US20100085627A1 - Microlens-assisted brightness enhancement in reflective image displays - Google Patents
Microlens-assisted brightness enhancement in reflective image displays Download PDFInfo
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- US20100085627A1 US20100085627A1 US12/161,048 US16104807A US2010085627A1 US 20100085627 A1 US20100085627 A1 US 20100085627A1 US 16104807 A US16104807 A US 16104807A US 2010085627 A1 US2010085627 A1 US 2010085627A1
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B26/00—Optical devices or arrangements for the control of light using movable or deformable optical elements
- G02B26/004—Optical devices or arrangements for the control of light using movable or deformable optical elements based on a displacement or a deformation of a fluid
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B5/00—Optical elements other than lenses
- G02B5/12—Reflex reflectors
- G02B5/126—Reflex reflectors including curved refracting surface
- G02B5/128—Reflex reflectors including curved refracting surface transparent spheres being embedded in matrix
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- FIG. 1A depicts a portion of a prior art reflective (i.e. front-lit) image display 10 in which total internal reflection (TIR) is electrophoretically modulated as described in U.S. Pat. Nos. 6,885,496 and 6,891,658.
- Display 10 includes a transparent outward sheet 12 formed by partially embedding a large plurality of high refractive index (e.g. ⁇ 1 > ⁇ 1.90) transparent spherical or approximately spherical beads 14 in the inward surface of a high refractive index (e.g. ⁇ 2 > ⁇ 1.75) polymeric material 16 having a flat outward viewing surface 17 which viewer V observes through an angular range of viewing directions Y.
- high refractive index e.g. ⁇ 1 > ⁇ 1.90
- a high refractive index e.g. ⁇ 2 > ⁇ 1.75
- Beads 14 are packed closely together to form an inwardly projecting monolayer 18 having a thickness approximately equal to the diameter of one of beads 14 . Ideally, each one of beads 14 touches all of the beads immediately adjacent to that one bead. Minimal interstitial gaps (ideally, no gaps) remain between adjacent beads.
- An electrophoresis medium 20 is maintained adjacent the portions of beads 14 which protrude inwardly from material 16 by containment of medium 20 within a reservoir 22 defined by lower sheet 24 .
- An inert, low refractive index (i.e. less than about 1.35), low viscosity, electrically insulating liquid such as FluorinertTM perfluorinated hydrocarbon liquid ( ⁇ 3 ⁇ 1.27) available from 3M, St. Paul, Minn. is a suitable electrophoresis medium.
- Other liquids, or water can also be used as electrophoresis medium 20 .
- a bead:liquid TIR interface is thus formed.
- Medium 20 contains a finely dispersed suspension of light scattering and/or absorptive particles 26 such as pigments, dyed or otherwise scattering/absorptive silica or latex particles, etc.
- Sheet 24 's optical characteristics are relatively unimportant: sheet 24 need only form a reservoir for containment of electrophoresis medium 20 and particles 26 , and serve as a support for backplane electrode 48 .
- the TIR interface between two media having different refractive indices is characterized by a critical angle ⁇ c .
- Light rays incident upon the interface at angles less than ⁇ c are transmitted through the interface.
- Light rays incident upon the interface at angles greater than ⁇ c undergo TIR at the interface.
- a small critical angle is preferred at the TIR interface since this affords a large range of angles over which TIR may occur.
- a voltage can be applied across medium 20 via electrodes 46 , 48 (shown as dashed lines) which can for example be applied by vapour-deposition to the inwardly protruding surface portion of beads 14 and to the outward surface of sheet 24 .
- Electrode 46 is transparent and substantially thin to minimize its interference with light rays at the bead:liquid TIR interface.
- Backplane electrode 48 need not be transparent. If electrophoresis medium 20 is activated by actuating voltage source 50 to apply a voltage between electrodes 46 , 48 as illustrated to the left of dashed line 28 , suspended particles 26 are electrophoretically moved into the region where the evanescent wave is relatively intense (i.e.
- Particles 26 need only be moved outside the thin evanescent wave region, by suitably actuating voltage source 50 , in order to restore the TIR capability of the bead:liquid TIR interface and convert each “dark” non-reflective absorption region or pixel to a “white” reflection region or pixel.
- the net optical characteristics of outward sheet 12 can be controlled by controlling the voltage applied across medium 20 via electrodes 46 , 48 .
- the electrodes can be segmented to control the electrophoretic activation of medium 20 across separate regions or pixels of sheet 12 , thus forming an image.
- FIG. 2 depicts, in enlarged cross-section, an inward hemispherical or “hemi-bead” portion 60 of one of spherical beads 14 .
- a light ray 62 perpendicularly incident (through material 16 ) on hemi-bead 60 at a radial distance a from hemi-bead 60 's centre C encounters the inward surface of hemi-bead 60 at an angle ⁇ 1 relative to radial axis 66 .
- Ray 68 encounters the inward surface of hemi-bead 60 at the critical angle ⁇ c (relative to radial axis 70 ), the minimum required angle for TIR to occur.
- Ray 68 is accordingly totally internally reflected, as ray 72 , which again encounters the inward surface of hemi-bead 60 at the critical angle ⁇ c .
- Ray 72 is accordingly totally internally reflected, as ray 74 , which also encounters the inward surface of hemi-bead 60 at the critical angle ⁇ c .
- Ray 74 is accordingly totally internally reflected, as ray 76 , which passes perpendicularly through hemi-bead 60 into the embedded portion of bead 14 and into material 16 .
- Ray 68 is thus reflected back as ray 76 in a direction approximately opposite that of incident ray 68 .
- FIGS. 3A , 3 B and 3 C depict three of hemi-bead 60 's reflection modes. These and other modes coexist, but it is useful to discuss each mode separately.
- FIG. 3A light rays incident within a range of distances a c ⁇ a ⁇ a 1 undergo TIR twice (the 2-TIR mode) and the reflected rays diverge within a comparatively wide arc ⁇ 1 centred on a direction opposite to the direction of the incident light rays.
- FIG. 3B light rays incident within a range of distances a 1 ⁇ a ⁇ a 2 undergo TIR three times (the 3-TIR mode) and the reflected rays diverge within a narrower arc ⁇ 2 ⁇ 1 which is again centred on a direction opposite to the direction of the incident light rays.
- TIR twice the 2-TIR mode
- the reflected rays diverge within a comparatively wide arc ⁇ 1 centred on a direction opposite to the direction of the incident light rays.
- FIG. 3B light rays incident within a range of distances a 1 ⁇ a ⁇ a 2 undergo TIR three times (the 3-TIR mode) and the reflected rays diverge
- Hemi-bead 60 thus has a “semi-retro-reflective,” partially diffuse reflection characteristic, causing display 10 to have a diffuse appearance akin to that of paper.
- Display 10 has relatively high apparent brightness, comparable to that of paper, when the dominant source of illumination is behind the viewer, within a small angular range. This is illustrated in FIG. 1B which depicts the wide angular range ⁇ over which viewer V is able to view display 10 , and the angle ⁇ which is the angular deviation of illumination source S relative to the location of viewer V. Display's 10 's high apparent brightness is maintained as long as ⁇ is not too large.
- the reflectance R of hemi-bead 60 i.e. the fraction of light rays incident on hemi-bead 60 that reflect by TIR
- ⁇ 1 is the refractive index of hemi-bead 60 and ⁇ 3 is the refractive index of the medium adjacent the surface of hemi-bead 60 at which TIR occurs.
- a reflectance R of about 36% is attained, whereas if hemi-bead 60 is formed of a high refractive index nano-composite material ( ⁇ 1 ⁇ 1.92) a reflectance R of about 56% is attained.
- FIGS. 4A-4G hemi-bead 60 's reflectance is maintained over a broad range of incidence angles, thus enhancing display 10 's wide angular viewing characteristic and its apparent brightness.
- FIG. 4A shows hemi-bead 60 as seen from perpendicular incidence—that is, from an incidence angle offset 0° from the perpendicular.
- the portion 80 of hemi-bead 60 for which a ⁇ a c appears as an annulus.
- the annulus is depicted as white, corresponding to the fact that this is the region of hemi-bead 60 which reflects incident light rays by TIR, as aforesaid.
- FIGS. 4B-4G show hemi-bead 60 as seen from incident angles which are respectively offset 15°, 30°, 45°, 60°, 75° and 90° from the perpendicular. Comparison of FIGS. 4B-4G with FIG. 4A reveals that the observed area of reflective portion 80 of hemi-bead 60 for which a ⁇ a c decreases only gradually as the incidence angle increases. Even at near glancing incidence angles (e.g. FIG. 4F ) an observer will still see a substantial part of reflective portion 80 , thus giving display 10 a wide angular viewing range over which high apparent brightness is maintained.
