Lumus Patent | Laser-illuminated displays with enhanced uniformity and/or eye safety

Which is lower compared to single combined spot. This will generate observable reduction of brightness to the eye if spots are spaced apart at least 0.5 degree.

The scanning pattern of the three spots in of FIG. 4B is shown in FIG. 4C. It is apparent that the pattern of all three lasers is almost overlapping. Consequently, same eye section is illuminated by all three lasers at very short interval. This aspect of the illumination is emphasized in FIG. 4D, that represents line 96 generated when scanner 25 malfunctions and does not move. In this case spots 80, 81 and 82 have approximately overlapping path thereby increasing the brightness impact. Therefore, the apparent brightness to the user can be approximated to be 3e6 NITs equivalent to FIG. 2C. Nevertheless, for spot separation of more than 0.5 degrees, the impact on the eye is lower than a single spot.

FIG. 4E shows the scan pattern if scanner 24 malfunctions and stops working. In this case, the line described in FIG. 2D is now split to three lines having ⅓ the intensity compared to the combined alternative of FIG. 2D.

A more optimal arrangement for reducing potential damage to the eye is shown in FIGS. 5A-5D. FIG. 5A shows the laser arrangement staggered at an angle relative to both scanning axes. The scanned field is shown in FIG. 5B. The scanned field for each laser 76, 77 and 78 is shifted vertically as well as horizontally relative to the other scanning patterns. Although the spacing is illustrated here schematically 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, and in some cases, in excess of 20, or even 50, pixel dimensions. The scanning patterns are most preferably completely non-overlapping (neglecting cross-overs), therefore substantially reducing local intensity in every pixel during normal operation. This is a substantial advantage for a display device intended to be used continuously.

FIGS. 5C and 5D illustrate the line scan patterns generated when scanner 25 or 24, respectively, malfunction. It is apparent that, with the tilted laser orientation, both cases avoid scanning overlap, so that local brightness is reduced by factor of 3 in both cases.

FIG. 6A shows a further non-limiting example of an architecture employing a combination of six lasers. A partial view of a device implementing such an architecture is illustrated in FIG. 6B as arrangement 132, where the laser assembly 19C is similar to 19B of FIG. 4A, but with two red lasers (R1, R2), two green (G1, G2) and two blue (B1, B2). Placement of the lasers is shown only schematically. FIG. 6A shows schematically a preferred alignment of the laser spots at the image plane in a diagonal arrangement. FIGS. 6C and 6D illustrate patterns 136 and 138, respectively, representing the line scans which would result from malfunction of the respective scanning mechanisms, and show the non-overlapping line patterns that would be generated by the lasers. Consequently, brightness is reduced by a factor of roughly six relative to a combined single spot architecture.

Other laser orientations are also possible. In all cases, particularly preferred implementations avoid having more than one laser located along a line parallel to either of the principal scanning axes.

Blue Laser Blurring for Eye Safety and Computation Load Reduction

It is noted that perception of color by the human eye is more sensitive to resolution at green wavelengths than it is at blue and red wavelengths of the visual spectrum. Therefore, image sharpness, as measured by the optical point-spread-function (PSF) in the image plane, is less important for blue and red illumination than it is for green illumination.

On the other hand, from a laser safety standpoint, the maximum permissible exposure (MPE) is approximately 0.1 mW/cm2 for long-term (e.g. several hours), direct ocular exposure to blue light and approximately 1.0 mW/cm2 for green light. (These figures are based upon Maini, A. K. (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.)

To take advantage of these observations, according to one aspect of the present invention, a display for displaying an image to a viewer employs a set of laser light sources including at least one of each of a red source R1, R2 generating a red light beam, a green source G1, G2 generating a green light beam, and a blue source B1, B2 generating a blue light beam. A scanning optical arrangement employing at least one scanning mirror 24, 25 generates 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 source and with the scanning arrangement, and is configured to modulate an intensity of each of the laser light sources synchronously with the scanning motion so as to generate an image at an image plane. A collimating optical arrangement collimates light from the image at the image plane for display to the viewer.

It is particular feature of certain embodiments of one aspect of the present invention that the blue source and/or the scanning optical arrangement are configured such that a 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 a point spread function of the green light beam.

The width of the point spread function (PSF) may be intuitively defined by the apparent width of the laser spot in the image plane, but can be more precisely defined by using the full width at half maximum (FWHM) in relation to the spot intensity.

