WO2006108285A1

WO2006108285A1 - Brightness enhancement in tir-modulated electrophoretic reflective image displays

Info

Publication number: WO2006108285A1
Application number: PCT/CA2006/000559
Authority: WO
Inventors: Lorne A. Whitehead; Michele Ann Mossman; Helge Seetzen
Original assignee: The University Of British Columbia
Priority date: 2005-04-15
Filing date: 2006-04-12
Publication date: 2006-10-19
Also published as: JP2008535005A; KR20070120608A; KR100949412B1

Abstract

A reflective display having a plurality of transparent hemi-beads (60), each having a reflective region (80) surrounding a non-reflective region (82). Light absorptive particles (26) are suspended in and moved toward or away from the hemi-beads (60), selectably frustrating or facilitating total internal reflection of light rays incident on the hemi-beads (60). The display's reflectance is increased by selectably reflecting light rays through the hemi-beads' non-reflective regions (82), e.g. by forming a patterned electrode (48) with an electrically conductive region (104) and a plurality of reflective regions (108); aligning the electrode's reflective regions (108) with the hemi-beads' non-reflective regions (82); and applying a voltage through the electrode (48) across the medium (20) to electrophoretically move the particles (26) between a position in which they substantially cover the hemi-beads (60), and another position in which they substantially cover the electrode's conductive region (104) without covering the electrode's reflective regions (108).

Description

BRIGHTNESS ENHANCEMENT IN TIR-MODULATED ELEC- TROPHORETIC REFLECTIVE IMAGE DISPLAYS

Reference to Related Applications [0001] This application claims the benefit of United States provisional patent application serial no. 60/671,538 filed 15 April 2005, and claims the benefit of United States provisional patent application serial no. 60/759,772 filed 17 January 2006.

Technical Field

[0002] This application pertains to brightness enhancement of reflective image displays of the type described in United States Patent Nos. 5,999,307; 6,064,784; 6,215,920; 6,865,011; 6,885,496 and 6,891 ,658; all of which are incorporated herein by reference.

Background

[0003] Figure IA depicts a portion of a prior art reflective (i.e. front-lit) electrophoretically frustrated total internal reflection (TIR) modulated display 10 of the type described in United States Patent 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. η_y > ~ 1.90) transparent spherical or approximately spherical beads 14 in the inward surface of a high refractive index (e.g. η₂ > - 1.75) polymeric material 16 having a flat outward 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 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. [0004] 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 Fluorinert™ perfluor- inated hydrocarbon liquid (^₃- 1.27) available from 3M, St. Paul, MN 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. [0005] As is well known, the TIR interface between two media having different refractive indices is characterized by a critical angle θ_c. Light rays incident upon the interface at angles less than θ_c are transmitted through the interface. Light rays incident upon the interface at angles greater than θ_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.

[0006] In the absence of electrophoretic activity, as is illustrated to the right of dashed line 28 in Figure IA, a substantial fraction of the light rays passing through sheet 12 and beads 14 undergoes TIR at the inward side of beads 14. For example, incident light rays 30, 32 are refracted through material 16 and beads 14. The rays undergo TIR two or more times at the bead:liquid TIR interface, as indicated at points 34, 36 in the case of ray 30; and indicated at points 38, 40 in the case of ray 32. The totally internally reflected rays are then refracted back through beads 14 and material 16 and emerge as rays 42, 44 respec- tively, achieving a "white" appearance in each reflection region or pixel.

[0007] 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. within 0.25 micron of the inward surfaces of inwardly protruding beads 14, or closer). When electrophoretically moved as aforesaid, particles 26 scatter or absorb light, thus frustrating or modulating TIR by modifying the imaginary and possibly the real component of the effective refractive index at the bead: liquid TIR interface. This is illustrated by light rays 52, 54 which are scattered and/or absorbed as they strike 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 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. [0008] As described above, 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 seg- mented to control the electrophoretic activation of medium 20 across separate regions or pixels of sheet 12, thus forming an image. - A -

[0009] Figure 2 depicts, in enlarged cross-section, an inward hemispherical or "hemi-bead" portion 60 of one of spherical beads 14. Hemi-bead 60 has a normalized radius r = 1 and a refractive index η_x. 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 O₁ relative to radial axis 66. For purposes of this theoretically ideal discussion, it is assumed that material 16 has the same refractive index as hemi-bead 60 (i.e. ηι = η₂), so ray 62 passes from material 16 into hemi-bead 60 without refraction. Ray 62 is refracted at the inward surface of hemi-bead 60 and passes into electrophoretic medium 20 as ray 64 at an angle θ₂ relative to radial axis 66. [0010] Now consider incident light ray 68 which is perpendicularly

incident (through material 16) on hemi-bead 60 at a distance a = —

from hemi-bead 60 's centre C. Ray 68 encounters the inward surface of hemi-bead 60 at the critical angle θ_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 θ_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 θ_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.

