WO2005064389A1

WO2005064389A1 - Transflective lcd device

Info

Publication number: WO2005064389A1
Application number: PCT/IB2004/052747
Authority: WO
Inventors: Emiel Peeters; Armanda C. Nieuwkerk; Sander J. Roosendaal; Dirk J. Broer
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2003-12-22
Filing date: 2004-12-10
Publication date: 2005-07-14
Also published as: TW200525238A

Abstract

A transfiective LCD device comprises a liquid crystal display cell including an active layer (110). The device is simultaneously operable in the reflective mode and in the transmissive mode. For this purpose, a rear stack (120) of optical elements comprises a partial mirror (126) for reflecting ambient light and passing light originating from the backlight system. The partial mirror (126) is arranged in between a reflective linear polarizer (122) which is preferably a wire-grid polarizer, and an absorbing linear polarizer (124) which is preferably a coatable absorbing polarizer provided inside the display cell. This configuration of a transflective LCD has a particularly high light efficiency in the transmissive mode, whereas the light efficiency in the reflective mode is hardly affected.

Description

Transfiective LCD device

The invention relates to a transfiective liquid crystal display (LCD) device.

Liquid Crystal Displays (LCDs) are increasingly used in computer monitors, television sets, handheld devices et cetera. For mobile applications, LCDs have become the standard display device due to low power consumption, reliability and low price. The operation of LCDs is based on light modulation in an active layer of a liquid crystalline (LC) material. By changing an electric field, the light modulation of the active layer is altered, and characteristics of the light passing through the LC layer are modified. Generally the active layer modifies a state of polarization of the passing light. The liquid crystal display cell comprises the active layer of the liquid crystalline material, optionally one or more in-cell optical elements, and in a color LCD also color filters. Generally the display cell is sandwiched between two substrates, namely a front substrate on the viewer side and a rear substrate on the opposing side. Additional optical elements may be attached to the outer surfaces of said substrates. The optical elements, both the in-cell optical elements and those on the outside of a substrate, are divided into a first arrangement on the backlight side, also referred to as the rear stack hereinafter, and a second arrangement on the viewer side, also referred to as the front stack hereinafter. LCDs are generally operable in one or both of two modes, namely a transmissive mode and a reflective mode. In a transmissive LCD, the active layer modulates light originating from a backlight system which is usually arranged adjacent to the rear substrate. Transmissive LCDs generally have a good contrast ratio, however when used in an outside environment the display becomes practically unreadable. The active layer in a reflective LCD modulates ambient light that impinges on the display. The reflective LCD relies on a reflector for reflecting the modulated ambient light back towards the viewer. Thus, in the reflective mode, ambient light generally passes through the active layer twice. The reflector is usually provided in the form of a mirror adjacent or on the rear substrate. However, a reflective LCD is difficult to read if ambient lighting is insufficient. Therefore, mobile devices may incorporate a so-called transfiective LCD, which operates in the transmissive and reflective modes at the same time. This has the advantage that the display is usable both under bright and dim ambient light conditions. In the latter case, light from the backlight system is used for viewing the display. A common type of transfiective LCD incorporates a reflector based on a partial mirror. The partial mirror is arranged for reflecting ambient light while at the same time passing light originating from the backlight system. In designing the partial mirror, generally a trade-off has to be made between sufficient performance of the display in the reflective and transmissive modes. For example, the partial mirror can be a reflective layer provided with holes for passing light from the backlight system. In this example, obtaining any appreciable reflective display effect requires that within each pixel of the LCD, about 70%-80% of the pixel area is available for reflecting ambient light. As a consequence, only about 20%-30% of the pixel area is available for passing light originating from the backlight system. Because of the relatively small hole dimensions, the light efficiency of the conventional transfiective LCD in the transmissive mode is inherently rather poor. The problem of inherently poor transmissive mode performance is common to any transfiective LCD based on a partial mirror. Generally, in order to have sufficient display performance in the transmissive mode, a backlight with a relatively high emission intensity is required. This is however undesirable in a mobile device because of the inherent high power consumption of such a bright backlight.