By way of a specific example, the power density of blue light illumination on the image plane is reduced by a factor of 10 by broadening the PSF (diameter) by a factor of about 10{circumflex over ( )}0.5=3.2, while maintaining the PSF of the green and preferably the red illumination at their optimal values. Consequently, 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 damage threshold to the eye, as determined by the MPE.

Broadening of the blue PSF also enables a reduction of the blue resolution provided by controller 228 (FIG. 10). As an example, if the green image has an array of 1000×1000 pixels, and a pixel size of 5×5 microns, then the blue image, after widening of the PSF, may have an array of 333×333 pixels, and a pixel size of about 15 microns. The reduced resolution of the blue image reduces the size and complexity of the electronics in controller 228, as well as the required bandwidth of fast pulse modulation in the blue laser driver. If the resolution of the red image is also reduced, then a similar reduction in size, complexity, and bandwidth applies also to controller 228 and the red laser driver. Thus, the display controller 228 will modulate the intensity of the green source according to pixel data corresponding to an image of a first resolution, and will modulate the intensity of the blue source, and optionally also the red source, according to pixel data corresponding to an image of a second resolution, the second resolution being lower than the first resolution.

Broadening the blue PSF can be implemented by various methods, in either the relay section (after MLA 29) or the laser section (before MLA 29) of the optical layout in FIG. 1. For example, chromatic aberration may be introduced in relay section so that the lens focal lengths corresponding to red, green, and blue illumination are different. By placing the focal plane of 32 (or 276) at a distance relative to 29 that is sub-optimal (defocused) for blue, and nearly optimal for green and red, the blue PSF is broadened and the blue image is of lower resolution at the image display.

However, the use of chromatic aberration for PSF broadening at the relay may not be the best approach, for two reasons. First, a lack of collimation in the blue optics gives rise to spatial non-uniformity of the blue image intensity, and possibly to other image artifacts. Furthermore, in some cases, the eye of an observer may compensate, or “accommodate”, the defocusing of the blue image and re-focus the lens of the eye to sharpen the blue laser image. In such a case, the power density on the retina of the eye, after accommodation, may exceed the MPE.

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 image plane at the MLA 29. This not only broadens the PSF as required, but also partially removes optical phase information after passage through a diffuser or MLA in image plane. Consequently, the image on the image display cannot be refocused by the eye of an observer, and the reduced power density on the retina of the eye are maintained. Furthermore, the blue illumination reaching entrance pupil 34 is well-collimated by lens 32 (or 276), and no non-uniformities appear in the blue image presented to the observer.

It should be noted that PSF broadening of the blue illumination may also be accomplished by shifting the position of the blue laser in FIG. 1 along the optical path so that to a location which is slightly out-of-focus at the image plane. All of the methods for PSF broadening may also be applied to the red illumination as well.

The aforementioned PSF broadening for the blue laser illumination may be implemented both in the case of side-by-side laser deployment or where the laser beams are combined along a common axis. The side-by-side configuration provides the added advantage of distributing the light intensity over a larger area, as already discussed, and is therefore typically preferred.

De-Flickering by Color-Fidelity Reduction

In FIG. 7A, the different color spots of FIG. 5A or 6A are presented having different shapes for clarity. The color laser spots 202 include circle with solid line-scan as red, square with dot-dash as green and rectangle with dashed line as blue.

The lasers are arranged at the focal plane having specific orientation considering the scanning speed so that during scan the traces (scan lines) do not overlap as shown. The gap between two scans of red 204 contains traces of blue 206 and green 208.

The human vision is insensitive to color at peripheral field, therefore as long as all colors are illuminated (some deviations from white) the observer will perceive all traces illuminated having no difference. Therefore, the perception will be that gap 204 is illuminated uniformly and there are no gaps at the edges of the field. Therefore, minimal flicker will be observed for images that have white or balanced gray levels (including a combination of colors) at the periphery. If, however, the peripheral region of a projected image has high coloration, this means that one or more of the other colors will be at low intensity, leaving non-illuminated regions that give rise to flicker.