[0011] All light rays which are incident on hemi-bead 60 at distances a≥a_c 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 characteris- tic giving it a white appearance, which is desirable in reflective display applications. Figures 3 A, 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. [0012] In Figure 3A, light rays incident within a range of distances a_c<a≤a₁ undergo TIR twice (the 2-TIR mode) and the reflected rays diverge within a comparatively wide arc φ_} centered on a direction opposite to the direction of the incident light rays. In Figure 3B, light rays incident within a range of distances a_} <a <a₂ undergo TIR three times (the 3-TIR mode) and the reflected rays diverge within a narrower arc φ₂ < φ₁ which is again centered on a direction opposite to the direction of the incident light rays. In Figure 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 centered 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, causing display 10 to have a diffuse appearance akin to that of paper. [0013] Display 10 has relatively high apparent brightness, compa- rable to that of paper, when the dominant source of illumination is behind the viewer, within a small angular range. This is illustrated in Figure IB which depicts the wide angular range a over which viewer V is able to view display 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):

2

R = I - % (1)

^■ 7, where η_x is the refractive index of hemi-bead 60 and 77₃ 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 (7Z₁- 1.59) and if the adjacent medium is Fluorinert (T7₃~ 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 (η_{~ 1.92) a reflectance R of about 56% is attained. When illumination source S (Figure IB) is positioned behind viewer Vs head, the apparent brightness of display 10 is further enhanced by the aforemen- tioned semi-retro-reflective characteristic.

[0014] As shown in Figures 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. For example, Figure 4 A shows hemi-bead 60 as seen from perpen- dicular incidence— that is, from an incidence angle offset 0° from the perpendicular. In this case, 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. The annulus surrounds a circu- lar 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. Figures 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 Figures 4B-4G with Figure 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. Figure 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. [0015] 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 Figure IA 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 Figure 5 yields a packing efficiency /*π/(6-tan 30°) ~ 90.7% assuming beads 14 are of uniform size.

[0016] 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 Figure 5 HCP arrangement of uniform size hemi-beads, and assumes the use of materials which yield a refractive index ratio 77/ η₃ = 1.5. These factors are not to be considered as limiting the scope of this disclosure. [0017] As previously explained in relation to Figure 2, a substantial portion of light rays which are perpendicularly incident on the flat outward face of hemi-bead 60 at distances a <a_c 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 by prior art display 10, yielding a dark non-reflective circular region 82 (Figures 4 A— 4G) on hemi-bead 60. Figure 5 depicts a plural- ity of these dark non-reflective regions 82, each of which is surrounded by a reflective annular region 80, as previously explained. [0018] 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, η_{l t} of hemi- bead 60 to the refractive index, η₃, 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 η_{l t} of hemi-bead 60 to that of the adjacent medium 77₃. For example, the average surface reflectance, R, of a hemispherical water drop (77_! ~ 1.33) in air (77₃ ~ 1.0) is about 43% ; the average surface reflectance, R, of a glass hemisphere (η_x ~ 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% .

[0019] 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 (Figure IA), interstitial gaps 84 (Figure 5) unavoidably remain between adjacent beads. Light rays incident upon any of gaps 84 are "lost" in the sense that they pass directly into electro- phoretic 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.

[0020] 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 R = I - Ih. by a "semi-retro-reflective enhancement factor" of about

⁷Ix

1.5 (see "A High Reflectance, Wide Viewing Angle Reflective Display Using Total Internal Reflection in Micro-Hemispheres," Mossman, M. A. et ai , Society for Information Display, 23rd International Display Research Conference, pages 233-236, September 15-18, 2003, Phoenix, AZ). For example, in a system where the refractive index ratio 77/ 77.₅ = 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. [0021] Individual hemi-beads 60 can be invisibly small, within the range of 2-50 μm in diameter, and as shown in Figure 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. In these regions 80, where TIR can occur, particles 26 (Figure IA) do not impede the reflection of incident light when they are not in contact with the inward, hemispherical portions of beads 14. However, in 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 η_j/η₃ 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.

