative polarizations desired for three or more beams of the same color, also satisfying the requirement that the resultant vector has a magnitude of zero. Such a resultant is also referred to as "orthogonal."

[0099] Thus, according to certain embodiments of this aspect of the invention, a display for displaying an image to a viewer includes an image generator having an illumination subsystem that generates illumination of at least a first color, the image generator utilizing the illumination to generate an image. A projection optics system projects the illumination from the image for display to the viewer. A particular feature of certain embodiments of this aspect of the invention is that the illumination subsystem includes a first laser that generates a first laser beam of the first color having a first polarization and a second laser that generates a second laser beam of the first color having a second polarization, the first polarization and the second polarization being orthogonal at at least one location within the projection optics.

[0100] Implementations of the depolarization aspect of the present invention can be broadly subdivided into two categories, depending on whether the image generator and / or image projection optics include polarizing elements. Thus, devices that utilize polarization-correcting spatial light modulators, such as LCOS modulators, or using PBS-based optics in an image projector, obviously cannot achieve depolarization until the light traverses the last polarizing element of the image generator and projector. One such example is shown in FIG. 12, where a PBS 31-based optical system is used as part of a collimated image projector.

[0101] To enable depolarization of the plane polarized laser illumination emerging from the collimated image projector, the first and second laser beams are preferably implemented with less than 5% slight difference in their respective wavelengths. This wavelength difference is small enough that each pair of lasers is perceived as the same color, but sufficient to provide different rotation rates of polarization as the image illumination propagates through a length of birefringent material, thereby allowing the two laser components to differentially rotate until their planes of polarization are substantially orthogonal.

[0102] In the embodiment of Figure 12, a depolarizer is placed in the optical path after the PBS plane 31. In this architecture, the initial polarization direction of all lasers is the same (indicated by the double-headed arrow), but each pair of two lasers of each color has slightly different wavelengths (e.g., 9 nanometers difference at a wavelength of 550 nanometers). A relatively thin birefringent window 266 (e.g., 1.7 mm quartz with its major axis at 45 degrees) separates the relative polarization of the two lasers by 90 degrees (indicated by the dot and double-headed arrow), thus producing complementary images 260 and 262, which are perceived as image 264. The spectral (color) differences of the two combined images become less noticeable.

[0103] Since the different color lasers use the same window, the spectral difference between every two lasers of every color is most preferably set to achieve about 90 degrees of polarization rotation over the same length. For example, when using a 1.7 mm quartz window, an optimal non-limiting example utilizes a spectral difference of 12 nm for a 640 nm wavelength, 9 nm for a 550 nm wavelength, and 7 nm for a 470 nm wavelength.

[0104] For this reason, if the two (here orthogonal) polarizations are oriented according to the waveguide orientation (perpendicular and parallel), it is best to introduce a wave plate adjacent to window 266 to set this orientation.

[0105] In some cases, the perpendicularity of the polarization of the two lasers may drift or otherwise become imprecise. Optionally, in such cases, an active polarization rotator (e.g., an LCD) can be introduced in window 266 in addition to the birefringent crystal.

[0106] To generate pairs of lasers with a predefined small offset in wavelength between them, several options are available. These include: 1. Structural changes: 1.1. Wavelength-selective resonators: Laser resonances can be set to resonate at different wavelengths by introducing wavelength-selective optics, such as selective mirrors or gratings, into the resonator. 1.2. External feedback: The laser can also be made active at a predefined wavelength using an external cavity optical reflector (mirror or grating) that reflects the light back into the resonator. 2. Changing activation parameters: 2.1. Temperature: By stabilizing two lasers at different temperatures, it is possible to transmit at two different wavelengths. 2. Different operating currents: The current of each laser shifts the wavelength. For example, one laser can deliver short, narrow pulses while the other laser can deliver longer pulses at lower intensity. 3. Change of gain medium: 3.1. By changing the bandgap of the semiconductor laser, the laser wavelength is changed. This can be achieved by slightly changing the doping. 3.2. The coating layer structure of the laser itself (e.g. quantum dot configuration) changes the laser output wavelength.

[0107] The choice of birefringent material to use depends on the magnitude of the wavelength difference and the optical path length available to achieve the required orthogonalization of the polarizations. In certain embodiments, a block of quartz is a preferred choice of material. Various birefringent polymeric materials are also suitable.

