WO1998032047A1

WO1998032047A1 - Ambient illuminated electro-optic display device

Info

Publication number: WO1998032047A1
Application number: PCT/US1998/000860
Authority: WO
Inventors: George T. Valliath; Robert B. Akins; Kevin W. Jelley
Original assignee: Motorola Inc.
Priority date: 1997-01-16
Filing date: 1998-01-16
Publication date: 1998-07-23
Also published as: AU6027998A; GB2325330B; DE19880175T1; GB2325330A; GB9820095D0

Abstract

An electro-optic display device (10) includes an electro-optic cell (14), such as a liquid crystal cell, a reflector (16), and a transmission holographic optical element (12). The electro-optic cell (14) includes a cell front (56), a cell back (58) opposite the cell front (56), and at least one cell region having a transparent mode. The reflector (16, 46) receives light that traverses the cell region and reflects the light toward the cell front (56). The transmission holographic optical element (12) is optically coupled to the cell front (56) and includes a front face (24). The holographic optical element (12) receives a light ray (26) at a first angle (31) and a second reflected light ray and diffracts the reflected light ray, such that the reflected light ray is emitted from the front face (24) at a third angle (34) that is distinct from the first angle (31), thereby increasing the contrast and apparent brightness of the display device (10).

Description

AMBIENT ILLUMINATED ELECTRO-OPTIC DISPLAY DEVICE

Field of the Invention

This invention relates generally to an electro- optic display device, such as a liquid crystal display device, or the like, wherein the display device is illuminated by reflected ambient light. More particularly, this invention relates to such device, wherein a transmission holographic optical element is utilized in combination with a reflector to direct light for enhanced brightness of the display.

Background of the Invention

A liquid crystal display (LCD) device includes a layer of liquid crystal material that is suitable for forming a display that is viewed through a front face of the device. Transparent electrodes are affixed to the inner surfaces of the liquid crystal cell to alter the light transmission properties of the liquid crystal material in the intermediate region of the cell. In this manner, the electrodes define pixels that create the display. As used herein, pixels refer to a region of the liquid crystal cell that forms a light or dark area of the display.

One approach for providing light utilizes ambient light and comprises a reflector facing the back face of the LCD device. Ambient light strikes the front surface of the front polarizer. The light is transmitted toward the liquid crystal cell, and passes through the liquid crystal cell. Depending on the electric field applied to the transparent electrodes defining a pixel, the state of the liquid crystal in the cell is altered, causing a polarization change in the light passing through the cell. In the transparent regions of the display, the polarization of the light is altered in such a manner as to cause most of the polarized light to pass through the back polarizer. In the opaque regions of the display, the polarization of the light is altered in such a manner as to cause most of the polarized light to be absorbed by the back polarizer. The light which passes through the back polarizer illuminates the reflector and is reflected by the reflector towards the liquid crystal cell.

The light reflected by the reflector strikes the back face of the back polarizer. Some of this light is absorbed, while the remaining light is transmitted toward and passes through the liquid crystal cell. In this manner, LCD devices alternate pixels between bright and opaque states, thereby allowing alphanumeric and other symbols to be displayed on the LCD device to convey information.

A benefit of reflective LCD devices is that they consume little power, since they do not need a backlight element and utilize ambient light to provide illumination for the display. However, problems exist regarding reflective LCD devices. A first problem is insufficient lighting produced by the reflector. Ambient light sources are typically localized sources with high luminance. The amount of light available to illuminate the display is fixed for a given location and orientation of the display, with respect to the light sources and viewer, by the number and luminance of the ambient light sources available to illuminate the display. Usually only one or two sources provide significant light to illuminate the display. Another problem with reflective LCD devices relates to glare inherent in a preferential viewing angle. As used herein, glare refers to a reflection of incident light off any interface in the display between the viewing side and the rear polarizer, which has no utility for viewing purposes and decreases the contrast of the display device. Conventional reflectors redirect light such that the direction of maximum brightness is at substantially the same angle as glare. Therefore the area of brightest illumination in current reflective LCD devices corresponds to the angle with the greatest amount of glare present. This glare greatly reduces the effectiveness of the display by reducing the contrast between bright and dark pixels, and causes users to view displays at angles away from glare, where the brightness is lower.