[0022] Disregarding the aforementioned semi-retro-reflective enhancement factor, a system having a refractive index ratio T)₁It]₃ = 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 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 (Figure IA). [0023] For example, since display 10' s maximum surface reflect- ance 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). [0024] Display 10's surface reflectance can be increased, as de- scribed below, improving the appearance of the display.

[0025] 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.

Brief Description of Drawings

[0026] 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. [0027] Figure IA is a greatly enlarged, not to scale, fragmented cross-sectional side elevation view, of a portion of an electrophoretically frustrated or modulated prior art reflective image display. [0028] Figure IB schematically illustrates the wide angle viewing range a of the Figure IA display, and the angular range β of the illumi- nation source. [0029] Figure 2 is a greatly enlarged, cross-sectional side elevation view of a hemispherical ("hemi-bead") portion of one of the spherical beads of the Figure IA apparatus.

[0030] Figures 3A, 3B and 3C depict semi-retro-reflection of light rays perpendicularly incident on the Figure 2 hemi-bead at increasing off- axis distances at which the incident rays undergo TIR two, three and four times respectively.

[0031] Figures 4A, 4B, 4C, 4D, 4E, 4F and 4G depict the Figure 2 hemi-bead, as seen from viewing angles which are offset 0°, 15°, 30°, 45°, 60°, 75° and 90° respectively from the perpendicular.

[0032] Figure 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 Figure 1 display, showing the spherical beads arranged in a hexagonal closest packed (HCP) structure. [0033] Figures 6 A and 6B are top plan views, on a greatly enlarged scale, of two alternative backplane electrode patterns for use with the Figure 5 structure.

[0034] Figures 7A and 7B are fragmented cross-sectional side elevation views, on a greatly enlarged scale, of a portion of an electro- phoretically frustrated (i.e. modulated) reflective image display incorporating the Figure 6A backplane electrode pattern. [0035] Figure 8 is a greatly enlarged, not to scale, cross-sectional side elevation view of a portion of an electrophoretically frustrated or modulated reflective image display incorporating electrophoretically suspended absorptive and reflective particles.

[0036] Figure 9 is a greatly enlarged, not to scale, cross-sectional side elevation view of a portion of an electrophoretically frustrated or modulated reflective image display incorporating a reflective porous membrane. [0037] Figure 10 is a greatly enlarged, not to scale, cross-sectional side elevation view of a portion of an electrophoretically frustrated or modulated reflective image display incorporating excess polymer material in the interstices between adjacent hemi-beads.

Description [0038] 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.

[0039] Backplane electrode 48 can be formed on sheet 24 using either one of patterns 100 or 102 depicted in Figures 6 A or 6B respectively. Black regions 104, 106 are electrically conductive regions, and may be either reflective or non-reflective. White regions 108, 110, 112 are reflective regions, and may be either electrically conductive or non- conductive— provided there is no electrical conductivity between regions 108, 110, 112 on the one hand and regions 104, 106 on the other hand. [0040] Reflective regions 108, 110 are each preferably circular in shape, and have a diameter greater than or equal to (preferably equal to) the diameter of one of the non-reflective, circular regions 82 of one of hemi-beads 60. Pattern 100's regions 104 have an overall size and shape substantially similar to the overall size and shape of regions 80, 84 of hemi-beads 60. [0041] The optical properties of regions 104, 106 are relatively unimportant, as are those of sheet 24. It may however be advantageous to provide a reflective outward surface on sheet 24 and to form regions 104 (or 106) thereon, with the remaining portions of sheet 24's reflective outward surface constituting regions 108 (or 110, 112). [0042] When used as explained below, patterned backplane electrode 100 decreases absorptive losses due to light absorption in regions 82, but does not decrease absorptive losses due to light absorption in gap regions 84. By contrast, when used as explained below, patterned backplane electrode 102 decreases absorptive losses due to light absorption in both regions 82 and 84. This is achieved by forming pattern 102 with each one of reflective regions 112 having a size and shape which is substantially similar to the size and shape of one of gaps 84, with each region 112 in the same location relative to its adjacent reflective regions 110 as the location of a corresponding one of gaps 84 relative to that gap's adjacent regions 82. [0043] Patterned backplane electrode 100 (or 102) is positioned with respect to monolayer 18 to align each circular reflective region 108 (or 110) with a corresponding one of non-reflective, circular regions 82; thereby also aligning electrically conductive region 104 (or 106) with reflective regions 80. [0044] When electrophoresis medium 20 is activated by actuating voltage source 50 to apply a voltage between electrodes 46 and 48, particles 26 substantially cover the inward surfaces of monolayer 18's hemi-beads 60 as shown in Figure 7 A (which depicts the non-reflective state utilizing patterned backplane electrode 100). Particles 26 absorb light rays (e.g. ray 114) which are incident upon reflective annular region 80 by frustrating or modulating TIR as aforesaid, and also absorb light rays (e.g. rays 116) which do not undergo TIR and which would otherwise pass through beads 14. Particles 26 need not completely cover the inward surfaces of hemi-beads 60, since as previously explained in relation to Figure 2 many incident light rays interact several times with hemi-bead 60 so substantial coverage results in an acceptable level of absorption.