- near glancing incidence angles e.g. FIG. 4F
- An estimate of the reflectance of an array of hemispheres corresponding to the inward “hemi-bead” portions of each one of spherical beads 14 depicted in FIG. 1A can be obtained by multiplying the reflectance of an individual hemi-bead by the hemi-beads' packing efficiency coefficient ⁇ .
- Calculation of the packing efficiency coefficient ⁇ of a closely packed structure involves application of straightforward geometry techniques which are well known to persons skilled in the art.
- the hexagonal closest packed (HCP) structure depicted in FIG. 5 yields a packing efficiency ⁇ /(6 ⁇ tan 30°) ⁇ 90.7% assuming beads 14 are of uniform size.
- the HCP structure yields the highest packing density for hemispheres, it is not necessary to pack the hemi-beads in a regular arrangement, nor is it necessary that the hemi-beads be of uniform size.
- a random distribution of non-uniform size hemi-beads having diameters within a range of about 1-50 ⁇ m has a packing density of approximately 80%, and has an optical appearance substantially similar to that of an HCP arrangement of uniform size hemi-beads.
- such a randomly distributed arrangement may be more practical to manufacture, and for this reason, somewhat reduced reflectance due to less dense packing may be acceptable.
- FIG. 5 depicts a plurality of these dark non-reflective regions 82 , each of which is surrounded by a reflective annular region 80 , as previously explained.
- Hemi-bead 60 's average surface reflectance, R is determined by the ratio of the area of reflective annulus 80 to the total area comprising reflective annulus 80 and dark circular region 82 . That ratio is in turn determined by the ratio of the refractive index, ⁇ 1 , of hemi-bead 60 to the refractive index, ⁇ 3 , of the medium adjacent the surface of hemi-bead 60 at which TIR occurs, in accordance with Equation (1). It is thus apparent that the average surface reflectance, R, increases with the ratio of the refractive index ⁇ 1 , of hemi-bead 60 to that of the adjacent medium ⁇ 3 .
- the average surface reflectance, R, of a hemispherical water drop ( ⁇ 1 ⁇ 1.33) in air ( ⁇ 3 ⁇ 1.0) is about 43%; the average surface reflectance, R, of a glass hemisphere ( ⁇ 1 ⁇ 1.5) in air is about 55%; and the average surface reflectance, R, of a diamond hemi-sphere ( ⁇ 1 ⁇ 2.4) in air exceeds 82%.
- interstitial gaps 84 ( FIG. 5 ) unavoidably remain between adjacent beads.
- Light rays incident upon any of gaps 84 are “lost” in the sense that they pass directly into electrophoretic medium 20 , producing undesirable dark spots on viewing surface 17 . While these spots are invisibly small, and therefore do not detract from display 10 's appearance, they do detract from viewing surface 17 's net average surface reflectance, R.
- Individual hemi-beads 60 can be invisibly small, within the range of 2-50 ⁇ m in diameter, and as shown in FIG. 5 they can be packed into an array to create a display surface that appears highly reflective due to the large plurality of tiny, adjacent, reflective annular regions 80 .
- regions 80 where TIR can occur, particles 26 ( FIG. 1A ) do not impede the reflection of incident light when they are not in contact with the inward, hemispherical portions of beads 14 .
- regions 82 and 84 where TIR does not occur, particles 26 may absorb incident light rays—even if particles 26 are moved outside the evanescent wave region so that they are not in optical contact with the inward, hemispherical portions of beads 14 .
- the refractive index ratio ⁇ 1 / ⁇ 3 can be increased in order to increase the size of each reflective annular region 80 and thus reduce such absorption losses.
- Non-reflective regions 82 , 84 cumulatively reduce display 10 's overall surface reflectance, R. Since display 10 is a reflective display, it is clearly desirable to minimize such reduction.
- R average surface reflectance
- the system's overall average surface reflectance is 91% of 55% or about 50%, implying a loss of about 50%. 41% of this loss is due to light absorption in circular non-reflective regions 82 ; the remaining 9% of this loss is due to light absorption in interstitial non-reflective gaps 84 .
- Display 10 's reflectance can be increased by decreasing such absorptive losses through the use of materials having specific selected refractive index values, optical microstructures or patterned surfaces placed on the outward or inward side(s) of monolayer 18 ( FIG. 1A ).
- Display 10 's surface reflectance can be increased, as described below, thereby further improving the appearance of the display.
- FIG. 1A is a greatly enlarged, not to scale, fragmented cross-sectional side elevation view of a portion of a prior art reflective image display in which TIR is electrophoretically modulated.
- FIG. 1B schematically illustrates the wide angle viewing range ⁇ of the FIG. 1A display, and the angular range ⁇ of the illumination source.
- FIG. 2 is a cross-sectional side elevation view, on a greatly enlarged scale, of a hemispherical (“hemi-bead”) portion of one of the spherical beads of the FIG. 1A apparatus.
- hemispherical hemispherical
- FIGS. 3A , 3 B and 3 C depict semi-retro-reflection of light rays perpendicularly incident on the FIG. 2 hemi-bead at increasing off-axis distances at which the incident rays undergo TIR two, three and four times respectively.
- FIGS. 4A , 4 B, 4 C, 4 D, 4 E, 4 F and 4 G depict the FIG. 2 hemi-bead, as seen from viewing angles which are offset 0°, 15°, 30°, 45°, 60°, 75° and 90° respectively from the perpendicular.
- FIG. 5 is a top plan (i.e. as seen from a viewing angle offset 0° from the perpendicular) cross-sectional view of a portion of the FIG. 1 display, showing the spherical beads arranged in a hexagonal closest packed (HCP) structure.
- HCP hexagonal closest packed
- FIG. 6A is a cross-sectional side elevation view, on a greatly enlarged scale, of a portion of a reflector incorporating a microlens array optically coupled to a reflective surface.
- FIG. 6B schematically depicts focusing of light rays by the FIG. 6A structure.
- FIG. 7 is a cross-sectional side elevation view, on a greatly enlarged scale, of a portion of another reflector incorporating a microlens array optically coupled to a reflective surface.
- FIGS. 8A and 8B respectively schematically depict electrophoretic modulation of the FIG. 6A structure, with FIG. 8A depicting the reflective state and FIG. 8B depicting the absorptive, non-reflective state.
- FIGS. 9A and 9B respectively schematically depict modulation of the FIG. 6A structure by electro-deformation of a liquid medium, with FIG. 9A depicting the reflective state and FIG. 9B depicting the absorptive, non-reflective state.
- FIGS. 10A and 10B respectively schematically depict electrophoretic modulation of the FIG. 7 structure, with FIG. 10A depicting the reflective state and FIG. 10B depicting the absorptive, non-reflective state.
- FIGS. 11A and 11B respectively schematically depict modulation of the FIG. 7 structure by electro-deformation of a liquid medium, with FIG. 11A depicting the reflective state and FIG. 11B depicting the absorptive, non-reflective state.
- a reflective display can be formed by providing a spatially variable small scale pattern on an optical surface.
- the value of ⁇ w can be modified on a small size scale to vary the pattern's net effective reflectance as a function of macroscopic position on the optical surface in order to produce a graphic image. This is typically achieved by placing a highly reflective surface immediately behind the optical surface—in which case the net effective reflectance is approximately equal to ⁇ w .
- Reflective displays based on this concept are for example disclosed by Hayes et al. in “Video-Speed Electronic Paper Based on Electrowetting,” Nature, Vol. 425, pp. 383-385, 25 Sep. 2003; Kishi et al. in “Development of In-Plane EPD,” pp. 24-27, Proceedings of the Society for Information Display Symposium 2000; and Swanson et al. in “High Performance Electrophoretic Displays,” pp. 29-31 Proceedings of the Society for Information Display Symposium 2000.
- the efficacy of such displays is limited by the value of ⁇ w , which cannot exceed 1, whereas in most practical displays it is difficult to achieve a value of ⁇ w exceeding about 0.6.
- FIG. 6A depicts a portion of a reflector 178 positionable on the rearward side of a reflective image display.
- Reflector 178 incorporates microlens array 180 and reflective optical surface 190 .
- Microlens array 180 in turn incorporates microlenses 182 , 184 , 186 , 188 which are positioned in front of (i.e. on an outward side of) reflective surface 190 and which protrude outwardly away from reflective surface 190 .
- an optically masked (i.e. invisible) annular region 80 A encircling a non-masked (i.e. visible) circular region 82 A is associated with each microlens.
- annular regions 80 A are not visible when viewed from the normal viewing direction, due to the refraction caused by the microlens; only circular regions 82 A are visible when viewed from the normal viewing direction.
- microlenses 182 - 188 matches that of a light absorptive pattern applied to surface 190 .
- annular pattern segment 192 is concentrically aligned beneath each one of microlenses 182 - 188 , as shown in FIGS. 6A and 6B .