To take advantage of these observations, according to one aspect of the present invention, a display for displaying an image to a viewer employs a set of laser light sources including at least one red source R1, R2 generating a red light beam, at least one green source G1, G2 generating a green light beam, and at least one blue source B1, B2 generating a blue light beam, the red, green and blue light beams being side-by-side. A scanning optical arrangement employs 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 source and with the scanning arrangement, and modulates an intensity of each of the laser light sources synchronously with the scanning motion so as to generate an image at an image plane. A collimating optical arrangement collimates light from the image at the image plane for display to the viewer.

It is particular feature of certain embodiments of one aspect of the present invention 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 sources so as to reduce a color saturation in at least part of the image outside the current region of interest. In other words, after determining which area(s) are not currently the region of interest, the color saturation in those regions can be reduced while maintaining the luminance of those regions, thereby at least partially equalizing the intensity of the three colors, and thereby filling gaps between the different color scans which might otherwise cause disturbing flicker.

This approach is illustrated in FIGS. 7B and 7C. FIG. 7B illustrates image illumination intensity along a vertical line 207 in FIG. 7A in a region of an image which is red only (two near crossings shown as single peek 208). The gap between scan lines is apparent. FIG. 7C shows image illumination intensity along the same line of the same image, but with suppressed color-fidelity (reduced color saturation but maintained luminance). In this case, the red (solid) continues to be the dominant color, but the green (dot-dash) and the blue (dash) are also illuminated so that the overall luminance envelope 210 has reduced non-uniformity. Since human peripheral vision has low sensitivity to colors, this will be perceived as uniform illumination, thereby reducing perceived flicker.

As mentioned, the above approach is predicated on the observation that, at the center of the visual field, the human eye is sensitive to color but not to flickering, while at the peripheral vision, the converse is true: the eye is insensitive to color but more sensitive to flickering. These eye properties can be used to suppress visibility of flickering without compromising on perceived color fidelity.

Various approaches may be used to define the “region of interest” within the image. In certain applications, particularly where different parts of a display are used to present different types of information, it may be possible to predefine certain regions of the display as being more critical for color fidelity and other regions as being less critical. More preferably, the “region of interest” is dynamically defined by one of a number of approaches. Options include, but are not limited to:

1. Use of an eye tracking arrangement deployed for tracking a current line of sight of the eye of the user. The display controller then determines the current region of interest based upon an input received from the eye tracking arrangement.

2. Where the image to be displayed is a video image, the display controller processes content of the video image to derive the current region of interest, for example, in regions in which there is significant motion.

3. In some cases, where the image is a video image, the display controller may receive along with the image data an associated data stream indicative of a current region of interest associated with the video image.

In each of these cases, since it is assumed that the observer is looking at the region of interest, the color fidelity within that region is unaltered. If flickering exists in this region, it will not be noticeable due to the insensitivity of the central visual field to flicker. The color-fidelity of the rest of the field can be reduced (according to the distance from the region of interest and the non-uniformity likely to occur in that region of the display) by image processing. In this manner, flickering will be suppressed while the color modification will not be noticeable. As the region of interest move across the field, so does the region in which color-fidelity is maintained, as described.

FIG. 7D is similar to FIG. 7A but shows an arrangement of 6 laser spots 212, two of each RGB color, similar to that illustrated in FIG. 6A, above. Other arrangements are possible. By arranging the traces not to overlap (by choice of lasers staggering orientation, spacing and scanning speeds), the gap 214 (in this example, between adjacent traces of the red lasers) is filled with traces of the other colors. Proper, tilt and setting of vertical scan speed will generate an even distribution of the traces. In this way, the aforementioned color-fidelity management as explained with reference to FIGS. 7B and 7C can be implemented very efficient to achieve de-flickering.

In the examples illustrated thus far, the scanning patterns employing two channels have illustrated the channels as being adjacent (FIG. 3D). FIG. 8A illustrates an alternative configuration in which the laser channels have a gap between them. Placing the laser channels adjacent has the advantage that the center of the field does not have gaps (region 75 of FIG. 3D), so there is no flickering at the center of the field, but suffers from the disadvantage that the gap at the side of the field is double (region 73 in FIG. 3D). In the configuration of FIG. 8, the two spaced channels have between them a gap, spaced as two pixels 216 with a gap of one empty pixel between them. FIG. 8B shows a combination trace also including the back scan. Because of the channel arrangement, the gap at the side of the field 218 is only one pixel (less flicker than from the gap 73 of FIG. 3D), but in this case, the center now also has a gap of one pixel (marked 220 in FIG. 8A), which introduces more flicker than would occur at region 75 of FIG. 3D.