[0045] In the reflective state— shown in Figure 7B— particles 26 are attracted to the electrically conductive regions 104 of patterned backplane electrode 100 (or to the electrically conductive regions 106 of a patterned backplane electrode 102). Since regions 104 are aligned with reflective annular regions 80, particles 26 are hidden from view (i.e. because light ray 114 which would otherwise illuminate particles 26 is reflected by regions 80). Light rays 116, which do not undergo TIR, but which are instead transmitted through hemi-beads 60, strike one of reflective regions 108 and are therefore also reflected.

[0046] If hemi-bead monolayer 18 is positioned an appropriate distance above reflective regions 108, the transmitted light rays are focused toward reflective annular regions 80, such that the light rays are returned approximately in the direction from which they came. This further enhances the display's semi-retro-reflective characteristic, and can result in a perceived reflectance value exceeding 100% . Even with the absorptive losses associated with a red-green-blue (RGB) colour filter array, patterned backplane electrodes 100, 102 facilitate production of reflective image displays having a brightness comparable to that of coloured ink on white paper.

[0047] Figure 8 depicts an alternative display brightness (i.e. reflectance) enhancement technique in which absorptive particles 26 are commingled, within electrophoretic medium 20, with a finely dispersed suspension of reflective beads or particles 118. The average diameter of reflective beads 118 is substantially greater (e.g. about 10 times greater) than the average diameter of absorptive particles 26. Reflective beads 118 can be electrostatically neutral so that they will not be affected by an applied electric field. Alternatively, reflective beads 118 can have an electrostatic charge opposite to that of absorptive particles 26, such that beads 118 will move in the opposite direction from particles 26 when subjected to an applied electric field. Although it may seem counterintuitive to maintain a stable suspension of oppositely-charged particles, this can be achieved by using suitable stabilizing dispersants (see Amund- son, K. , et ai , "Microencapsulated Electrophoretic Materials for Electronic Paper Displays," Society for Information Display, 20th Interna- tional Display Research Conference Proceedings, pages 84-87, September 25-28, 2000, Palm Beach, FL). Reflective beads 118 can be any substantially reflective (e.g. white) granular material having a suitable granular size distribution, although high refractive index materials such as titanium dioxide {η ~ 2.4) are preferred.

[0048] In the absence of electrophoretic activity, as is illustrated to the left of dashed line 28 in Figure 8, the smaller absorptive particles 26 tend to settle toward lower sheet 24, beneath the larger reflective beads 118. Reflectance is thus increased, since incident light rays (e.g. ray 120) which would otherwise have been absorbed by non-reflective circular regions 82 are instead reflected (e.g. ray 122) by beads 118. Light rays (e.g. ray 124) which are incident upon reflective annular regions 80 are totally internally reflected (e.g. ray 126) as previously explained. [0049] When a voltage is applied across medium 20, as is illustrated to the right of dashed line 28 in Figure 8, the smaller absorptive particles 26 are electrophoretically moved through the interstices between beads 118 to the inward surfaces of hemi-beads 60. When so moved into this absorptive state, particles 26 absorb light rays (e.g. ray 128) which are incident upon reflective annular region 80 by frustrating or modulating TIR as aforesaid, and also absorb light rays (e.g. ray 130) which do not undergo TIR and which would otherwise pass through beads 14. Reflective beads 118 accordingly form a porous filter, allowing absorptive particles 26 to move outwardly into contact with hemi-beads 60 in the absorptive state; and to move inwardly away from hemi-beads 60 in the reflective state, thus obscuring absorptive particles 26 from direct view in the reflective state. Persons skilled in the art will understand that although Figure 8 depicts reflective beads 118 as spherically shaped, such shape is not essential— beads 118 can be of arbitrary shape. [0050] The Figure 8 technique affords benefits besides brightness enhancement. For example, if reflective beads 118 are provided in sufficiently high density, they tend to impede long term lateral movement of absorptive particles 26, thus slowing agglomeration of absorptive particles 26. Such agglomeration can cause image degradation in electrophoretic image displays. [0051] The brightness enhancement (i.e. reflectance) attainable via the Figure 8 technique can be estimated. For example, if reflective beads 118 are assumed to have a diffuse reflectance of about 40% , and if reflective beads 118 are also assumed to affect the entirety of the previously explained 50% absorptive loss area, a brightness enhancement of about 20% (i.e. 50% of 40%) is attained.