[0108] The configuration of FIG. 12 requires the aforementioned wavelength offset to achieve orthogonal polarization of the two components using a relatively short optical path in the birefringent material after the light leaves the final polarizing beam splitter (PBS). By using PBS-based optics in the projector, an inherently polarized output is produced, and thus orthogonal polarization can only be produced at a subsequent stage in the optical configuration. In contrast, FIG. 13A shows a configuration that utilizes non-polarizing optics, such as free space optics, to scan (if relevant), direct, and collimate the image illumination. In this case, it becomes feasible to generate a composite beam consisting of combining pairs of laser beams with orthogonal polarizations into a single beam, allowing the same wavelength to be used for the two beams. In this case, it is typically not necessary to further modify the polarization properties of the beam when it enters the waveguide, except in some cases where the axis is aligned with the main axis of the waveguide as described above.

[0109] 13A thus illustrates the case of an optical system that does not incorporate a polarization component in collimating optics 276. One set of RGB laser light P polarization is rotated by waveplate 268 to S polarization (shown as a dot), while the other laser set is transmitted to PBS 270 in P polarization (shown as a double-headed arrow). From this point on, the light throughout the optical system contains both polarizations and no depolarizer is required.

[0110] The combination of all sets of RGB lasers can include a dichroic combiner 20 (as shown) or can be arranged side-by-side on all sides of the PBS 270, producing separate side-by-side spots as shown in Figure 13B (showing only the lasers combined into the PBS). The same depolarization can also be achieved if all six lasers are side-by-side, as in laser assembly 19C shown in Figure 6B, having spots as shown in Figure 6A.

[0111] The invention of depolarization by combining two lasers is also applicable when the laser is used to flood illuminate or scan an image generating matrix such as an amplitude or phase modulated LCOS (polarization sensitive) or a micromirror matrix such as Digital Light Processing (DLP) by Texas Instrument.

[0112] In scanning applications, the need for two orthogonally polarized lasers to have overlapping beams is alleviated by the fact that the lasers scan the field of view through the same optical aperture (scanner mirror). Depolarization can therefore be applied in configuration 162 as shown in Figure 12. In this figure, configuration 162 shows an architecture equivalent to device 100 of Figure 2A, with laser assembly 19C and an additional birefringent window 266 placed between the collimating optics and output aperture 34.

[0113] Fig. 14 shows a further display implementation according to the teachings of an embodiment of the present invention. In this case, a laser portion 280 is collimated by a lens 282 and scanned by scanners 24 and 25 (a single two-axis scanner is presented in this drawing). The light from the scanner is focused by an optical system 284 onto an LCOS 286. From the LCOS, the reflected light is collimated to an exit aperture 34, which is the entrance to the LOE (not to scale here). In this example, the optical system is based on a polarizing beam splitter, so depolarization is performed by introducing into the pair of laser portions 280 of wavelength-differentiated lasers of the same perceived color and of the same polarization, as described with reference to Fig. 12. Here, a birefringent window 266 is placed immediately after the PBS.

[0114] An equivalent system without a PBS can be implemented using a DLP configuration, where the optical portion can be equivalent to the laser shown in Figure 13A or 13B. Side-by-side lasers with selective polarization rotation can also be implemented.

[0115] Although shown above with a birefringent depolarizing window deployed as part of the projection optics immediately after the PBS, the birefringent component may alternatively be integrated with the waveguide. Such an embodiment is shown here in FIG. 15A. This embodiment may enable the use of a relatively long depolarizing block of birefringent material, which can achieve effective depolarization even with a single laser beam (without the wavelength offset mentioned above).

[0116] Thus, Figure 15A shows the implementation of a birefringent section 280 integrated into a waveguide 250, whereby depolarization (or polarization rotation) is achieved over a relatively long optical path. Injected light enters a coupling prism 258 attached to the transparent section of the birefringent material 280. The light is then passed to the section 250 containing the reflective facets. The orientation of the principal axis 282 of the birefringent section 280 (Figure 15B) should be parallel to the principal axes of the waveguide orientation, and the laser illumination should be injected into the waveguide with a polarization plane at approximately 45 degrees to those axes.