FIG. 9 illustrates this spaced-apart approach applied to a set of two three-color channels 156. A non-overlapping arrangement of the traces preferably fills the gaps at the side of the field 222 (similar to region 214 of FIG. 7D, but with narrower gaps), and also fills the gaps at the center of the field 224.

In this arrangement 156, the aforementioned foveated reduced color-fidelity may be applied to advantage across the entire field (not only at the edges), so if the current region of interest is located at the side of the field, then color fidelity may advantageously be reduced in the rest of the image, including the center. Because of the smaller gap between the traces, less color fidelity reduction is needed.

It should be noted that here, and throughout the drawings, scanning patterns are illustrated schematically as if the laser spots are close to each other, either one pixel or two pixels apart. However, for reasons of eye-safety discussed above, as well as for convenience of manufacture, the physical distance between the laser spots is preferably much larger, and for optimal eye safety, preferably corresponds to half a degree or more. The traces of motion of those laser spots however do have the proximity (e.g., adjacent pixels, or spaced-apart by one pixel) as represented in the schematic illustrations. This larger spacing of the laser spots generates correspondingly larger margins above and/or to the sides of the frame described in FIGS. 4C and 5B. These margins are not shown here in most of the drawings for clarity of presentation in the out-of-scale schematic drawings.

FIG. 10 shows a block diagram illustrating schematically a laser driver suitable for implementing this and other aspects of the present invention, particularly for the example of a two-channel laser scanning system. The image is received or generated by the CPU processor 228. The CPU may also receive from an eye tracker 226 a signal indicative of a gaze direction of the observer, thereby defining a 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 a color-fidelity modification where needed, and transfers the image to pre-processor 230 (which may include two separate pre-processors corresponding to the two laser channels). The preprocessor 230 does image modifications associated with the specific channel. This includes white balance and continuous color-fidelity modification. Hardware 232 coordinates pulses according to mirror position information from a scanner driver 238, and a laser driver 234 drives the electrical power to lasers 236.

Depolarization

By way of introduction, a further aspect of the present invention relates to optical assemblies, systems and methods for reducing non-uniformity of an image coupled out of a light-guide optical element (LOE or substrate or waveguide) in cases where the illumination of the image is derived from one or more laser, and where the coupling-out configuration is sensitive to the polarization of the light propagating within the LOE, as is the case with coupling-out based on obliquely-angled partially-reflective internal surfaces and diffractive coupling-out configurations. The principles of this aspect of the present invention are applicable to a wide range of displays which are illuminated by light from one or more laser, including displays based on rapid scanning of a modulated laser beam to successively illuminate sequential pixels and displays in which 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. In the case of illuminating an SLM, the illumination may be static “flood” illumination, or may be illumination which is scanned across a surface of the SLM.

Injecting unpolarized light into a waveguide typically generates a uniform image. (There can be additional causes for dark stripes within the projected image due to various limitations of certain optical designs, but such problems can be avoided proper optical design and are not addressed here.) According to an aspect of the present invention, effectively unpolarized light may be generated by introducing illumination from two uncorrelated lasers having approximately the same wavelength but orthogonal polarizations. Any deviation in wavelength between the two lasers (discussed further below) is preferably sufficiently small that the two wavelengths are perceived chromatically as the same color.

FIG. 11 shows a waveguide configuration 158 having two sets of internal reflectors 250 and 252 to achieve two-dimensional aperture expansion for projecting an image towards an eye of an observer. The light from a projector is injected into the waveguide 254 through prism 258 and guided by total internal reflection (TIR) until being reflected by reflectors 250. The redirected light continues to be guided and reflectors 252 reflect the light out 256 to observer eye. Reflectors 250 and 252 are polarization selective. A first polarized orientation of laser light that is injected into waveguide 254 will generate non-uniform image as shown in FIG. 11B, image 260. A second orthogonally polarized laser light injected into waveguide 254 generates another non-uniform but complementary image 262. By injecting, simultaneously, the two orthogonally polarized lasers, a uniform image is generated, as shown in image 264 of FIG. 11B. These two orthogonally polarized couples of lasers are preferably implemented on each color (R, G and B), requiring a total of 6 lasers.