[0052] Figure 9 depicts a further alternative display brightness (i.e. reflectance) enhancement technique in which a reflective, porous, membrane 140 is provided between the inward surfaces of hemi-beads 60 and lower sheet 24. The average diameter of the pores in membrane 140 is substantially greater (e.g. about 10 times greater) than the average diameter of absorptive particles 26. The pores in membrane 140 constitute a sufficiently large fraction (e.g. at least 20%) of the total surface area of membrane 140 to permit substantially unimpeded passage of absorptive particles 26 through membrane 140. Membrane 40 can be formed of a porous membrane material such as a polycarbonate or fibre- weave membrane. Membrane 140's outward surface 142 is highly reflective, and may be either diffusely or specularly reflective. A suitably reflective membrane 140 can be formed from an intrinsically refiec- tive material such as a multilayer broadband reflector (e.g. Multilayer Optical Film available from 3M, St. Paul, MN) or aluminized Mylar™ flexible film, or by coating outward surface 142 with a reflective (e.g. aluminum) film using standard vapour deposition techniques. [0053] In the absence of electrophoretic activity, as is illustrated to the left of dashed line 28 in Figure 9, the smaller absorptive particles 26 tend to settle through membrane 140's pores, toward lower sheet 24. Reflectance is thus increased, since incident light rays (e.g. ray 144) which would otherwise have been absorbed by non-reflective circular regions 82 are instead reflected (e.g. ray 146) by membrane 140's reflective outward surface 142. Light rays (e.g. ray 148) which are incident upon reflective annular regions 80 are totally internally reflected (e.g. ray 150) as previously explained.

[0054] When a voltage is applied across medium 20, as is illustrated to the right of dashed line 28 in Figure 9, absorptive particles 26 are electrophoretically moved through membrane 140's pores to the inward surfaces of hemi-beads 60. When so moved into this absorptive state, particles 26 absorb light rays (e.g. ray 152) which are incident upon reflective annular region 80 by frustrating or modulating TIR as aforesaid, and also absorb light rays (e.g. ray 154) which do not undergo TIR and which would otherwise pass through beads 14. Membrane 140's pores allow absorptive particles 26 to move outwardly into contact with hemi-beads 60 in the absorptive state; and to move inwardly away from hemi-beads 60 in the reflective state, thus obscuring absorptive particles 26 from direct view in the reflective state. [0055] The brightness enhancement (i.e. reflectance) attainable via the Figure 9 technique can be estimated. For example, if membrane 140's outward surface 142 is assumed to have an overall reflectance of about 60% , and is also assumed to affect the entirety of the previously explained 50% absorptive loss area, a brightness enhancement of about 30% (i.e. 50% of 60%) is attained.

[0056] Figure 10 depicts another alternative display brightness (i.e. reflectance) enhancement technique in which outward sheet 12 's interstitial regions 160 between hemi-beads 60 are modified to increase reflectance. This is achieved by partially embedding spherical beads 14 in outward sheet 12 such that the reflective polymer material used to form sheet 12 protrudes inwardly, as indicated at 162, in an approximately hemispherical shape, through interstitial regions 160 and between the hemi-bead portions 60 of spherical beads 14.