- the optical characteristics of segments 192 are relatively unimportant, but segments 192 may be electrically conductive to facilitate controllable attraction toward segments 192 of a light absorber, as explained below.
- Each segment 192 has approximately the same size and shape as, and is concentrically aligned with, the optically masked annular region 80 A of the adjacent microlens.
- Each segment 192 encloses a circular, reflective portion 194 of surface 190 .
- Each portion 194 has approximately the same size and shape as, and is aligned with, the non-optically masked circular region 82 A of the adjacent microlens.
- each microlens focuses (i.e. converges) incident light rays 195 (i.e. light rays which arrive from approximately the typical viewing direction) toward a focal point 197 beneath the microlens.
- Surface 190 is positioned between microlens array 180 and focal point 197 , and is spaced an appropriate distance d 1 beneath microlens array 180 , such that light rays 195 are focused by the microlens onto the circular, reflective portion 194 of surface 190 underlying the microlens.
- Distance d 1 is selected such that substantially all light rays which pass through any one of microlenses 182 - 188 are focused onto the circular, reflective portion 194 beneath that microlens, and such that substantially no light rays which pass through any one of microlenses 182 - 188 reach the annular pattern segment 192 beneath that microlens.
- surface 190 would reflect only a fraction of the incident light rays, corresponding to the ratio of the total areas of reflective portions 194 and surface 190 respectively.
- microlens array 180 focuses substantially all incident light rays onto surface 190 's circular, reflective portions 194 , substantially all incident light rays are reflected, thus enhancing the apparent brightness of surface 190 . Since relatively few light rays reach annular pattern segments 192 , they are effectively hidden from view (i.e. optically masked, as aforesaid).
- incident light ray 196 is focused by microlens 184 onto the circular, reflective portion 194 beneath microlens 184 , which reflects the ray back through microlens 184 such that the ray emerges as reflected ray 198 .
- incident light ray 200 is similarly focused by microlens 184 onto the circular, reflective portion 194 beneath microlens 184 , which again reflects the ray back through microlens 184 such that the ray emerges as reflected ray 202 .
- Reflective surface 190 may be diffusely reflective, or specularly reflective, or semi-retro-reflective, or retro-reflective.
- the magnitude of the brightness enhancement of surface 190 will depend on its reflectivity characteristic. For example, the brightness of surface 190 will be enhanced if surface 190 is diffusely reflective, but the magnitude of the brightness enhancement will be relatively small since only a fraction of the diffusely reflected light rays will be reflected within the angular range of viewing directions Y ( FIG. 6A ) through which viewer V observes reflector 178 .
- the magnitude of the brightness enhancement will be greater if surface 190 is specularly reflective, but that magnitude will depend on the viewing angle.
- the magnitude of the brightness enhancement will be even greater if surface 190 is semi-retro-reflective, since light rays will be semi-retro-reflected in a manner which is relatively independent of the viewing angle.
- the magnitude of the brightness enhancement will be greater still if surface 190 is retro-reflective, since retro-reflected light rays return to the viewer essentially independently of the viewing angle.
- Semi-retro-reflective and specularly reflective characteristics are desirable because they facilitate significant brightness enhancement, without highly collimating the reflected light which accordingly appears white.
- FIGS. 6A and 6B depict only one possible configuration for microlens array 180 .
- microlenses 182 - 188 need not be hemispherical. Any microlens shape capable of focusing a substantial fraction of incident light rays onto a reflective region beneath the microlens will suffice.
- Persons skilled in the art will understand that there are many different appropriate microlens configurations—the configuration shown in FIGS. 6A and 6B should not be considered to be limiting.
- FIGS. 6A and 6B selectively redirects light rays back into the direction from which they came, and diffuses the rays sufficiently to impart a bright white appearance to reflective surface 190 .
- Most light rays which pass through microlens array 180 to reach surface 190 within about 30° of the normal direction are reflected by surface 190 back toward microlens array 180 within the same angular range, as shown in FIG. 6A .
- the incident light rays typically fall within about a 30° angular range of the normal direction.
- the embodiment of FIGS. 6A and 6B enhances net effective reflectance by a factor of about 2 relative to that of a highly reflective diffuse material. The magnitude of the enhancement factor depends on the geometry of microlens array 180 , its distance from surface 190 and the reflectance characteristics of surface 190 .
- FIG. 7 depicts a semi-diffuse and semi-retro-reflective reflector 220 having a wide effective angular viewing range.
- Reflector 220 incorporates an array 222 of microlenses 224 , 226 , 228 ; and a reflective surface 230 positioned near the effective focal points 231 of the respective microlenses 224 , 226 , 228 .
- Microlenses 224 , 226 , 228 are positioned in front of (i.e. on an outward side of) reflective surface 230 and protrude inwardly toward surface 230 , which may be diffusely or specularly reflective. Incident light rays (i.e.
- light rays which arrive from approximately the typical viewing direction such as rays 232 , 234 , 236 which undergo TIR within microlenses 224 , 226 , 228 respectively are semi-retro-reflected back through microlenses 224 , 226 , 228 respectively as previously described in relation to rays 68 , 72 , 74 , 76 depicted in FIG. 2 ; whereas light rays which pass through microlens array 222 —such as rays 238 , 240 , 242 —and which reach reflective surface 230 are reflected by surface 230 from near one of focal points 231 and hence are also semi-retro-reflected back through microlens array 222 .
- light rays which arrive from approximately the typical viewing direction such as rays 232 , 234 , 236 which undergo TIR within microlenses 224 , 226 , 228 respectively are semi-retro-reflected back through microlenses 224 , 226 , 228 respectively as
- the degree of sharpness or clarity of retro-reflection attained by reflector 220 can be modified by adjusting the distance d 2 between microlens array 222 and reflective surface 230 .
- the degree of retro-reflection is too sharp, the display's luminance is reduced due to the absence of a source of light rays collinear with the viewing direction—the viewer's head will obstruct such rays.
- reflective (i.e. front-lit) displays rely upon an external light source which may be obstructed by the viewer's head in some cases.
- an optimal intermediate degree of retro-reflection should be selected.
- it is desirable for the periodicity of any microstructures incorporated in reflective surface 230 to be large compared to the periodicity of microlens array 222 , but this is not always necessary.
- FIGS. 8A and 8B schematically depict electrophoretic modulation of the embodiment of FIGS. 6A and 6B , with FIG. 8A depicting the reflective state and FIG. 8B depicting the absorptive, non-reflective state.
- An air or liquid electrophoresis medium 204 is contained between microlens array 180 and reflective surface 190 .
- Light absorbing particles e.g. pigment particles—not shown
- a voltage is applied across electrophoresis medium 204 as previously described in relation to FIG. 1A , with annular pattern segments 192 constituting the backplane electrode.
- the particles are attracted to and clumped around annular pattern segments 192 .
- FIGS. 9A and 9B schematically depict modulation of the embodiment of FIGS. 6A and 6B by electro-deformation of a liquid medium 208 such as oil containing a light absorptive dye or dye mixture.
- a liquid medium 208 such as oil containing a light absorptive dye or dye mixture.
- FIG. 9A depicts the electro-deformed, reflective state.
- FIG. 9B depicts the relaxed, absorptive (non-reflective) state.
- Liquid electro-deformation is described by Aggarwal et al. in “Liquid Transport Based on Electrostatic Deformation of Fluid Interfaces” Journal of Applied Physics 99, 104904 published online 25 May 2006.
- a voltage is applied across liquid medium 208 as previously described in relation to FIG. 1A , with the backplane electrode conforming to the shape of annular pattern segments 192 .
- Liquid medium 208 always remains on reflective surface 190 —liquid medium 208 does not cross the gap between microlens array 180 and reflective surface 190 .
- liquid medium 208 is moved away from the circular, reflective portions 194 of surface 190 beneath the respective microlenses in array 180 , and is redistributed in approximately hemi-toroidal shapes atop each of annular pattern segments 192 as seen in FIG. 9A .
- FIGS. 10A and 10B schematically depict electrophoretic modulation of the FIG. 7 embodiment, with FIG. 10A depicting the reflective state and FIG. 10B depicting the absorptive, non-reflective state.
- An air or liquid electrophoresis medium 250 is contained between transparent sheets 252 , 254 on the outward side of microlens array 222 .
- Light absorbing particles e.g. pigment particles—not shown
- a voltage is applied across electrophoresis medium 250 via transparent electrodes on the internally opposed sides of sheets 252 , 254 .
- the electrode on inward sheet 254 may conform to the shape of microlens array 222 's reflective annular regions (analogous to hemi-bead 60 's reflective, annular region 80 shown in FIGS. 4A-4G ) or may be formed as an array of thin lines, etc.
- the particles are attracted to and clumped around the electrode segments on inward sheet 254 , as indicated at 256 .
