Method and apparatus for improved color filter saturation

A reflective image display comprising of a reflection enhancing layer comprising of a plurality of approximately spherical indentations is placed adjacent a sheet comprising of a plurality of hemispherical protrusions. The radii of curvature of the spherical indentations substantially coincides with the center of curvature of the adjacently located hemispheres to enhance the white paper-like appearance of the display while efficiently enabling optional color filters to yield saturated color.

FIELD

This disclosure relates to a method and apparatus for improved color filter saturation. More specifically, the disclosure relates to enhanced reflective displays to produce images having a white appearance through total internal reflection.

BACKGROUND

Semi-retro-reflection refers to a reflection property of an approximately macroscopically planar structure wherein the planar structure reflects a substantial fraction of the light that strikes it, and does so with a special directional characteristic, especially for incident light that is within a range of directions deviating by less than approximately 45 degrees from the direction perpendicular to the macroscopically planar structure. The special directional characteristic is that for each incident light ray, the reflected light propagates backward in approximately the reverse direction, in other words, back towards the point of origin, with a deviation from the reverse direction that is macroscopically random and primarily less than a predetermined maximum deviation, wherein the predetermined maximum deviation being less than approximately 45 degrees. This semi-retro-reflection exhibits what is often called “optical gain”, meaning that it increases the apparent reflectance of the surface under common illumination and viewing conditions. Furthermore, the manner in which light is reflected from this structure results in a paper-like white appearance. A paper-like white appearance is generally preferable to the metallic luster normally observed in optical systems that exhibit optical gain.

A semi-retro-reflective characteristic can be approximated in a reflective display incorporating an array of convex or hemi-spherical protrusions or hemi-spheres (it should be noted that the terms “convex protrusions” and “hemi-spherical protrusions” and “hemi-spheres” will henceforth be used interchangeably). Depicted inFIG. 1is a front sheet100of a reflective display with an outward front surface102facing the viewer and an inward surface104comprising of a plurality of hemi-spherical protrusions106which reflects light by means of total internal reflection (TIR) within the individual hemi-spheres108. Typically only about half of the incident light rays on sheet100are totally internally reflected, impeding attainment of a white appearance, as depicted in enlarged detail of a portion of a hemi-spherical array inFIG. 2. In theFIG. 2example, the incident light rays110(depicted as solid lines) are either totally internally reflected and emerge as reflected light rays112(depicted as dotted lines) back towards the viewer or they pass through the dark pupil region as non-totally reflected light rays114. The incident light rays110deviate by about 30 degrees from the perpendicular direction, which represents a typical operating condition where high quality semi-retro-reflection is desired but not achieved due to the large fraction of light rays114that do not undergo total internal reflection within the hemi-spheres largely due to passing through the non-reflective dark pupil region as previously explained, for example, in U.S. Pat. No. 6,885,496.

One approach to recovering a substantial portion of the light rays114that pass through the dark pupil region is to place a planar reflective element116beneath the hemi-spherical array as shown inFIG. 3to improve reflectivity. However, although the planar reflector is able to cause most incident light rays110(depicted as solid lines) to undergo net reflection, most light rays reflected by the planar reflector do not have the desired semi-retro-reflection characteristic and emerge as light rays118(depicted as dotted lines) that instead are reflected away from the viewer and the source of incident light. High optical gain can be achieved by the system shown inFIG. 3if the incident light rays are incident perpendicular to the hemispherical array's planar outward surface, but this has limited usefulness in practice.

Another approach to reflecting light rays that pass through the dark pupil regions of the individual convex or hemi-spherical protrusions as shown inFIG. 2but in a semi-retro-reflective manner is to place a reflective element beneath the hemi-spherical array such that it reflects the light substantially back towards the direction of origin of the light rays. This may be achieved by a reflective element which incorporates an array of approximately spherical indentations. The approximately spherical indentations each has a radius of curvature that substantially coincides with the center of curvature of the hemi-sphere located directly above it. The invention described in this application is directed to “recycling” of such light rays in a semi-retro-reflective manner to enhance the brightness in TIR-based displays. Furthermore, the invention described enables high efficiency and high color saturation in a reflective color display comprising of a color filter array.

DESCRIPTION

FIG. 4depicts a reflective structure described herein in which a reflective element placed beneath the hemi-spherical array incorporates an array of approximately spherical indentations illustrating how light rays that pass through the dark pupil regions are semi-retro-reflected.FIG. 4depicts a transparent sheet100with an outward surface102facing the viewer and an inward surface104opposite the viewer. Sheet100is further comprised of a plurality of convex protrusions or hemi-spherical protrusions or hemi-spheres106which reflects light by means of total internal reflection (TIR) within the individual hemi-spheres108. Hemi-spheres108may also be hemi-spherical beads or hemi-beads. The reflective structure inFIG. 4is further comprised of reflective element120with a reflective surface122placed behind sheet100and adjacent surface104. Reflective element120is comprised of a plurality of spherical indentations124, each of which has a radius of curvature that substantially coincides with the center of curvature of the hemi-sphere located directly above it. The ratio of the two radii of curvature influences the degree of angular deviation associated with the retro-reflection (where (radius of curvature of a spherical indentation)/(radius of curvature of a hemisphere)). Incident light rays110(depicted as solid lines) that pass through sheet100typically are totally internally reflected at the surface104of the hemi-spherical protrusions108about half of the time and are retro-reflected predominantly but not necessarily directly back towards the light source as seen in emerged light rays112(depicted as dotted lines). The remaining light rays that are not totally internally reflected114pass through the dark pupil of the individual hemi-spherical protrusions108and are semi-retro-reflected at the contoured surface122of the reflective element comprising of the plurality of spherical indentations124. Unlike a planar reflective element described inFIG. 3where light rays118are reflected away from the light source when the direction of the incident light rays is in a non-perpendicular direction to the surface of said planar reflective element, instead light rays114are substantially directed back towards the hemi-sphere by which they originated and emerge as light rays112directed towards the light source in a semi-retro-reflective manner. These light rays combine with the totally internally reflected light rays to enhance the reflectivity.