[0057] If reflective polymeric structures 162 each have a "perfect" hemispherical shape (which is theoretically ideal, but unattainable in practice), then the light reflecting and absorption characteristics of polymeric structures 162 will be identical to those of hemi-beads 60 as explained above. Although polymeric structures 162 are preferably hemispherically shaped in order to achieve the desired reflectance characteristics, they need not be perfectly hemispherical. Polymeric structures 162 need only be substantially hemispherical in that their inward surfaces should have sufficiently high curvature to cause TIR of incident light rays. TIR which occurs in polymeric structures 162 can be frustrated by absorptive particles 26 in the same manner as previously described in relation to hemi-beads 60. [0058] TIR does not normally occur in interstitial regions 160, thus reducing sheet 12' s overall reflectance. If hemi-beads 60 have a hexag- onally closest packed arrangement, their overall average surface reflectance is 91 % as previously explained, with the remaining 9% being lost due to light absorption in interstitial regions 160. By facilitating TIR in interstitial regions 160, the Figure 10 brightness enhancement technique reduces this 9% loss by theoretically increasing to close to 100% the percentage of sheet 12 which bears useful light reflecting structures. [0059] Instead of partially embedding spherical beads 14 in outward sheet 12, brightness can be enhanced by minimizing the size of interstitial regions 160. For example, by employing a polymer material such as polycarbonate having plastic deformation characteristics such that the uncured or softened resin material inherently forms hemispherical structures, one may fabricate both hemi-beads 60 and polymeric structures 162 as a single integral array, avoiding the need for high precision casting molds. [0060] The Figure 10 brightness enhancement technique can be used in combination with any one of the Figure 7A-7B, 8 or 9 brightness enhancement techniques to further enhance display brightness. [0061] 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. It is therefore intended that the following appended claims and claims hereafter introduced are interpreted to include all such modifications, permutations, additions and sub-combinations as are within their true spirit and scope.

Claims

WHAT IS CLAIMED IS:

1. A reflective display, comprising:

(a) a plurality of transparent hemi-beads (60) protruding in- wardly from an inward surface of a transparent sheet (12) having an outward viewing surface (17), each hemi-bead (60) having a reflective region (80) surrounding a non-reflective region (82);

(b) a second sheet (24) spaced inwardly from the transparent sheet (12) to define a reservoir between the transparent sheet

(12) and the second sheet (24);

(c) an electrophoresis medium (20) within the reservoir;

(d) a plurality of light absorptive particles (26) suspended in the medium; and (e) means for selectably reflecting light rays from the second sheet (24) through the non-reflective regions (82) of the hemi-beads (60).

2. A reflective display as defined in claim 1, the means for selectably reflecting light rays further comprising an electrode (48) formed on an outward side of the second sheet (24) in a pattern (100 or 102) comprising:

(i) an electrically conductive region (104 or 106); and (b) a first plurality of reflective regions (108 or 110); each one of the first plurality of reflective regions (108 or 110) of the second sheet (24) corresponding to and aligned with a corresponding one of the non-reflective regions (82) of the hemi-beads (60).

3. A reflective display as defined in claim 2, wherein each one of the first plurality of reflective regions (108 or 110) of the second sheet (24) has a size and shape substantially similar to a size and shape of the corresponding one of the non-reflective regions (82) of the hemi-beads (60).

4. A reflective display as defined in claim 3, wherein the electrically conductive region (104 or 106) has an overall size and shape substantially similar to an overall size and shape of the reflective regions (80) of the hemi-beads (60).

5. A reflective display as defined in claim 4, wherein: each one of the hemi-beads (60) is adjacent to another one or more of the hemi-beads (60), the display further comprising a non- reflective gap (84) between each adjacent one or more of the hemi- beads (60); the pattern (100 or 102) further comprising a second plurality of reflective regions (112) on the outward side of the second sheet (24); and each one of the second plurality of reflective regions (112) corresponds to and is aligned with a corresponding one of the gaps (84).

6. A reflective display as defined in claim 5, wherein each one of the second plurality of reflective regions (112) has a size and shape substantially similar to a size and shape of the corresponding one of the gaps (84).

7. A reflective display as defined in claim 4, wherein: each one of the non-reflective regions (82) of the hemi-beads (60) has a circular shape having a first diameter; and each one of the first plurality of reflective regions (108 or 110) of the pattern (100 or 102) has a circular shape having a second diameter substantially equal to the first diameter.

8. A reflective display as defined in claim 7, wherein each one of the reflective regions (80) of the hemi-beads (60) has an annular shape.

