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EP-4740057-A2 - IMPROVED COLOR UNIFORMITY IN DISPLAYS HAVING LIGHTGUIDE OPTICAL ELEMENTS

EP4740057A2EP 4740057 A2EP4740057 A2EP 4740057A2EP-4740057-A2

Abstract

A display (100) includes a lightguide optical element (102) with internal partially reflecting surfaces (106) for delivering an image from an image projector (100) to the eye of a viewer. Chromatic variations in the appearance of white pixels across the output image are reduced by using a bro ad- spectrum white light source (114W) to provide at least part of the illumination for the white pixels. Additionally, or alternatively, chromatic corrections for groups of white pixels, or for individual pixels, are provided by delivering corresponding white-balance corrective illumination from red, blue and/or green light sources (114R, 114G, 114B). The corrections are derived from maps of chromatic variations for the display when activated to deliver a uniform white image as calculated, or preferably measured, at one or more locations within an eye-motion box (108) from which the image is to be viewed.

Inventors

  • EISENFELD, Tsion
  • SHARLIN, Elad

Assignees

  • Lumus Ltd.

Dates

Publication Date
20260513
Application Date
20240705

Claims (20)

  1. 1. A display for presenting an image to an eye of a user, the display comprising: (a) a lightguide optical element (LOE) having a pair of mutually parallel major surfaces for supporting propagation of light corresponding to an image by internal reflection at said major surfaces, said LOE including a set of mutually parallel partially reflecting coupling-out surfaces, internal to said LOE and angled obliquely to said major surfaces, for coupling out the light corresponding to the image towards the eye of the user; and (b) an image projector optically coupled to said LOE for introducing light corresponding to the image into said LOE so as to propagate within said LOE, said image projector including light sources of at least four distinct spectral properties including a red light source, a green light source, a blue light source and a white light source, and a controller configured to actuate said image projector to generate the light corresponding to the image, wherein said controller is configured to operate in at least a first mode in which at least part of an intensity of at least a subset of pixels of the image is generated by said white light source.
  2. 2. The display of claim 1, wherein said subset of pixels is the subset of pixels of the image set to white.
  3. 3. The display of claim 1, wherein said image projector further comprises a spatial light modulator (SLM), and wherein, in said first mode, said controller actuates said light sources to sequentially illuminate said SLM with pulses of light from said red, green and blue light sources and with a pulse generated at least in part by said white light source.
  4. 4. The display of claim 3, wherein a first group of at least one white pixel is illuminated by said white light source plus a first white-balance corrective illumination from said red light source, said green light source and/or said blue light source, and wherein a second group of at least one white pixel is illuminated by said white light source plus a second white-balance corrective illumination from said red light source, said green light source and/or said blue light source, said second white-balance corrective illumination being different from said first whitebalance corrective illumination.
  5. 5. The display of claim 4, wherein said white pulse together with said first white-balance corrective illumination is delivered as a first white pulse illuminating said SLM and said white pulse together with said second white-balance corrective illumination is delivered as a second white pulse illuminating said SLM.
  6. 6. The display of claim 4, wherein said first white-balance corrective illumination and said second white-balance corrective illumination are generated by setting pixels of said SLM to corresponding brightness values during illumination pulses from said red, green and blue illumination sources.
  7. 7. The display of claim 4, wherein a composition of said first white-balance corrective illumination and a composition of said second white-balance corrective illumination are derived from a calibration map of chromatic aberration introduced by said LOE for white light reaching an eye motion box from which the image is to be viewed.
  8. 8. The display of claim 4, wherein a composition of said first white-balance corrective illumination and a composition of said second white-balance corrective illumination are derived as a function of eye position from calibration maps of chromatic aberration introduced by said LOE for white light reaching multiple locations within an eye motion box from which the image is to be viewed.
  9. 9. The display of claim 4, wherein each group of pixels is a single pixel and each pixel has its own composition of white-balance corrective illumination.
  10. 10. The display of claim 3, wherein said controller is further configured to operate in a second mode in which said controller actuates said light sources to sequentially illuminate said SLM with pulses of light from said red, green and blue light sources and said white light source is not activated, and wherein said controller switches between said second mode and said first mode according to a non-uniformity visibility criterion.
  11. 11. The display of claim 1, wherein said white light source includes a white LED having a blue spectral peak at a first wavelength, and wherein said blue light source has a spectral peak at a second wavelength that is longer than said first wavelength.
  12. 12. The display of claim 1, wherein said white light source includes a white LED having a blue spectral peak at a wavelength between 460 and 490 nm.
  13. 13. The display of claim 1, wherein said white light source includes a white LED having a color temperature between 3500 K and 4500 K, and wherein said controller is configured to actuate said white light source together with said blue light source to generate illumination with a color temperature of at least 6000 K.
  14. 14. A display for presenting an image to an eye of a user, the display comprising: (a) a lightguide optical element (LOE) having a pair of mutually parallel major surfaces for supporting propagation of light corresponding to an image by internal reflection at said major surfaces, said LOE including a set of mutually parallel partially reflecting coupling-out surfaces, internal to said LOE and angled obliquely to said major surfaces, for coupling out the light corresponding to the image towards the eye of the user; and (b) an image projector optically coupled to said LOE for introducing light corresponding to the image into said LOE so as to propagate within said LOE, said image projector including light sources of at least three distinct spectral properties including a red light source, a green light source and a blue light source, and a controller configured to actuate said image projector to generate the light corresponding to the image, wherein said controller is configured to operate in at least a first mode in which said controller: (i) receives pixel image data corresponding to an image to be displayed; (ii) identifies within the pixel data a subset of pixels that are set to a uniform white; (iii) generates modified pixel data in which data for at least the subset of pixels are modified to correct for chromatic aberration introduced by said LOE; and (iv) actuates said image projector to generate the image according to the modified pixel data.
  15. 15. The display of claim 14, wherein said modified pixel data are generated based on a calibration map of chromatic aberration introduced by said LOE for white light reaching an eye motion box from which the display is to be viewed.
  16. 16. The display of claim 14, wherein said modified pixel data are generated as a function of eye position from calibration maps of chromatic aberration introduced by said LOE for white light reaching multiple locations within an eye motion box from which the display is to be viewed.
  17. 17. The display of claim 14, wherein said image projector includes a plurality of scanned laser sources.
  18. 18. The display of claim 14, wherein said image projector further comprises a spatial light modulator (SLM), and wherein, in said first mode, said controller actuates said light sources to sequentially illuminate said SLM with pulses of light from said red, green and blue light sources and with a pulse of white light.
  19. 19. The display of claim 18, wherein the pulse of white light is generated by simultaneous operation of said red, green and blue light sources.
  20. 20. The display of claim 18, wherein the pulse of white light is generated at least in part by a white light source.