A third problem with reflective LCD devices relates to shadowing. As used herein, shadowing refers to the shadow cast on the reflector by a dark pixel. The shadow can be seen by the user as a dark border underneath and slightly offset from the dark pixels. The pixel shadow becomes more apparent when the distance from the reflector to the pixel increases or if the pixel size decreases. Therefore, a need exists for an electro-optic display device that consumes little power and is capable of providing a bright, well-contrasted display. Further, a need exists for an electro-optic display device that decreases the effect of glare on the brightness of a display and lessens the effects of shadowing.

Brief Description of the Drawings FIG. 1 is a cross-sectional view of an electro- optic display device in accordance with one embodiment of the present invention;

FIG. 2 is a plot depicting the functional diffraction properties of the transmission holographic optical element in FIG. 1 ; and

FIG. 3 is a cross-sectional view of an electro- optic display device in accordance with another embodiment of the present invention.

Detailed Description of a Preferred Embodiment

The present invention provides an electro-optic display device that includes an electro-optic cell, which is a liquid crystal cell in the preferred embodiment. The cell includes a cell front, a cell back opposite the cell front, and at least one cell region having a transparent mode. The electro-optic display device further includes a reflector optically coupled to the cell back to receive light traversing the cell region and to reflect the light toward the cell. The electro-optic display device further includes a transmission holographic optical element optically coupled to the cell front.

The transmission holographic optical element includes a front face and defines an axis perpendicular to the front face. The holographic optical element can include a diffraction grating adapted to receive a light ray at the front face at a first angle and to diffract the light ray such that the light ray is emitted toward the cell front at a second angle. Additionally, the holographic optical element can include a diffusing layer in proximity to the front face. The transmission holographic optical element is also adapted to receive a reflected light ray and to diffract the reflected light ray such that the reflected light ray is emitted from the front face at a third angle distinct from the first angle. The difference between the third angle and the first angle provides for glare avoidance, thereby increasing the contrast and apparent brightness of the display device.

The present invention can be better understood with reference to FIGs. 1 and 2. In accordance with a preferred embodiment of this invention, FIG. 1 depicts an electro-optic display device 10 that comprises, as major components, transmission holographic optical element 12, electro-optic cell 14 (which includes a front polarizer 18, and a back polarizer 20), and a reflector 16. Display device 10 produces a display that is illuminated by ambient light upon front side 22 and is viewed through a front side 22 of device 10. In a preferred embodiment, front side 22 corresponds to a transmission holographic optical element front face 24.

As used herein, ambient light refers to a bundle of light rays emanating from an ambient light source, propagating in substantially a parallel direction, with small angular extent. For purposes of better understanding this embodiment, ambient light ray 26 is a central light ray contained within a bundle of light rays. Light ray 26 is sufficient to describe the propagation of ambient light through the display, and should be understood to represent the entire bundle. Ambient light sources include those sources found in a well-lit room or outdoors. Several ambient sources may irradiate the electro-optic display device 10 from one or more directions and with varying intensity. The ambient light coming from multiple sources and multiple directions cooperates in illuminating the display for viewing.

Transmission holographic optical element 12 comprises a polymeric material and includes front face 24 and back face 28 opposite front face 24. Transmission holographic optical element 12 is effective to transmit ambient light incident on front face 24 toward electro-optic cell 14, which is optically coupled thereto. As used herein, optically coupled refers to elements which can bi-directionally transmit and receive light passing from one element to another. Transmission holographic optical element 12 is also effective to transmit light received from electro-optic cell 14. The reflected light is transmitted toward device front side 22 in a preferential viewing cone 30.

Transmission holographic optical element 12 redirects ambient light incident on front face 24 through second angle 32 to illuminate electro-optic cell 14. Transmission holographic optical element 12 also redirects light received from electro-optic cell 14 and incident on back face 28 through third angle 34 to illuminate viewing cone 30. Viewing cone 30 is represented by a central light ray 36 contained within the bundle of light rays defining viewing cone 30. Light ray 36 is sufficient to describe the orientation of viewing cone 30. Second angle 32 and third angle 34 cooperate such that viewing cone 30 is directed at an angle that is different from a glare angle 54. Glare angle 54 is the angle of incident light 42 reflected from the front side 22 of the display device 10. In a preferred embodiment, viewing cone 30 is about thirty degrees offset from glare angle 54, thereby significantly increasing the apparent brightness of display device 10. The incident portion of light ray 26 is shown on the front face 24 of holographic element 12. The transmitted portion of light ray 26 is depicted on the back face 28 of holographic element 12. Light ray 26 is incident on holographic element 12 at first angle 31. First angle 31 is defined as the angle between the incident portion of light ray 26 and an axis 40. Axis 40 is defined to be perpendicular to holographic optical element front face 24. Light ray 26 is transmitted through holographic element 12 and redirected by holographic element 12 into a direction described by second angle 32. Second angle 32 is defined as the angle between the transmitted portion of light ray 26 and axis 40. As shown, holographic element 12 is effective to steer light ray 26 into a unique second angle 32.