9. A reflective display as defined in claim 1, the means for selectably reflecting light rays further comprising a plurality of reflective particles (118) suspended in the medium (20).

10. A reflective display as defined in claim 9, wherein the reflective particles (118) have an average diameter substantially greater than an average diameter of the absorptive particles (26).

11. A reflective display as defined in claim 10, wherein the reflective particles (118) have an average diameter about 10 times greater than the average diameter of the absorptive particles (26).

12. A reflective display as defined in claim 10, wherein the reflective particles (118) are electrostatically neutral.

13. A reflective display as defined in claim 10, wherein the reflective particles (118) have an electrostatic charge opposite to an electro- static charge of the absorptive particles (26).

14. A reflective display as defined in claim 10, wherein the reflective particles (118) are white granules.

15. A reflective display as defined in claim 10, wherein the reflective particles (118) are titanium dioxide particles.

16. A reflective display as defined in claim 1, the means for selectably reflecting light rays further comprising a reflective, porous, membrane (140) between the hemi-beads (60) and the second sheet (24).

17. A reflective display as defined in claim 16, wherein the membrane (140) further comprises pores having an average diameter substantially greater than an average diameter of the absorptive particles (26).

18. A reflective display as defined in claim 17, wherein the pores have an average diameter about 10 times greater than the average diameter of the absorptive particles (26).

19. A reflective display as defined in claim 17, wherein the membrane (140) has a surface area, the pores comprising a sufficiently large fraction of the surface area to permit substantially unimpeded passage of the absorptive particles (26) through the membrane (140).

20. A reflective display as defined in claim 16, wherein the membrane (140) further comprises a diffusely reflective outward surface (142).

21. A reflective display as defined in claim 16, wherein the membrane (140) further comprises a specularly reflective outward surface (142).

22. A reflective display as defined in claim 1, wherein each one of the hemi-beads (60) is adjacent to another one or more of the hemi- beads (60), the means for selectably reflecting light rays further comprising a reflective structure (162) between each adjacent one or more of the hemi-beads (60).

23. A reflective display as defined in claim 22, wherein each one of the reflective structures (162) protrudes inwardly from the inward surface of the transparent sheet (16).

24. A reflective display as defined in claim 23, wherein each one of the reflective structures (162) has an approximately hemispherical shape.

25. A reflective display as defined in claim 23, wherein each one of the reflective structures (162) has sufficiently high curvature to cause total internal reflection of a substantial fraction of light rays incident on the reflective structure (162).

26. A method of increasing the reflectance of a reflective display having a plurality of transparent hemi-beads (60) protruding inwardly from an inward surface of a transparent sheet (16) having an outward viewing surface (17), each hemi-bead (60) having a reflective region (80) surrounding a non-reflective region (82), a second sheet (24) spaced inwardly from the transparent sheet (16) to define a reservoir between the transparent sheet (16) and the second sheet (24), an electrophoresis medium (20) within the reservoir, and a plurality of light absorptive particles (26) suspended in the medium (20), the method comprising selectably reflecting light rays through the non-reflective regions of the hemi- beads (60).

27. A method as defined in claim 26, wherein selectably reflecting light rays through the non-reflective regions of the hemi-beads (60) further comprises: forming an electrode (48) on an outward side of the second sheet (24) in a pattern (100 or 102) comprising:

(i) an non-reflective region (104 or 106); (ii) a first plurality of reflective regions (108 or 110); aligning each one of the first plurality of reflective regions (108 or 110) with a corresponding one of the non-reflective regions (82) of the hemi-beads (60); and applying a voltage across the medium (20) to selectably, electrophoretically move the absorptive particles (26) between a first position in which the absorptive particles (26) substantially cover inward surfaces of the hemi-beads (60), and a second posi- tion in which the absorptive particles (26) substantially cover the non-reflective region (104 or 106) of the electrode (48) without covering the first plurality of reflective regions (108 or 110).

28. A method as defined in claim 27, further comprising spacing the transparent sheet (12) at a distance from the second sheet (24), the distance selected such that an incident light ray reflected by one of the first plurality of reflective regions (108 or 110) of the electrode (48) is reflected in a direction substantially opposite to an incidence direction of the incident light ray.

29. A method as defined in claim 27, further comprising forming each one of the first plurality of reflective regions (108 or 110) in a size and shape substantially similar to a size and shape of one of the non-reflective regions (82) of the hemi-beads (60).