Description

Improved Color Uniformity in Displays Having Lightguide Optical Elements FIELD AND BACKGROUND OF THE INVENTION The present invention relates to displays, in particular displays having micro image projectors that generate collimated image illumination for injection into a lightguide optical element (LOE) with reflective or partially reflective couplers. Optical arrangements used in a near eye display (NED) or a head up display (HUD), for example as part of augmented reality (AR) or virtual reality (VR) applications, require a large aperture to cover the area where the eye of the observer is located (referred to as the eye motion box, or EMB). Certain optical arrangement technologies, such as those of Lumus Ltd., employ a lightguide optical element (LOE), also referred to as a “light-transmitting substrate” or simply “substrate”, having a series of internal mutually parallel partially reflective surfaces that are inclined obliquely to major external surfaces of the LOE. A micro image projector that is optically coupled to the LOE generates illumination corresponding to a collimated image, and the collimated image illumination is injected into the LOE by an optical coupling-in arrangement (for example a reflective surface or a coupling prism) so as to propagate through the LOE by internal reflection (in many cases, total internal reflection or TIR) at the major external surfaces of the LOE. The propagating image is progressively coupled out of the LOE toward the viewer’s eye by reflection at the series of partially reflective surfaces, thereby expanding the effective optical aperture of the image at the EMB. Reflectivity of the partially reflective surfaces (as well as of coupling-in reflectors) is sensitive to various parameters of the incident illumination, including the spectral range of the illumination, polarization state, and angle of incidence. The partially reflective surfaces are typically coated with optical coatings to generate a desired reflectivity pattern as a function of incident angle and are intended to have similar reflective properties for different wavelengths of illumination across the entire visible spectrum. The micro image projector can be implemented in various ways, including implementations that employ back-lit and front- lit architectures. In one example, the image projector employs illumination sources (typically a set of colored light sources such as LEDs or lasers, e.g., red (R), green (G), blue (B) LEDs or lasers), a spatial light modulator such as a liquid crystal on silicon (LCoS) chip that applies spatial light modulation to the LED illumination, and collimating optics that collimates the modulated illumination, typically all arranged on surfaces of one or more polarization selective beamsplitter (PBS) cube or other prism arrangement. The image projector operates as a color sequential imaging device (CSD), alternatively referred to as field sequential color (FSC), whereby primary color information (e.g., R, G, B) is transmitted to the optical coupling-in arrangement in successive images at high frame rates, and the human visual perception combines the primary color images (coupled-out by the partially reflective surfaces) into a single perceived picture of true color. Typically, when generating images with non-white pixels, or images with white pixels that do not require high accuracy or uniformity of white color, the image projector components can produce uniform color images from the primary colors (e.g., R, G, B) that are of high quality. In order to produce white pixels in images of white objects or a white uniform background, the intensities of the primary colors can be adjusted as required. However, issues may arise with the ability of the optical coatings of the partially reflective surfaces to render, with fidelity, white pixels across the image. Specifically, since the performance of the optical coatings are dependent both on angle and on wavelength, the coating reflectance and transmittance of the primary colors (e.g., R, G, B) vary in different ways across the image. It is possible to balance the intensities of the RGB sources in order to obtain a perfect white for a single pixel in the image. But this balance for a single pixel will lead to variation in the white tone for other pixels in the image because the white color for those other pixels is obtained with the same RGB source balance but with different coating performances. These white tonal variations across the pixels in the image may be noticeable to the viewer, which can reduce the quality of the viewer’s viewing experience. SUMMARY OF THE INVENTION The present invention is a display for presenting an image to the eye of a user. According to the teachings of an embodiment of the present invention there is provided, a display for presenting an image to an eye of a user, the display comprising: (a) a lightguide optical element (LOE) having a pair of mutually parallel major surfaces for supporting propagati