As illustrated in FIG. 1 , light ray 26 has a first section 100 impinging on holographic element 12, a second section 101 diffracted by holographic element 12 and impinging on reflector 16, a third section 102 reflected from reflector 16 and impinging on holographic element 12, and a fourth section 103 diffracted from holographic element 12. The angle of incidence of segment 101 on reflector 16 is equal to the angle of reflection of segment 102. The reflector can be a diffuse reflector, so that the reflected light is diffused over an angular band, which is in turn refracted for an enhanced viewing angle.

The action of holographic element 12 can be described by a functional relationship relating second angle 32 to first incident angle 31 , and designated by the letter f. One possible functional relationship, f, of second angle 32 on first angle 31 for holographic optical element 12 is shown in FIG. 2. Provided that the functional relationship / is not an odd function, holographic element 12 will be effective such that first angle 31 and second angle 32 cooperate in a manner that preferential viewing cone 30 is directed at an angle that is different from the glare angle 54. As used herein, the function f is odd if the relationship f(-x)=-f(x) holds for all values of x. The functional dependence f need not be the same for all locations on the holographic optical element 12, but may be tailored to provide for uniform illumination across the display.

With respect to the function illustrated in FIG. 2, function f includes a function g and a function f. Function f describes the diffraction of central light ray 36 in holographic element 12 from section 100 to section 101 . While, function g describes the diffraction of central light ray 36 in holographic element 12 from section 102 to section 103. The dashed line, designated as function h, is a reference function depicting diffraction of a central light ray in a body of glass suspended in air. Function f illustrates the non-symmetry of light diffraction by holographic element 12 when compared to the symmetrical function h for a glass body.

Now referring again to FIG. 1 , transmission holographic optical element 12 is preferably a volume holographic optical element composed of a photopolymeric film having regions of differing indices of refraction that cooperate, in a manner similar to a diffraction grating, to redirect light in an interference pattern. The diffraction grating is adapted to receive a light ray 26 at a first angle 31 , such as an ambient light ray, and to diffract the light ray 26 to a second angle 32. A light ray striking transmission holographic optical element 12 on the opposite surface at the negative second angle is diffracted to a third angle 34 that is distinct from first angle 31. A suitable holographic optical element is commercially available from the Polaroid Corporation and includes a layer composed of a photopolymer having the trade designation "DMP-128", which is exposed to laser light and developed to form regions of varying indices of refraction that are effective to redirect light in an interference pattern corresponding to a preferential viewing cone 30 for use in liquid crystal display device 10. Transmission holographic optical element 12 can be a diffuse transmission holographic optical element having asymmetrical diffusion characteristics. To reduce image blurring, the diffuse transmission holographic optical element will diffuse light entering the cell, but not when the light is reflected back out of the cell.

In accordance with a preferred embodiment, electro-optic cell 14 comprises a twisted nematic effect liquid crystal cell, commonly referred to as a TN or STN cell. Electro-optic cell 14 comprises front polarizer 18, a front substrate 48, a front electrode 44, liquid crystal layer 38, a back electrode 46, a back substrate 50, and back polarizer 20. Front electrode 44 is adjacent to liquid crystal layer 38. Back electrodes 46 are affixed to inner surface 52 of liquid crystal layer 38. Preferably, front electrode 44 and back electrodes 46 are formed of a transparent indium-tin oxide material. Electrodes 44 and 46 are connected to an external power supply, not shown. Transparent electrodes 44 and 46 are effective to alter the light transmission properties of the liquid crystal material in the adjacent region of the layer. In this manner, electrodes 44 and 46 define pixels that create the display. Front and back substrates 48 and 50 are transparent glass or plastic and function to protect liquid crystal layer 38 from external contamination.