30. A method as defined in claim 27, further comprising forming the non-reflective region (104 or 106) in an overall size and shape substantially similar to an overall size and shape of the reflective regions (80) of the hemi-beads (60).

31. A method as defined in claim 30, wherein: each one of the hemi-beads (60) is adjacent to another one or more of the hemi-beads (60); a non-reflective gap (84) exists between each adjacent one or more of the hemi-beads (60); the pattern further comprising a second plurality of reflective regions (112), each one of the second plurality of reflective regions (112) having a size and shape substantially similar to a size and shape of one of the gaps (84).

32. A method as defined in claim 30, wherein each one of the non- reflective regions (82) of the hemi-beads (60) has a circular shape having a first diameter, the method further comprising forming each one of the first plurality of reflective regions (108 or 110) in a circular shape having a second diameter substantially equal to the first diameter.

33. A method as defined in claim 26, wherein selectably reflecting light rays through the non-reflective regions (82) of the hemi-beads (60) further comprises: suspending a plurality of reflective particles (118) in the medium (20); applying a voltage across the medium (20) to selectably, electrophoretically move the absorptive particles (26) through the reflective particles (118) between a first position in which the absorptive particles (26) substantially cover inward surfaces of the hemi-beads (60), and a second position in which the reflective particles (118) are between the hemi-beads (60) and the absorptive particles (26).

34. A method as defined in claim 33, wherein the reflective particles (118) have an average diameter substantially greater than an average diameter of the absorptive particles (26).

35. A method as defined in claim 34, wherein the reflective particles (118) have an average diameter about 10 times greater than the average diameter of the absorptive particles (26).

36. A method as defined in claim 33, further comprising electrostatically neutrally charging the reflective particles (118).

37. A method as defined in claim 33, further comprising charging the reflective particles (118) with an electrostatic charge opposite to an electrostatic charge of the absorptive particles (26).

38. A method as defined in claim 26, wherein selectably reflecting light rays through the non-reflective regions (82) of the hemi-beads (60) further comprises: providing a reflective, porous, membrane (140) in the medium (20), between the hemi-beads (60) and the second sheet (24); applying a voltage across the medium (20) to selectably, electrophoretically move the absorptive particles (26) through the membrane (140) between a first position in which the absorptive particles (26) substantially cover inward surfaces of the hemi-beads (60), and a second position in which the membrane (140) is between the hemi-beads (60) and the absorptive particles (26).

39. A method as defined in claim 38, wherein the membrane (140) further comprises pores having an average diameter substantially greater than an average diameter of the absorptive particles (26).

40. A method as defined in claim 39, wherein the pores have an average diameter about 10 times greater than the average diameter of the absorptive particles (26).

41. A method as defined in claim 38, wherein the membrane (140) has a surface area, the pores comprising a sufficiently large fraction of the surface area to permit substantially unimpeded passage of the absorptive particles (26) through the membrane (140).

42. A method as defined in claim 26, wherein: 5 each one of the hemi-beads (60) is adjacent to another one or more of the hemi-beads (60); selectably reflecting light rays through the non-reflective regions (82) of the hemi-beads further comprises: providing a reflective structure (162) between each adjacent 0 one or more of the hemi-beads (60); and applying a voltage across the medium (20) to selectably, electrophoretically move the absorptive particles (26) between a first position in which the absorptive particles (26) substantially cover inward surfaces of the hemi-beads (60) and the reflective 5 structures (162), and a second position in which the absorptive particles (26) do not cover the hemi-beads (60) or the reflective structures (162).

43. A method as defined in claim 42, further comprising forming the Cι reflective structures (162) to protrude inwardly from the inward surface of the transparent sheet (12).

44. A method as defined in claim 43, further comprising forming the reflective structures (162) in an approximately hemispherical shape.

5 45. A method as defined in claim 43, further comprising forming the reflective structures (162) with sufficiently high curvature to cause total internal reflection of a substantial fraction of light rays incident on the reflective structures (162).

C¹ 46. A method as defined in claim 43, further comprising forming the hemi-beads (60) and the reflective structures (162) by: softening the transparent sheet (12); and partially embedding closely spaced spherical beads in the transparent sheet (12). 5

47. A method as defined in claim 43, further comprising forming the hemi-beads (60) and the reflective structures (162) by: providing a circular apertured mesh; softening the transparent sheet (12); 0 applying pressure to the transparent sheet (12); and pressing the mesh into the transparent sheet (12).