[0117] Light entering section 280 is refracted according to the refractive index of all polarizations, as shown in polarization 170 (P and S polarization). Because the principal axes are orthogonal, TIR does not cause coupling between the polarizations. When the light leaves the birefringent section 280 and enters the next section 250, the light beams of the two polarizations become parallel again. Using the relatively long propagation in the birefringent section, depolarization can be effectively achieved even with a single laser light beam, albeit with a narrow spectral width.

[0118] It will be appreciated that the above description is intended to serve as an example only, and that many other embodiments are possible within the scope of the invention as defined in the appended claims.

Claims

1. A display for displaying an image to a viewer, comprising: (a) an image generator having an illumination subsystem that generates illumination of at least a first color, the image generator utilizing the illumination to generate an image; (b) a projection optical system for projecting illumination from the image for display to the viewer, the projection optical system comprising: (i) collimating optics for collimating the illumination from the image generator to produce a collimated image; (ii) a light-guide optical element having a pair of parallel outer major surfaces for directing the collimated image within the light-guide optical element by internal reflection, the light-guide optical element having a coupling-out configuration for redirecting at least a portion of the collimated image to the viewer; a first laser generating a first laser beam of the first color having a first polarization and a second laser generating a second laser beam of the first color having a second polarization, the first laser and the second laser being two of a set of at least three lasers in the illumination subsystem generating a laser beam of the first color, the at least three lasers having mutually orthogonal polarizations at at least one location within the projection optics, and the first polarization and the second polarization are orthogonal at at least one location within the projection optics.

2. 10. The display of claim 1, wherein the first laser beam and the second laser beam are arranged to produce side-by-side beams having mutually orthogonal polarizations.

3. 2. The display of claim 1, wherein the first laser beam and the second laser beam are combined into a single beam by a polarizing beam combiner of the illumination subsystem to form a single beam comprising two orthogonal polarizations.

4. A display for displaying an image to a viewer, comprising: (a) an image generator having an illumination subsystem that generates illumination of at least a first color, the image generator utilizing the illumination to generate an image; (b) a projection optical system for projecting illumination from the image for display to the viewer, the projection optical system comprising: (i) collimating optics for collimating the illumination from the image generator to produce a collimated image; (ii) a light-guide optical element having a pair of parallel outer major surfaces for directing the collimated image within the light-guide optical element by internal reflection, the light-guide optical element having a coupling-out configuration for redirecting at least a portion of the collimated image to the viewer; 1. A display comprising: the illumination subsystem comprising a first laser generating a first laser beam of the first color having a first polarization and a second laser generating a second laser beam of the first color having a second polarization, the first polarization and the second polarization being orthogonal at at least one location within the projection optics, the first laser beam and the second laser beam having first and second wavelengths, respectively, a difference between the first wavelength and the second wavelength being less than 5%, and the projection optics configured to differentially rotate the first polarization and the second polarization from a parallel state to a substantially orthogonal state, the display including a length of birefringent material.

5. The display of claim 4 , wherein the image generator comprises a polarization-modulating spatial light modulator illuminated by the first and second laser beams.

6. 6. The display of claim 5, wherein the image generator further comprises a scanning optical arrangement comprising at least one scanning mirror, the scanning arrangement being arranged to generate a scanning motion of the first and second laser beams in at least one direction so as to sequentially illuminate different areas of the spatial light modulator.

7. 5. The display of claim 4, wherein the projection optical system comprises a collimating optical system for generating a collimated image and a light-guiding optical element for transmitting the collimated image to the viewer, and the birefringent material is disposed in an optical path from the collimating optical system to the light-guiding optical element.

8. The display of claim 7 , wherein the collimating optics comprises at least one polarizing beam splitter.

9. The display of claim 4 wherein the birefringent material is a block of quartz.

10. 2. The display of claim 1, wherein the image generator further comprises a scanning optical arrangement comprising at least one scanning mirror, the scanning arrangement being arranged to generate a scanning motion of the first and second laser beams in at least one direction.

11. 11. The display of claim 1, wherein the illumination subsystem further comprises a third laser and a fourth laser generating a laser beam of a second color having an orthogonal polarization at the image, and a fifth laser and a sixth laser generating a laser beam of a third color having an orthogonal polarization at the image.

Images