- Some light rays e.g. those labelled 258 and 260 in FIG. 10A ) are lost due to absorption, so it is desirable to minimize the area occupied by the electrode segments on inward sheet 254 .
- the particles are attracted to and are distributed more or less uniformly across the inward surface of sheet 252 , as indicated at 262 .
- Light rays which pass through sheet 252 i.e. substantially all incident light rays) are absorbed by the particles at the inward surface of sheet 252 as shown in FIG. 10B .
- FIGS. 11A and 11B schematically depict modulation of the FIG. 7 embodiment by electro-deformation of a liquid medium 270 such as oil containing a light absorptive dye or dye mixture.
- Liquid-medium 270 is contained between transparent sheets 252 , 254 on the outward side of microlens array 222 .
- FIG. 11A depicts the electro-deformed, reflective state.
- FIG. 11B depicts the relaxed, absorptive (non-reflective) state.
- a voltage is applied across liquid medium 270 via transparent electrodes on the internally opposed sides of sheets 252 , 254 (similar to the electrodes described above in relation to FIGS. 10A and 10B ).
- FIG. 11A depicts the electro-deformed, reflective state.
- liquid medium 270 is redistributed in approximately hemi-toroidal shapes 272 atop the electrode segments on inward sheet 254 .
- Some light rays e.g. those labelled 258 and 260 in FIG. 11A ) are lost due to absorption, so it is desirable to minimize the area occupied by the electrode segments on inward sheet 254 .
- liquid medium 270 is distributed more or less uniformly across the inward surface of sheet 252 , as indicated at 274 .
- Light rays which pass through sheet 252 i.e. substantially all incident light rays) are absorbed by liquid medium 270 at the inward surface of sheet 252 as shown in FIG. 11B .
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Abstract
A reflective display having a light reflector (178) incorporating a microlens array (180) and a reflective surface (190). The microlenses redirect light onto reflective regions (194) of the surface. An electrode on the surface has a plurality of annular segments (192), each segment being aligned with a microlens. An electrophoresis medium (204) is contained between the array and the reflective surface. Light absorptive particles are suspended in the medium. An electrical potential source applies an electrical potential across the medium. In the reflective state, the particles are attracted to the electrode segments, leaving the reflective regions substantially unobstructed, and permitting reflection of light by the reflective regions. In the absorptive, non-reflective state the particles are attracted to and distributed across an inward surface (206) of the microlens array. Light which passes through the array is absorbed by the particles at the inward surface of the microlens array.
Description
- This application claims the benefit of U.S. provisional patent application Ser. No. 60/759,772 filed 17 Jan. 2006.
- This application pertains to brightness enhancement of reflective image displays of the type described in U.S. Pat. Nos. 5,999,307; 6,064,784; 6,215,920; 6,865,011; 6,885,496 and 6,891,658; in United States Patent Application Publication No. 2006-0209418-A1; and in International Patent Publication No. WO 2006/108285 all of which are incorporated herein by reference.
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FIG. 1A depicts a portion of a prior art reflective (i.e. front-lit)image display 10 in which total internal reflection (TIR) is electrophoretically modulated as described in U.S. Pat. Nos. 6,885,496 and 6,891,658.Display 10 includes a transparent outward sheet 12 formed by partially embedding a large plurality of high refractive index (e.g. η1>˜1.90) transparent spherical or approximatelyspherical beads 14 in the inward surface of a high refractive index (e.g. η2>˜1.75)polymeric material 16 having a flatoutward viewing surface 17 which viewer V observes through an angular range of viewing directions Y. The “inward” and “outward” directions are indicated by double-headed arrow Z.Beads 14 are packed closely together to form an inwardly projectingmonolayer 18 having a thickness approximately equal to the diameter of one ofbeads 14. Ideally, each one ofbeads 14 touches all of the beads immediately adjacent to that one bead. Minimal interstitial gaps (ideally, no gaps) remain between adjacent beads. - An
electrophoresis medium 20 is maintained adjacent the portions ofbeads 14 which protrude inwardly frommaterial 16 by containment ofmedium 20 within areservoir 22 defined bylower sheet 24. An inert, low refractive index (i.e. less than about 1.35), low viscosity, electrically insulating liquid such as Fluorinert™ perfluorinated hydrocarbon liquid (η3˜1.27) available from 3M, St. Paul, Minn. is a suitable electrophoresis medium. Other liquids, or water can also be used aselectrophoresis medium 20. A bead:liquid TIR interface is thus formed.Medium 20 contains a finely dispersed suspension of light scattering and/orabsorptive particles 26 such as pigments, dyed or otherwise scattering/absorptive silica or latex particles, etc.Sheet 24's optical characteristics are relatively unimportant:sheet 24 need only form a reservoir for containment ofelectrophoresis medium 20 andparticles 26, and serve as a support forbackplane electrode 48. - As is well known, the TIR interface between two media having different refractive indices is characterized by a critical angle θc. Light rays incident upon the interface at angles less than θc are transmitted through the interface. Light rays incident upon the interface at angles greater than θc undergo TIR at the interface. A small critical angle is preferred at the TIR interface since this affords a large range of angles over which TIR may occur.
- In the absence of electrophoretic activity, as is illustrated to the right of dashed
line 28 inFIG. 1A , a substantial fraction of the light rays passing through sheet 12 andbeads 14 undergoes TIR at the inward side ofbeads 14. For example,incident light rays material 16 andbeads 14. The rays undergo TIR two or more times at the bead:liquid TIR interface, as indicated atpoints ray 30; and indicated atpoints ray 32. The totally internally reflected rays are then refracted back throughbeads 14 andmaterial 16 and emerge asrays - A voltage can be applied across medium 20 via
electrodes 46, 48 (shown as dashed lines) which can for example be applied by vapour-deposition to the inwardly protruding surface portion ofbeads 14 and to the outward surface ofsheet 24. Electrode 46 is transparent and substantially thin to minimize its interference with light rays at the bead:liquid TIR interface.Backplane electrode 48 need not be transparent. Ifelectrophoresis medium 20 is activated by actuatingvoltage source 50 to apply a voltage betweenelectrodes line 28, suspendedparticles 26 are electrophoretically moved into the region where the evanescent wave is relatively intense (i.e. within 0.25 micron of the inward surfaces of inwardly protrudingbeads 14, or closer). When electrophoretically moved as aforesaid,particles 26 scatter or absorb light, thus frustrating TIR by modifying the imaginary and possibly the real component of the effective refractive index at the bead:liquid TIR interface. This is illustrated bylight rays particles 26 inside the thin (˜0.5 μm) evanescent wave region at the bead:liquid TIR interface, as indicated at 56, 58 respectively, thus achieving a “dark” appearance in each TIR-frustrated non-reflective absorption region or pixel.Particles 26 need only be moved outside the thin evanescent wave region, by suitably actuatingvoltage source 50, in order to restore the TIR capability of the bead:liquid TIR interface and convert each “dark” non-reflective absorption region or pixel to a “white” reflection region or pixel. - As described above, the net optical characteristics of outward sheet 12 can be controlled by controlling the voltage applied across medium 20 via
electrodes medium 20 across separate regions or pixels of sheet 12, thus forming an image. -
FIG. 2 depicts, in enlarged cross-section, an inward hemispherical or “hemi-bead”portion 60 of one ofspherical beads 14. Hemi-bead 60 has a normalized radius r=1 and a refractive index η1. A light ray 62 perpendicularly incident (through material 16) on hemi-bead 60 at a radial distance a from hemi-bead 60's centre C encounters the inward surface of hemi-bead 60 at an angle θ1 relative toradial axis 66. For purposes of this theoretically ideal discussion, it is assumed thatmaterial 16 has the same refractive index as hemi-bead 60 (i.e. η1=η2), so ray 62 passes frommaterial 16 into hemi-bead 60 without refraction. Ray 62 is refracted at the inward surface of hemi-bead 60 and passes intoelectrophoretic medium 20 asray 64 at an angle θ2 relative toradial axis 66. - Now consider
incident light ray 68 which is perpendicularly incident (through material 16) on hemi-bead 60 at a distance -
- from hemi-