In accordance with an alternate embodiment, electro-optic cell 14 is a polymer dispersed liquid crystal cell, commonly referred to as a PDLC cell. PDLC cells comprise a layer of polymeric material, containing a plurality of droplets of liquid crystal material suspended therein, sandwiched between transparent plates. The liquid crystal droplets suspended in the polymer dispersed liquid crystal layer contain a small amount of dichroic dye, dispersed throughout the liquid crystal. In the absence of an electric field of a predetermined strength, the liquid crystal material and dichroic dye contained in the droplets are randomly aligned from one droplet to another, and are effective to absorb most incident light. In the presence of an electric field of a predetermined strength, the liquid crystal material and dichroic dye contained in the droplets are aligned in the direction of the applied field, and are effective to transmit substantial amounts of incident light. In this manner, a pixel of polymer dispersed liquid crystal cell can be switched from a relatively transparent state to a significantly less transparent state.

Electro-optic cell 14 can also be comprised of a heterogeneously aligned liquid crystal display, commonly referred to as a reflective optically compensated bend cell. Further, electro-optic cell 14 can be a cholesteric liquid crystal cell, in which the liquid crystal is also reflective, relieving the need for a separate reflector.

The above embodiments serve to illustrate possible electro-optic cells. Those skilled in the art will recognize that other electro-optic cells can be utilized with the present device. Such electro-optic cells include other nematic liquid crystal cells such as pi cells, electrically controlled birefringence cells, guest host cells, other non-nematic liquid crystal cells such as surface-stabilized ferroelectric liquid crystal cells, anti-ferroelectric liquid crystal cells, and other non-liquid crystal cells such as electrochromic cells, electrophoretic cells, and suspended particle cells.

Electro-optic cell 14 operates bi-directionally. As used herein, bi-directional operation means that electro-optic cell 14 is effective to impart a pattern of light and dark regions corresponding to the regions defined by the pixels on light incident on device front side 22, where such pattern is visible on the light emitted from the cell back 58, and is also effective to impart a pattern of light and dark regions on light incident on the cell back 58, where such pattern is visible on the light emitted from the cell front 56. Reflector 16 is optically coupled to electro- optic cell 14, and is effective to receive light emitted from electro-optic cell 14, and to redirect light back toward cell 14. In a preferred embodiment, reflector 16 is a metallic gain reflector. The surface properties of reflector 16 can be altered in a predetermined manner to effect the amount of diffusion imparted to the ambient light, and in this manner effect the angular distribution of light redirected from the illuminated regions of reflector 16. In this manner, reflector 16 can be a diffuse reflector.

To create a display, front side 22 of liquid crystal display device 10 is illuminated by ambient light. Approximately 4% of the incident ambient light is reflected at a glare angle 54. The remaining light enters the front face 24 and emerges from the back face 28 of transmission holographic optical element 12 at second angle 32. The transmission hologram 12 effectively steers the ambient light at an angle that is different from the angle that the light would follow without the transmission hologram 12.

Subsequently, front polarizer 18 polarizes the ambient light and transmits linearly polarized light having a first axis of polarization. The linearly polarized light from front polarizer 12 is received by electro-optic cell 14. As the light passes through cell 14, the liquid crystal material changes the polarization state. The polarization state is dependent upon the electrical potential applied by front electrode 44 and back electrode 46 to the pixel. Back polarizer 20 receives the light from electro-optic cell 14 and emits linearly polarized light. The linearly polarized light passes to reflector 16 which redirects the light back towards the back polarizer 20. Reflector 16 may redirect the light in a specular manner as mirror or it may diffuse the light into a preferential diffusion pattern.

The redirected light passes through back polarizer 20 and is transmitted to cell 14 and subsequently through front polarizer 18. Front polarizer 18 emits light that is linearly polarized. The linearly polarized light then encounters back face 28 of the holographic optical element 12, and emerges through the front face 24, having been steered through a preferential exit angle 34 into the preferential viewing cone 30. In a preferred embodiment the viewing cone 30 is oriented thirty degrees away from the angle of glare. To ensure that the holographic optical element 12 does provide glare avoidance, it is important that the beam steering of the light on the way out of the display does not undo the effect of beam steering the light on the way in. As discussed, when the incident light traverses the holographic optical element 12 it is steered by a second angle 32. If the third angle 34 is of the same magnitude but acts to undo the second angle 32 then the next effect is zero beam steering. The viewing cone will consequently be aligned with glare, thereby providing no increase in brightness. Consequently, the offset must be enough to eliminate the deleterious effects of glare.