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. It is common to describe polarization parameters as vectors on the mathematically-defined Poincare-sphere. The above definition of two laser beams having orthogonal polarization can be generalized mathematically as referring to two opposite vectors on the Poincare-sphere thereby having combined Poincare-vector of Zero magnitude. This same mathematical description can be used to define the desired relative polarizations for more than two beams of same color to also satisfy the requirement that the combined vector is of zero magnitude. Such combinations are also referred to as “orthogonal.”

Thus, according to certain embodiments of this aspect of the present invention, a display for displaying an image to a viewer includes an image generator having an illumination subsystem generating illumination of at least a first color, the image generator employing the illumination to generate an image. Projection optics projects illumination from the image for display to the viewer. It is a particular feature of certain embodiments of this aspect of the present invention that the illumination subsystem includes a first laser generating a first laser beam of the first color with a first polarization and a second laser generating a second laser beam of the first color with a second polarization, and that the first and second polarizations are orthogonal at at least one location within the projection optics.

Implementations of the depolarization aspect of the present invention can be subdivided broadly into two categories according to whether the image generator and/or image projecting optics include polarizing elements. Thus, for example, devices employing a polarization-modifying spatial light modulator, such as an LCOS modulator, or that uses PBS-based optics in an image projector, clearly cannot achieve depolarization until the light has traversed the last polarizing element of the image generator and the projector. One such example is illustrated in FIG. 12, where optics based on a PBS 31 are used as part of the collimating image projector.

In order to allow depolarization of the plane-polarized laser illumination emerging from the collimating image projector, the first and second laser beams are preferably implemented with a small difference of less than 5% in their respective wavelengths. This differential in wavelength is sufficiently small that each pair of lasers is perceived as the same color, but is sufficient to provide different rates of rotation of the polarization as the image illumination propagates through a length of birefringent material, thereby allowing the two laser components to be differentially rotated until their polarization planes are substantially orthogonal.

In the implementation of FIG. 12, a depolarizer is placed in the light path after PBS plane 31. In this architecture all lasers have the same initial polarization orientation (shown as double headed arrows), but each pair of two lasers of each color have slightly different wavelength (for example 9 nanometers difference at wavelength of 550 nanometer). The relatively thin birefringent window 266 (for example 1.7 mm crystal quartz where principle axes are at 45 degrees) will diverge the relative polarizations of the two lasers to be 90 degree apart (shown as dots and double headed arrows) and therefore generate the complimentary images 260 and 262 to be perceived as 264. The spectral (color) difference of the two combined images will not be noticeable.

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 approximately 90 degrees polarization rotation over the same length. For example, if using a 1.7 mm crystal quartz window, then an optimal non-limiting example would employ a spectral difference of 12 nm for 640 nm wavelength, 9 nm for 550 nm and 7 nm for 470 nm wavelength.

It is best if the two (now orthogonal) polarizations be oriented according to waveguide orientation (perpendicular and parallel) for this a waveplate can be introduced adjacent to window 266 to set this orientation.

In some cases, the perpendicularity of the polarization of the two lasers may drift or otherwise be inaccurate. Optionally in such cases, an active polarization rotator (for example LCD) may be introduced at 266 in addition to the birefringent crystal.

A number of options are available for generating pairs of lasers with a predefined small offset in wavelength between them. These include:

1. Structural modification:

1.1. Wavelength selective resonator: Laser resonance can be set to resonate at different wavelength by introducing wavelength selective optics in the resonator such as selective mirrors or gratings.

1.2. External feedback: using external cavity optical reflectors (mirror or grating) that reflect the light back into the resonator, can also cause the laser to be active at a predefined wavelength. 2. Modification of an activation parameter:

2.1. Temperature: Stabilizing two lasers at different temperatures will enable transmission at two different wavelengths.

2.2. Different operating current: the current for each laser causes a shift in wavelength. For example, one laser can transmit short and narrow pulses while the other longer with less intensity. 3. Modification of gain medium:

3.1. Changing the bandgap in a semiconductor laser will change the laser wavelength. Slight modification of doping can achieve this.

3.2. Coating layers structure of the laser itself (quantum dot configuration as an example) will modify laser output wavelength.

The choice of what birefringent material to use will depend on the magnitude of the wavelength differential and the available light path length to achieve the required orthogonalization of the polarization. In certain implementations, a block of quartz crystal is a preferred choice of material. Various birefringent polymer materials are also suitable.