bead 60's centre C. Ray 68 encounters the inward surface of hemi-bead 60 at the critical angle θc (relative to radial axis 70), the minimum required angle for TIR to occur. Ray 68 is accordingly totally internally reflected, asray 72, which again encounters the inward surface of hemi-bead 60 at the critical angle θc. Ray 72 is accordingly totally internally reflected, asray 74, which also encounters the inward surface of hemi-bead 60 at the critical angle θc. Ray 74 is accordingly totally internally reflected, asray 76, which passes perpendicularly through hemi-bead 60 into the embedded portion ofbead 14 and intomaterial 16. Ray 68 is thus reflected back asray 76 in a direction approximately opposite that ofincident ray 68. - All light rays which are incident on hemi-
bead 60 at distances a≧ac from hemi-bead 60's centre C are reflected back (but not exactly retro-reflected) toward the light source; which means that the reflection is enhanced when the light source is overhead and slightly behind the viewer, and that the reflected light has a diffuse characteristic giving it a white appearance, which is desirable in reflective display applications.FIGS. 3A , 3B and 3C depict three of hemi-bead 60's reflection modes. These and other modes coexist, but it is useful to discuss each mode separately. - In
FIG. 3A , light rays incident within a range of distances ac<a≦a1 undergo TIR twice (the 2-TIR mode) and the reflected rays diverge within a comparatively wide arc φ1 centred on a direction opposite to the direction of the incident light rays. InFIG. 3B , light rays incident within a range of distances a1<a≦a2 undergo TIR three times (the 3-TIR mode) and the reflected rays diverge within a narrower arc φ2<φ1 which is again centred on a direction opposite to the direction of the incident light rays. InFIG. 3C , light rays incident within a range of distances a2<a≦a3 undergo TIR four times (the 4-TIR mode) and the reflected rays diverge within a still narrower arc φ3<φ2 also centred on a direction opposite to the direction of the incident light rays. Hemi-bead 60 thus has a “semi-retro-reflective,” partially diffuse reflection characteristic, causingdisplay 10 to have a diffuse appearance akin to that of paper. -
Display 10 has relatively high apparent brightness, comparable to that of paper, when the dominant source of illumination is behind the viewer, within a small angular range. This is illustrated inFIG. 1B which depicts the wide angular range α over which viewer V is able to viewdisplay 10, and the angle β which is the angular deviation of illumination source S relative to the location of viewer V. Display's 10's high apparent brightness is maintained as long as β is not too large. At normal incidence, the reflectance R of hemi-bead 60 (i.e. the fraction of light rays incident on hemi-bead 60 that reflect by TIR) is given by equation (1): -
- where η1 is the refractive index of hemi-
bead 60 and η3 is the refractive index of the medium adjacent the surface of hemi-bead 60 at which TIR occurs. Thus, if hemi-bead 60 is formed of a lower refractive index material such as polycarbonate (η1˜1.59) and if the adjacent medium is Fluorinert (η3˜1.27), a reflectance R of about 36% is attained, whereas if hemi-bead 60 is formed of a high refractive index nano-composite material (η1˜1.92) a reflectance R of about 56% is attained. When illumination source S (FIG. 1B ) is positioned behind viewer V's head, the apparent brightness ofdisplay 10 is further enhanced by the aforementioned semi-retro-reflective characteristic. - As shown in
FIGS. 4A-4G , hemi-bead 60's reflectance is maintained over a broad range of incidence angles, thus enhancingdisplay 10's wide angular viewing characteristic and its apparent brightness. For example,FIG. 4A shows hemi-bead 60 as seen from perpendicular incidence—that is, from an incidence angle offset 0° from the perpendicular. In this case, theportion 80 of hemi-bead 60 for which a≧ac appears as an annulus. The annulus is depicted as white, corresponding to the fact that this is the region of hemi-bead 60 which reflects incident light rays by TIR, as aforesaid. The annulus surrounds acircular region 82 which is depicted as dark, corresponding to the fact that this is the non-reflective region of hemi-bead 60 within which incident rays are absorbed and do not undergo TIR.FIGS. 4B-4G show hemi-bead 60 as seen from incident angles which are respectively offset 15°, 30°, 45°, 60°, 75° and 90° from the perpendicular. Comparison ofFIGS. 4B-4G withFIG. 4A reveals that the observed area ofreflective portion 80 of hemi-bead 60 for which a≧ac decreases only gradually as the incidence angle increases. Even at near glancing incidence angles (e.g.FIG. 4F ) an observer will still see a substantial part ofreflective portion 80, thus giving display 10 a wide angular viewing range over which high apparent brightness is maintained. - An estimate of the reflectance of an array of hemispheres corresponding to the inward “hemi-bead” portions of each one of
spherical beads 14 depicted inFIG. 1A can be obtained by multiplying the reflectance of an individual hemi-bead by the hemi-beads' packing efficiency coefficient ƒ. Calculation of the packing efficiency coefficient ƒ of a closely packed structure involves application of straightforward geometry techniques which are well known to persons skilled in the art. The hexagonal closest packed (HCP) structure depicted inFIG. 5 yields a packing efficiency ƒ∞π/(6·tan 30°)˜90.7% assuming beads 14 are of uniform size. - Although the HCP structure yields the highest packing density for hemispheres, it is not necessary to pack the hemi-beads in a regular arrangement, nor is it necessary that the hemi-beads be of uniform size. A random distribution of non-uniform size hemi-beads having diameters within a range of about 1-50 μm has a packing density of approximately 80%, and has an optical appearance substantially similar to that of an HCP arrangement of uniform size hemi-beads. For some reflective display applications, such a randomly distributed arrangement may be more practical to manufacture, and for this reason, somewhat reduced reflectance due to less dense packing may be acceptable. However, for simplicity, the following description focuses on the
FIG. 5 HCP arrangement of uniform size hemi-beads, and assumes the use of materials which yield a refractive index ratio η1/η3=1.5. These factors are not to be considered as limiting the scope of this disclosure. - As previously explained in relation to
FIG. 2 , a substantial portion of light rays which are perpendicularly incident on the flat outward face of hemi-bead 60 at distances a<ac from hemi-bead 60's centre C do not undergo TIR and are therefore not reflected by hemi-bead 60. Instead, a substantial portion of such light rays are scattered and/or absorbed byprior art display 10, yielding a dark non-reflective circular region 82 (FIGS. 4A-4G ) on hemi-bead 60.FIG. 5 depicts a plurality of these darknon-reflective regions 82, each of which is surrounded by a reflectiveannular region 80, as previously explained. - Hemi-
bead 60's average surface reflectance, R, is determined by the ratio of the area ofreflective annulus 80 to the total area comprisingreflective annulus 80 and darkcircular region 82. That ratio is in turn determined by the ratio of the refractive index, η1, of hemi-bead 60 to the refractive index, Θ3, of the medium adjacent the surface of hemi-bead 60 at which TIR occurs, in accordance with Equation (1). It is thus apparent that the average surface reflectance, R, increases with the ratio of the refractive index η1, of hemi-bead 60 to that of the adjacent medium η3. For example, the average surface reflectance, R, of a hemispherical water drop (η1˜1.33) in air (η3˜1.0) is about 43%; the average surface reflectance, R, of a glass hemisphere (η1˜1.5) in air is about 55%; and the average surface reflectance, R, of a diamond hemi-sphere (η1˜2.4) in air exceeds 82%. - Although it may be convenient to fabricate
display 10 using spherically (or hemispherically) shaped beads as aforesaid, even if spherical (or hemispherical)beads 14 are packed together as closely as possible within monolayer 18 (FIG. 1A ), interstitial gaps 84 (FIG. 5 ) unavoidably remain between adjacent beads. Light rays incident upon any ofgaps 84 are “lost” in the sense that they pass directly intoelectrophoretic medium 20, producing undesirable dark spots on viewingsurface 17. While these spots are invisibly small, and therefore do not detract fromdisplay 10's appearance, they do detract from viewingsurface 17's net average surface reflectance, R. - The above-described “semi-retro-reflective” characteristic is important in a reflective display because, under typical viewing conditions where light source S is located above and behind viewer V, a substantial fraction of the reflected light is returned toward viewer V. This results in an apparent reflectance which exceeds the value
-
- by a “semi-retro-reflective enhancement factor” of about 1.5 (see “A High Reflectance, Wide Viewing Angle Reflective Display Using Total Internal Reflection in Micro-Hemispheres,” Mossman, M. A. et al., Society for Information Display, 23rd International Display Research Conference, pages 233-236, Sep. 15-18, 2003, Phoenix, Ariz.). For example, in a system where the refractive index ratio η1/η3=1.5, the average surface reflectance, R, of 55% determined in accordance with Equation (1) is enhanced to approximately 85% under the semi-retro-reflective viewing conditions described above.