Another embodiment of the invention is illustrated in FIG. 3. An electo-optic display device 1 1 includes an electro-optic cell 14 in which a reflector resides in intimate contact with the liquid crystal layer. Placing a reflector in direct contact with the liquid crystal layer provides a display having enhanced color performance and eliminates shadowing. Electro-optic cell 14 includes a front polarizer 18, a front substrate 48, a front electrode 44, a liquid crystal layer 38, a reflector 46, and a back substrate 50. As illustrated, reflector 46 also functions as a back electrode of electo-optic cell. In certain circumstances, the combination of the reflector and the back electrode reduces production costs and simplifies device fabrication. However, those skilled in the art will recognize that a separate back electrode could be fabricated to reside on or under reflector 46. Also, the back electrode could be separated from the reflector by an intermediate layer.

Electo-optic device 1 1 functions in a similar manner to the previously described embodiments. Light ray 26 has a first section 100 impinging on holographic element 12, a second section 101 diffracted by holographic element 12 and impinging on reflector 46, a third section 102 reflected from reflector 46 and impinging on holographic element 12, and a forth section 103 diffracted from holographic element 12. The angle of incidence of segment 101 on reflector 46 is equal to the angle of reflection of segment 102. The reflector can be a diffuse reflector, so that the reflected light is diffused over an angular band, which is in turn refracted for an enhanced viewing angle. Alternatively, the reflector can be mirror-like where the hologram is a diffuse transmission optical element.

The present invention provides a liquid crystal display device that uses ambient light for device illumination and does not require a back light. Consequently, the device consumes less power than a device illuminated by a back light. Further, the apparent brightness and the contrast of the display device are greatly increased by offsetting the emitted angle from the glare angle. This offset is accomplished by the use of a transmission holographic optical element, which diffracts light entering the display device at a first angle to a second angle, and diffracts light exiting the display device into a third angle that is distinct from the negative of the first angle. In this manner, the brightest output angle is offset from the glare angle, thereby providing an enhanced contrast and brighter display device. While this invention has been described in terms of certain examples thereof, it is not intended that it be limited to the above description, but rather only to the extent set forth in the claims that follow.

Claims

The embodiments of this invention in which an exclusive property or privilege is claimed are defined as follows:

1 . An electro-optic display device comprising: an electro-optic cell having a cell front and at least one cell region having a transparent mode; a reflector configured to receive light that impinges on the cell front and that traverses the cell region, and to reflect the light toward the cell front; and a transmission holographic optical element affixed to the cell front, said element comprising a front face, said transmission holographic optical element adapted to receive a light ray at the front face at a first angle and to diffract the light ray such that the light ray is emitted toward the cell front at a second angle, and further to receive a reflected light ray and to diffract the reflected light ray such that the reflected light ray is emitted from the front face at a third angle distinct from the first angle.

2. An electro-optic display device in accordance with claim 1 , further comprising a diffraction grating, wherein the diffraction grating is adapted to diffuse the light ray, and wherein the transmission holographic optical element produces a diffuse pattern.

3. An electro-optic display device in accordance with claim 1 further comprising a cell back opposite the cell front, and wherein the reflector is optically coupled to the cell back.

4. An electro-optic display device in accordance with claim 1 , wherein the reflector functions as a back electrode.

5. An electro-optic display device in accordance with claim 1 , wherein the transmission holographic optical element is asymmetrically diffusive.

6. A liquid crystal display device comprising: an electro-optic cell having a cell front, a cell back opposite the cell front, and at least one cell region having a transparent mode; a reflector optically coupled to the cell back and configured to receive light admitted to the electro- optic cell through the cell front that traverses the cell region, and to reflect the light toward the cell front; and a transmission holographic optical element affixed to the cell front, said element comprising a front face and having, said transmission holographic optical element comprising a diffraction grating adapted to receive a light ray at the front face at a first angle and to diffract the light ray, such that the light ray is emitted toward the cell front at a second angle, and further to receive a reflected light ray and to diffract the reflected light ray such that the reflected light ray is emitted from the front face at a third angle distinct from the first angle.

7. A liquid crystal display device in accordance with claim 6, wherein the liquid crystal cell is a cholesteric liquid crystal cell.

8. A liquid crystal display device in accordance with claim 6, wherein the liquid crystal cell is a polymer dispersed liquid crystal cell.

9. A liquid crystal display device in accordance with claim 6, wherein the liquid crystal cell comprises a liquid crystal layer comprising liquid crystal material sandwiched between transparent electrodes.

10. A liquid crystal display device in accordance with claim 6, wherein the reflector is a diffuse reflector and wherein the transmission holographic optical element is a diffuse transmission holographic optical element.