The structure described inFIG. 4can be incorporated into a reflective display to enhance the reflectance and apparent brightness of the display.FIG. 5embodiment depicts incorporation of a reflective structure depicted inFIG. 4into a reflective display. Display200inFIG. 5is comprised of a transparent front sheet202composed of, for example, glass or a polymer with an outer surface204facing the viewer. Display200further comprises a second transparent sheet206composed of, for example, glass or a polymer which forms a cavity208wherein said cavity comprises an optical modulation layer. On the opposite side of sheet206from cavity208is a rear reflective structure210similar to that described inFIG. 4. Reflective structure210is comprised of a transparent sheet212further comprised of a plurality of convex protrusions or hemi-spherical protrusions (i.e. hemi-spheres)214which reflects light by means of total internal reflection (TIR) within the individual hemi-spheres216. The plurality of hemi-spheres214forms a contoured surface218wherein a reflective element220comprising of a plurality of spherical indentations with reflective surface222is placed adjacent. In one embodiment the ratio of the two radii of curvature is about 0.5 to 5 (where (radius of curvature of the spherical indentation)/(radius of curvature of a hemisphere)≈0.5-5). In another embodiment the ratio of the two radii of curvature is about 1 to 3. In another embodiment the ratio of the two radii of curvature is about 1 to 2. Though not shown, the display200inFIG. 5may further comprise an optional front light.

The optical modulating layer208of display200inFIG. 5allows or prevents incident light from passing through sheet206towards the rear reflective element214. The optical modulating layer may be comprised of an optical shutter based on any number of techniques that do not rely on polarization of reflected light such as, but not limited to, micro-electromechanical system (MEMS), electro-wetting system or electrophoretically mobile particles or a combination thereof. Display200depicts an optical modulating layer comprising of a liquid medium224with suspended electrophoretically mobile particles226. Within cavity208and located on the front surface of transparent sheet206is an electrode layer228that may be comprised of a thin film transistor array, patterned electrode array or a combination thereof. Electrode layer228in combination with a voltage source (not shown) controls the optical modulation layer. It may be envisioned that the electrode layer could also be located on the rear surface of the color filter layer adjacent to liquid medium224. Display200further comprises an optional color filter array layer230further comprised of red, green and blue sub-pixels denoted R, G and B, respectively inFIG. 5. Alternatively, the sub-pixels could be composed of cyan, magenta and yellow.

As depicted in display200inFIG. 5, the optical modulation layer is set to transmit light. A voltage of appropriate polarity is applied such that the electrophoretically mobile particles226localize or group at specific locations at the electrode layer228such that they allow incident and reflected light rays232to pass. Incident light rays are depicted by the solid lines and the reflected light rays by the dotted lines. Light rays that pass through the several layers of display200either totally internally reflect at the convex protrusion or hemi-spherical array214or pass through the dark pupil regions of the individual hemi-spheres216. The light rays that pass through the dark pupils are retro-reflected at the surface222of reflective element220such that the light rays are substantially reflected back towards the viewer from the direction by which they came (i.e. origin) thus enhancing the apparent brightness of the display.

The depiction shown inFIG. 5illustrates an important property of the overall design described herein. Not only does it achieve high efficiency (i.e. high reflectivity of incident light rays) but the retro-reflection characteristic causes the incident light that is shown passing through the green sub-pixel filter to return substantially through the same green sub-pixel filter in the color filter layer230and back towards the direction of the incident light. This is critical to achieve high efficiency and high color saturation in a reflective color display. The reflective structure210facilitates production of a bright, white, paper-like appearance while efficiently enabling color filters to yield saturated color. These characteristics are preserved over a wide range of incident viewing angles, providing an excellent ergonomic viewing characteristic.

As depicted in display200inFIG. 6, the optical modulation layer comprising of a liquid medium224containing suspended electrophoretically mobile particles226is set to prevent transmission of light. In this example, the particles are delocalized into a substantially uniform layer onto the electrode layer228by application of an electric field of appropriate polarity such that incident light rays232that pass through the outer transparent sheet202and the optional color filter layer230are absorbed by the layer of particles226preventing light being reflected back resulting in a dark state of the display. Alternatively, one could envision the electrode layer228being located on the inward surface of the color filter layer230in display200.

In the display embodiments described herein, they may be used in applications such as in, but not limited to, electronic book readers, portable computers, tablet computers, wearables, cellular telephones, smart cards, signs, watches, shelf labels, flash drives and outdoor billboards or outdoor signs.

Embodiments described above illustrate but do not limit this application. While a number of exemplary aspects and embodiments have been discussed above, those of skill in the art will recognize certain modifications, permutations, additions and sub-combinations thereof. Accordingly, the scope of this disclosure is defined only by the following claims.