- Individual hemi-
beads 60 can be invisibly small, within the range of 2-50 μm in diameter, and as shown inFIG. 5 they can be packed into an array to create a display surface that appears highly reflective due to the large plurality of tiny, adjacent, reflectiveannular regions 80. In theseregions 80, where TIR can occur, particles 26 (FIG. 1A ) do not impede the reflection of incident light when they are not in contact with the inward, hemispherical portions ofbeads 14. However, inregions particles 26 may absorb incident light rays—even ifparticles 26 are moved outside the evanescent wave region so that they are not in optical contact with the inward, hemispherical portions ofbeads 14. The refractive index ratio η1/η3 can be increased in order to increase the size of each reflectiveannular region 80 and thus reduce such absorption losses.Non-reflective regions display 10's overall surface reflectance, R. Sincedisplay 10 is a reflective display, it is clearly desirable to minimize such reduction. - Disregarding the aforementioned semi-retro-reflective enhancement factor, a system having a refractive index ratio η1/η3=1.5 has an average surface reflectance, R, of 55%, as previously explained. Given the HCP arrangement's aforementioned packing efficiency of about 91%, the system's overall average surface reflectance is 91% of 55% or about 50%, implying a loss of about 50%. 41% of this loss is due to light absorption in circular
non-reflective regions 82; the remaining 9% of this loss is due to light absorption in interstitialnon-reflective gaps 84.Display 10's reflectance can be increased by decreasing such absorptive losses through the use of materials having specific selected refractive index values, optical microstructures or patterned surfaces placed on the outward or inward side(s) of monolayer 18 (FIG. 1A ). - For example, since
display 10's maximum surface reflectance is determined by the ratio of the refractive index values of hemi-bead 60 and electrophoretic medium 20, the reflectance can be increased by substituting air (refractive index=1.0) as electrophoretic medium 20 instead of a low refractive index liquid (refractive index less than 1.35). -
Display 10's surface reflectance can be increased, as described below, thereby further improving the appearance of the display. - The foregoing examples of the related art and limitations related thereto are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.
- Exemplary embodiments are illustrated in referenced figures of the drawings. It is intended that the embodiments and figures disclosed herein are to be considered illustrative rather than restrictive.
-
FIG. 1A is a greatly enlarged, not to scale, fragmented cross-sectional side elevation view of a portion of a prior art reflective image display in which TIR is electrophoretically modulated. -
FIG. 1B schematically illustrates the wide angle viewing range α of theFIG. 1A display, and the angular range β of the illumination source. -
FIG. 2 is a cross-sectional side elevation view, on a greatly enlarged scale, of a hemispherical (“hemi-bead”) portion of one of the spherical beads of theFIG. 1A apparatus. -
FIGS. 3A , 3B and 3C depict semi-retro-reflection of light rays perpendicularly incident on theFIG. 2 hemi-bead at increasing off-axis distances at which the incident rays undergo TIR two, three and four times respectively. -
FIGS. 4A , 4B, 4C, 4D, 4E, 4F and 4G depict theFIG. 2 hemi-bead, as seen from viewing angles which are offset 0°, 15°, 30°, 45°, 60°, 75° and 90° respectively from the perpendicular. -
FIG. 5 is a top plan (i.e. as seen from a viewing angle offset 0° from the perpendicular) cross-sectional view of a portion of theFIG. 1 display, showing the spherical beads arranged in a hexagonal closest packed (HCP) structure. -
FIG. 6A is a cross-sectional side elevation view, on a greatly enlarged scale, of a portion of a reflector incorporating a microlens array optically coupled to a reflective surface.FIG. 6B schematically depicts focusing of light rays by theFIG. 6A structure. -
FIG. 7 is a cross-sectional side elevation view, on a greatly enlarged scale, of a portion of another reflector incorporating a microlens array optically coupled to a reflective surface. -
FIGS. 8A and 8B respectively schematically depict electrophoretic modulation of theFIG. 6A structure, withFIG. 8A depicting the reflective state andFIG. 8B depicting the absorptive, non-reflective state. -
FIGS. 9A and 9B respectively schematically depict modulation of theFIG. 6A structure by electro-deformation of a liquid medium, withFIG. 9A depicting the reflective state andFIG. 9B depicting the absorptive, non-reflective state. -
FIGS. 10A and 10B respectively schematically depict electrophoretic modulation of theFIG. 7 structure, withFIG. 10A depicting the reflective state andFIG. 10B depicting the absorptive, non-reflective state. -
FIGS. 11A and 11B respectively schematically depict modulation of theFIG. 7 structure by electro-deformation of a liquid medium, withFIG. 11A depicting the reflective state andFIG. 11B depicting the absorptive, non-reflective state. - Throughout the following description specific details are set forth in order to provide a more thorough understanding to persons skilled in the art. However, well known elements may not have been shown or described in detail to avoid unnecessarily obscuring the disclosure. Accordingly, the description and drawings are to be regarded in an illustrative, rather than a restrictive, sense.
- In general, it is known that a reflective display can be formed by providing a spatially variable small scale pattern on an optical surface. Light rays are substantially non-absorbed by a first area fraction, ƒw, of the pattern, and substantially absorbed by a second, complementary, area fraction, ƒb=1−ƒw, OF the pattern. The value of ƒw can be modified on a small size scale to vary the pattern's net effective reflectance as a function of macroscopic position on the optical surface in order to produce a graphic image. This is typically achieved by placing a highly reflective surface immediately behind the optical surface—in which case the net effective reflectance is approximately equal to ƒw.
- Reflective displays based on this concept are for example disclosed by Hayes et al. in “Video-Speed Electronic Paper Based on Electrowetting,” Nature, Vol. 425, pp. 383-385, 25 Sep. 2003; Kishi et al. in “Development of In-Plane EPD,” pp. 24-27, Proceedings of the Society for Information Display Symposium 2000; and Swanson et al. in “High Performance Electrophoretic Displays,” pp. 29-31 Proceedings of the Society for Information Display Symposium 2000. However, the efficacy of such displays is limited by the value of ƒw, which cannot exceed 1, whereas in most practical displays it is difficult to achieve a value of ƒw exceeding about 0.6. This is problematic because a net effective reflectance of about 2 is required to implement red-green-blue (RGB) filtering in a colour reflective display having a brightness comparable to that of coloured ink on white paper. (It is possible, in principle, to attain a net effective reflectance of about 2 in a reflective display, because the incident light is usually derived from a restricted range of directions. A selectively reflective surface can accordingly produce an enhanced effective reflectance value.)
-
FIG. 6A depicts a portion of areflector 178 positionable on the rearward side of a reflective image display.Reflector 178 incorporatesmicrolens array 180 and reflectiveoptical surface 190.Microlens array 180 in turn incorporatesmicrolenses reflective surface 190 and which protrude outwardly away fromreflective surface 190. As shown inFIG. 6B , an optically masked (i.e. invisible)annular region 80A encircling a non-masked (i.e. visible)circular region 82A is associated with each microlens. As explained below,annular regions 80A are not visible when viewed from the normal viewing direction, due to the refraction caused by the microlens; onlycircular regions 82A are visible when viewed from the normal viewing direction. - The periodicity of microlenses 182-188 matches that of a light absorptive pattern applied to
surface 190. Specifically, anannular pattern segment 192 is concentrically aligned beneath each one of microlenses 182-188, as shown inFIGS. 6A and 6B . The optical characteristics ofsegments 192 are relatively unimportant, butsegments 192 may be electrically conductive to facilitate controllable attraction towardsegments 192 of a light absorber, as explained below. Eachsegment 192 has approximately the same size and shape as, and is concentrically aligned with, the optically maskedannular region 80A of the adjacent microlens. Eachsegment 192 encloses a circular,reflective portion 194 ofsurface 190. Eachportion 194 has approximately the same size and shape as, and is aligned with, the non-optically maskedcircular region 82A of the adjacent microlens. - As shown schematically in
FIG. 6B , each microlens focuses (i.e. converges) incident light rays 195 (i.e. light rays which arrive from approximately the typical viewing direction) toward afocal point 197 beneath the microlens.Surface 190 is positioned betweenmicrolens array 180 andfocal point 197, and is spaced an appropriate distance d1 beneathmicrolens array 180, such thatlight rays 195 are focused by the microlens onto the circular,reflective portion 194 ofsurface 190 underlying the microlens. Distance d1 is selected such that substantially all light rays which pass through any one of microlenses 182-188 are focused onto the circular,reflective portion 194 beneath that microlens, and such that substantially no light rays which pass through any one of microlenses 182-188 reach theannular pattern segment 192 beneath that microlens. Withoutmicrolens array 180,surface 190 would reflect only a fraction of the incident light rays, corresponding to the ratio of the total areas ofreflective portions 194 andsurface 190 respectively. However, sincemicrolens array 180 focuses substantially all incident light rays ontosurface 190's circular,reflective portions 194, substantially all incident light rays are reflected, thus enhancing the apparent brightness ofsurface 190. Since relatively few light rays reachannular pattern segments 192, they are effectively hidden from view (i.e. optically masked, as aforesaid). - More particularly, as shown in
FIG. 6A , incidentlight ray 196 is focused by microlens 184 onto the circular,reflective portion 194 beneathmicrolens 184, which reflects the ray back throughmicrolens 184 such that the ray emerges as reflectedray 198.Incident light ray 200 is similarly focused by microlens 184 onto the circular,reflective portion 194 beneathmicrolens 184, which again reflects the ray back throughmicrolens 184 such that the ray emerges as reflectedray 202. - It can thus be seen that light rays are converged by microlenses 182-188 away from
annular pattern segments 192 ontosurface 190's circular,reflective portions 194. Although only a fractional portion, ƒw, of surface 190 (i.e. the circular,reflective portions 194 thereof which are devoid of annular pattern segments 192) is reflective, essentially all light rays of interest are directed toward that fractional portion, increasing the effective value of ƒw to approximately 1. -
Reflective surface 190 may be diffusely reflective, or specularly reflective, or semi-retro-reflective, or retro-reflective. The magnitude of the brightness enhancement ofsurface 190 will depend on its reflectivity characteristic. For example, the brightness ofsurface 190 will be enhanced ifsurface 190 is diffusely reflective, but the magnitude of the brightness enhancement will be relatively small since only a fraction of the diffusely reflected light rays will be reflected within the angular range of viewing directions Y (FIG. 6A ) through which viewer V observesreflector 178. The magnitude of the brightness enhancement will be greater ifsurface 190 is specularly reflective, but that magnitude will depend on the viewing angle. The magnitude of the brightness enhancement will be even greater ifsurface 190 is semi-retro-reflective, since light rays will be semi-retro-reflected in a manner which is relatively independent of the viewing angle. The magnitude of the brightness enhancement will be greater still ifsurface 190 is retro-reflective, since retro-reflected light rays return to the viewer essentially independently of the viewing angle. Semi-retro-reflective and specularly reflective characteristics are desirable because they facilitate significant brightness enhancement, without highly collimating the reflected light which accordingly appears white. -
FIGS. 6A and 6B depict only one possible configuration formicrolens array 180. For example, although they are depicted as hemispherical, microlenses 182-188 need not be hemispherical. Any microlens shape capable of focusing a substantial fraction of incident light rays onto a reflective region beneath the microlens will suffice. Persons skilled in the art will understand that there are many different appropriate microlens configurations—the configuration shown inFIGS. 6A and 6B should not be considered to be limiting. - The embodiment of
FIGS. 6A and 6B selectively redirects light rays back into the direction from which they came, and diffuses the rays sufficiently to impart a bright white appearance toreflective surface 190. Most light rays which pass throughmicrolens array 180 to reachsurface 190 within about 30° of the normal direction are reflected bysurface 190 back towardmicrolens array 180 within the same angular range, as shown inFIG. 6A . In a reflective display application the incident light rays typically fall within about a 30° angular range of the normal direction. The embodiment ofFIGS. 6A and 6B enhances net effective reflectance by a factor of about 2 relative to that of a highly reflective diffuse material. The magnitude of the enhancement factor depends on the geometry ofmicrolens array 180, its distance fromsurface 190 and the reflectance characteristics ofsurface 190. -
FIG. 7 depicts a semi-diffuse and semi-retro-reflective reflector 220 having a wide effective angular viewing range.Reflector 220 incorporates anarray 222 ofmicrolenses reflective surface 230 positioned near the effectivefocal points 231 of therespective microlenses Microlenses reflective surface 230 and protrude inwardly towardsurface 230, which may be diffusely or specularly reflective. Incident light rays (i.e. light rays which arrive from approximately the typical viewing direction) such asrays microlenses microlenses rays FIG. 2 ; whereas light rays which pass throughmicrolens array 222—such asrays reflective surface 230 are reflected bysurface 230 from near one offocal points 231 and hence are also semi-retro-reflected back throughmicrolens array 222. Thus, almost all of the incident light rays are reflected toward the viewer, within the desired angular range. - The degree of sharpness or clarity of retro-reflection attained by
reflector 220 can be modified by adjusting the distance d2 betweenmicrolens array 222 andreflective surface 230. Generally, if the degree of retro-reflection is too sharp, the display's luminance is reduced due to the absence of a source of light rays collinear with the viewing direction—the viewer's head will obstruct such rays. Recall that reflective (i.e. front-lit) displays rely upon an external light source which may be obstructed by the viewer's head in some cases. However, if the degree of retro-reflection is insufficiently sharp, inadequate retro-reflectivity is attained, so an optimal intermediate degree of retro-reflection should be selected. Generally, it is desirable for the periodicity of any microstructures incorporated inreflective surface 230 to be large compared to the periodicity ofmicrolens array 222, but this is not always necessary. -
FIGS. 8A and 8B schematically depict electrophoretic modulation of the embodiment ofFIGS. 6A and 6B , withFIG. 8A depicting the reflective state andFIG. 8B depicting the absorptive, non-reflective state. An air orliquid electrophoresis medium 204 is contained betweenmicrolens array 180 andreflective surface 190. Light absorbing particles (e.g. pigment particles—not shown) are suspended inelectrophoresis medium 204. A voltage is applied acrosselectrophoresis medium 204 as previously described in relation toFIG. 1A , withannular pattern segments 192 constituting the backplane electrode. In the reflective state (FIG. 8A ) the particles are attracted to and clumped aroundannular pattern segments 192. This leaves circular,reflective portions 194 ofsurface 190 substantially unobstructed by the particles, thus permitting reflection of light rays by the circular,reflective portions 194 ofsurface 190 as previously explained in relation toFIG. 6A . In the absorptive, non-reflective state (FIG. 8B ) the particles are attracted to and are distributed more or less uniformly across theinward surface 206 of microlens array 180 (which bears a transparent electrode—not shown). Light rays which pass throughmicrolens array 180 are absorbed by the particles at theinward surface 206 ofmicrolens array 180. -
FIGS. 9A and 9B schematically depict modulation of the embodiment ofFIGS. 6A and 6B by electro-deformation of aliquid medium 208 such as oil containing a light absorptive dye or dye mixture.FIG. 9A depicts the electro-deformed, reflective state.FIG. 9B depicts the relaxed, absorptive (non-reflective) state. Liquid electro-deformation is described by Aggarwal et al. in “Liquid Transport Based on Electrostatic Deformation of Fluid Interfaces” Journal of Applied Physics 99, 104904 published online 25 May 2006. - A voltage is applied across liquid medium 208 as previously described in relation to
FIG. 1A , with the backplane electrode conforming to the shape ofannular pattern segments 192.Liquid medium 208 always remains onreflective surface 190—liquid medium 208 does not cross the gap betweenmicrolens array 180 andreflective surface 190. In the electro-deformed, reflective state (FIG. 9A )liquid medium 208 is moved away from the circular,reflective portions 194 ofsurface 190 beneath the respective microlenses inarray 180, and is redistributed in approximately hemi-toroidal shapes atop each ofannular pattern segments 192 as seen inFIG. 9A . This leaves circular,reflective portions 194 ofsurface 190 substantially unobstructed by absorptiveliquid medium 208, thus permitting reflection of light rays by the circular,reflective portions 194 ofsurface 190 as previously explained in relation toFIGS. 6A and 6B. In the relaxed, absorptive (non-reflective) state (FIG. 9B )liquid medium 208 is distributed more or less uniformly acrossreflective surface 190, obstructing substantially allreflective portions 194 ofsurface 190. Light rays which are converged throughmicrolens array 180 towardsurface 190 are thus absorbed wherever they encountersurface 190. -
FIGS. 10A and 10B schematically depict electrophoretic modulation of theFIG. 7 embodiment, withFIG. 10A depicting the reflective state andFIG. 10B depicting the absorptive, non-reflective state. An air orliquid electrophoresis medium 250 is contained betweentransparent sheets microlens array 222. Light absorbing particles (e.g. pigment particles—not shown) are suspended inelectrophoresis medium 250. A voltage is applied acrosselectrophoresis medium 250 via transparent electrodes on the internally opposed sides ofsheets inward sheet 254 may conform to the shape ofmicrolens array 222's reflective annular regions (analogous to hemi-bead 60's reflective,annular region 80 shown inFIGS. 4A-4G ) or may be formed as an array of thin lines, etc. - In the reflective state (
FIG. 10A ) the particles are attracted to and clumped around the electrode segments oninward sheet 254, as indicated at 256. This leaves circular regions oninward sheet 254 substantially unobstructed by the particles, thus permitting semi-retro-reflection of light rays which are transmitted throughmicrolens array 222 and reflected bysurface 230 back throughmicrolens array 222. Some light rays (e.g. those labelled 258 and 260 inFIG. 10A ) are lost due to absorption, so it is desirable to minimize the area occupied by the electrode segments oninward sheet 254. - In the absorptive, non-reflective state (
FIG. 10B ) the particles are attracted to and are distributed more or less uniformly across the inward surface ofsheet 252, as indicated at 262. Light rays which pass through sheet 252 (i.e. substantially all incident light rays)) are absorbed by the particles at the inward surface ofsheet 252 as shown inFIG. 10B . -
FIGS. 11A and 11B schematically depict modulation of theFIG. 7 embodiment by electro-deformation of aliquid medium 270 such as oil containing a light absorptive dye or dye mixture. Liquid-medium 270 is contained betweentransparent sheets microlens array 222.FIG. 11A depicts the electro-deformed, reflective state.FIG. 11B depicts the relaxed, absorptive (non-reflective) state. A voltage is applied acrossliquid medium 270 via transparent electrodes on the internally opposed sides ofsheets 252, 254 (similar to the electrodes described above in relation toFIGS. 10A and 10B ). In the electro-deformed, reflective state (FIG. 11A )liquid medium 270 is redistributed in approximately hemi-toroidal shapes 272 atop the electrode segments oninward sheet 254. This leaves circular regions oninward sheet 254 substantially unobstructed, thus permitting-semi-retro-reflection of light rays which are transmitted throughmicrolens array 222 and reflected bysurface 230 back throughmicrolens array 222. Some light rays (e.g. those labelled 258 and 260 inFIG. 11A ) are lost due to absorption, so it is desirable to minimize the area occupied by the electrode segments oninward sheet 254. - In the relaxed, absorptive (non-reflective) state (
FIG. 11B )liquid medium 270 is distributed more or less uniformly across the inward surface ofsheet 252, as indicated at 274. Light rays which pass through sheet 252 (i.e. substantially all incident light rays)) are absorbed by liquid medium 270 at the inward surface ofsheet 252 as shown inFIG. 11B . - 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. All such modifications, permutations, additions and sub-combinations are within the true spirit and scope of the invention.
Claims (20)
1. A reflective display, comprising a rearward light reflector the light reflector further comprising a microlens array.
2. A reflective display as defined in claim 1 , wherein the microlens array further comprises a plurality of hemispherical microlenses.
3. A reflective display as defined in claim 1 , wherein the microlens array further comprises a plurality of approximately hemispherical microlenses.
4. A reflective display as defined in claim 1 , further comprising:
a reflective surface spaced inwardly from the microlens array;
an electrode on the reflective surface, the electrode having a plurality of annular segments, each annular segment aligned with one of the microlenses;
a light absorptive liquid medium on the reflective surface;
an electrical potential source electrically connected to apply an electrical potential across the liquid medium; and
wherein the liquid medium is oil.
5. A reflective display as defined in claim 1 , further comprising:
a reflective surface spaced inwardly from the microlens array;
an electrode on the reflective surface, the electrode having a plurality of annular segments, each annular segment aligned with one of the microlenses;
an electrophoresis medium contained between the microlens array and the reflective surface;
a plurality of light absorptive particles suspended in the electrophoresis medium; and
an electrical potential source electrically connected to apply an electrical potential across the electrophoresis medium.
6. A reflective display as defined in claim 5 , wherein the electrophoresis medium is a liquid.
7. A reflective display as defined in claim 5 , wherein the electrophoresis medium is air.
8. A reflective display as defined in claim 1 , wherein:
the electrophoresis medium is a liquid;
the liquid medium is oil;
the reflector further comprises a specular reflector; and
the specular reflector is spaced inwardly from the microlens array such that a substantial fraction of light rays refracted through the microlens array toward the specular reflector are approximately retro-reflected by the specular reflector through the microlens array.
9. A reflective display as defined in claim 3 , further comprising:
a reflective surface spaced inwardly from the microlens array;
a light absorptive liquid medium contained on an outward side of the microlens array between outward and inward transparent sheets;
an electrode on the inward sheet, the electrode having a plurality of segments, each segment aligned with one of the microlenses; and
an electrical potential source electrically connected to apply an electrical potential across the liquid medium.
10. A reflective display as defined in claim 9 , wherein the liquid medium is oil.
11. A reflective display as defined in claim 5 , wherein:
the reflector further comprises a specular reflector; and
the specular reflector is spaced inwardly from the microlens array such that a substantial fraction of light rays refracted through the microlens array toward the specular reflector are approximately retro-reflected by the specular reflector through the microlens array.
12. A reflective display as defined in claim 1 , wherein:
the microlens array further comprises a plurality of microlenses shaped such that a substantial fraction of light rays incident on the microlens array are redirected onto a reflective region beneath the microlenses;
the reflector further comprises a specular reflector; and
the specular reflector is spaced inwardly from the microlens array such that a substantial fraction of light rays refracted through the microlens array toward the specular reflector are approximately retro-reflected by the specular reflector through the microlens array.
13. A reflective display as defined in claim 11 , wherein the microlens array further comprises a plurality of approximately hemispherical microlenses.
14. A reflective display as defined in claim 2 , further comprising:
for substantially each one of the microlenses, a reflective region aligned with the one of the microlenses, and wherein substantially each one of the microlenses is shaped such that a substantial fraction of light rays incident on the one of the microlenses are redirected onto the reflective region aligned with the one of the microlenses.
15. A reflective display as defined in claim 2 , further comprising:
a reflective surface spaced inwardly from the microlens array;
an electrophoresis medium contained on an outward side of the microlens array between outward and inward transparent sheets;
an electrode on the inward sheet, the electrode having a plurality of segments, each segment aligned with one of the microlenses;
a plurality of light absorptive particles suspended in the electrophoresis medium;
an electrical potential source electrically connected to apply an electrical potential across the electrophoresis medium;
the reflector further comprises a specular reflector; and
the specular reflector is spaced inwardly from the microlens array such that a substantial fraction of light rays refracted through the microlens array toward the specular reflector are approximately retro-reflected by the specular reflector through the microlens array.
16. A reflective display as defined in claim 1 , further comprising:
for substantially each one of the microlenses, a reflective region aligned with the one of the microlenses, and wherein substantially each one of the microlenses is shaped such that a substantial fraction of light rays incident on the one of the microlenses are redirected onto the reflective region aligned with the one of the microlenses;
a reflective surface spaced inwardly from the microlens array;
a light absorptive liquid medium contained on an outward side of the microlens array between outward and inward transparent sheets;
an electrode on the inward sheet, the electrode having a plurality of segments, each segment aligned with one of the microlenses;
an electrical potential source electrically connected to apply an electrical potential across the liquid medium; and
wherein the liquid medium is oil.
17. A reflective display as defined in claim 15 , the microlens array further compriseing a plurality of microlenses shaped such that a substantial fraction of light rays incident on the microlens array are redirected onto a reflective region beneath the microlenses.
18. A reflective display as defined in claim 2 , the microlens array further comprising a plurality of microlenses shaped such that a substantial fraction of light rays incident on the microlens array are redirected onto a reflective region beneath the microlenses;
further comprising, for substantially each one of the microlenses, a reflective region aligned with the one of the microlenses, and wherein substantially each one of the microlenses is shaped such that a substantial fraction of light rays incident on the one of the microlenses are redirected onto the reflective region aligned with the one of the microlenses;
further comprising:
a reflective surface spaced inwardly from the microlens array;
a light absorptive liquid medium contained on an outward side of the microlens array between outward and inward transparent sheets;
an electrode on the inward sheet, the electrode having a plurality of segments, each segment aligned with one of the microlenses; and
an electrical potential source electrically connected to apply an electrical potential across the liquid medium.
19. A reflective display as defined in claim 2 , the microlens array further comprising a plurality of microlenses shaped such that a substantial fraction of light rays incident on the microlens array are redirected onto a reflective region beneath the microlenses;
further comprising, for substantially each one of the microlenses, a reflective region aligned with the one of the microlenses, and wherein substantially each one of the microlenses is shaped such that a substantial fraction of light rays incident on the one of the microlenses are redirected onto the reflective region aligned with the one of the microlenses;
further comprising:
a reflective surface spaced inwardly from the microlens array;
an electrophoresis medium contained on an outward side of the microlens array between outward and inward transparent sheets;
an electrode on the inward sheet, the electrode having a plurality of segments, each segment aligned with one of the microlenses;
a plurality of light absorptive particles suspended in the electrophoresis medium; and
an electrical potential source electrically connected to apply an electrical potential across the electrophoresis medium.
20. A reflective display as defined in claim 19 , wherein the electrophoresis medium is air.
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PCT/CA2007/000061 WO2007082370A1 (en) | 2006-01-17 | 2007-01-15 | Microlens-assisted brightness enhancement in reflective image displays |
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US12/161,045 Active 2027-07-24 US7760417B2 (en) | 2006-01-17 | 2007-01-15 | Brightness enhancement by fluid interface deformation in TIR-modulated displays |
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US20100128340A1 (en) | 2010-05-27 |
US7760417B2 (en) | 2010-07-20 |
WO2007082369A1 (en) | 2007-07-26 |
WO2007082370A1 (en) | 2007-07-26 |
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