It is an object of the invention to have a transfiective LCD device with an improved light efficiency in the transmissive mode, whereby light efficiency in the reflective mode is maintained as much as possible. This object has been achieved by means of a transfiective LCD device according to the invention as specified in the independent Claim 1. Further advantageous embodiments are defined in dependent Claims 2-7. According to the invention, the first arrangement of optical elements, i.e. the rear stack, includes a first absorbing linear polarizer, a partial mirror for reflecting ambient light and for passing light originating from the backlight system, and a first reflecting linear polarizer, in this order as seen from the active layer. Thus, the partial mirror is arranged in between the first reflecting linear polarizer and the first absorbing linear polarizer, and the first reflecting linear polarizer is arranged closest to the backlight system. This reflective polarizer passes light having a desired polarization direction while reflecting light having a polarization direction perpendicular to the desired polarization direction. This light is reflected back towards the backlight system and is recycled thereby so as to re-enter the display cell and contribute to the brightness of the transmissive mode. After passing the reflecting polarizer, the light that has the desired polarization direction either directly passes through the partial mirror, or is reflected back towards the backlight system and recycled thereby until it ultimately passes through the partial mirror. Between reflections, little light is lost in the reflective polarizer as the light is already linearly polarized in correspondence with the polarization direction of the reflecting polarizer. As a consequence, a relatively large fraction of light emitted by the backlight system ultimately passes the partial mirror. Preferably, the absorbing linear polarizer and the reflecting linear polarizer have substantially parallel polarization axes. In this case, light that passed the partial mirror is passed as well as possible by the absorbing linear polarizer arranged behind the partial mirror, because this light is already linearly polarized in correspondence with the polarization direction of the absorbing polarizer. In the reflective mode, the brightness of the LCD device in its white state is slightly reduced, because the first absorbing linear polarizer is present between the active layer and the partial mirror from which ambient light that passed the active layer is reflected. The optical mode of the active layer should be such, that ambient light having passed the active layer in the white state of the LCD device, has a linear polarization direction parallel to the transmission axis of the first absorbing linear polarizer. In this case, the transfiective LCD device according to the invention maintains about 80% of the brightness in the reflective mode as compared to the prior art. For this purpose, preferably, the optical mode of the active layer has half- wave (λ/2) retardation in the white state of the LCD device and zero retardation in the black state of the LCD device, whereby the active layer is arranged between crossed linear polarizers, i.e. linear polarizers having perpendicular transmission axes are arranged on either side of the active layer. The new transfiective LCD device is slightly less efficient in the reflective mode. However, this disadvantage is outweighed by a 2 to 8 times higher efficiency in the transmissive mode as compared to traditional transfiective LCD designs, without the need to incorporate brightness enliancement foils. The increase in transmissive light efficiency can be utilized to increase the brightness of the LCD, or to reduce power consumption by decreasing brightness of the backlight system. WO 94/11776 discloses a transmissive LCD having an absorbing polarizer and a reflective polarizer on the backlight side of the display cell. However, WO 94/11776 does not teach how to efficiently incorporate the absorbing and reflective rear polarizers into a transfiective LCD including a partial mirror, whereas the transfiective LCD device according to the invention has particularly high light efficiency in both the reflective and transmissive modes. In a preferred embodiment, a second arrangement of optical elements (also referred to as front stack) on a viewing side of the active layer includes a second reflecting linear polarizer and a second absorbing linear polarizer, in this order as seen from the display cell. Preferably, the second reflecting polarizer and the second absorbing polarizer have substantially parallel transmission axes. This preferred embodiment has the advantage that, in a black state of the active layer, light being transmitted through the active layer is reflected back towards the backlight system by means of the second-reflecting polarizer, and can be recycled to contribute to the brightness of other picture elements in the white state or an intermediate state. In a black pixel, the light emitted by the backlight will now be reflected by the second reflecting polarizer, as its polarization direction does not match the transmission axis of the front polarizer. In a conventional LCD, such light would be absorbed by the second absorbing polarizer so as to cause a black appearance of a display pixel, and consequently be lost. By virtue of the second reflecting polarizer, such light is now reflected back to the backlight system and can be recycled thereby. However, the preferred embodiment slightly decreases the light efficiency in the bright state of the reflective mode as the light has to pass the additional second reflecting polarizer two times. It is estimated that in this preferred embodiment, the efficiency of the reflective mode is decreased by a further 20%, whereas the efficiency of the transmissive mode is increase by approximately the same amount. Suitable reflective linear polarizers include multilayer birefringent polymer films, or cholesteric polarizers in combination with a half wave retarder. However, the use of wire-grid polarizers is preferred since they perform better than other reflective polarizers in terms of polarization efficiency, single piece transmittance, thickness and homogeneity. The absorbing polarizer may be a conventional linear polarizer glued to the exterior of the display cell, or preferably a coatable linear polarizer inside the display cell. Also, the reflective polarizer and partial mirror may be provided within the display cell, that is in between the front substrate and the rear substrate. This can be advantageous, since both types of optical elements generally include metallic features, and the elements are therefore suitable to simultaneously act as a pixel electrode, or as a part thereof. Preferably, the backlight system incorporates a light guide and a specular reflector for reflecting light between the backlight system and the first reflective linear polarizer. A specular reflector is particularly efficient for this purpose leading to a particularly high light efficiency in the transmissive mode. However, a backlight system with a specular reflector requires that the light guide in the backlight system is very thin, that is, less than one millimeter. Such a backlight system should preferably be positioned as closely as possible to the first reflective linear polarizer in order to prevent parallax problems. Preferably, the backlight system with specular reflector is directly adjacent the first reflective linear polarizer, or alternatively a quarter wave retarder may be provided in between. These and other aspects of the invention will now be elucidated further with reference to the accompanying drawings. Herein: Figs. 1A and IB represent an optical configuration of a first embodiment of a transfiective LCD device according to the invention, and Fig. 2 represents an optical configuration of a second embodiment of the transfiective LCD device.

In the drawings, like reference numerals represent like elements. The optical configuration of a single picture element 100 of the first embodiment is shown in the white state in Fig. 1A and in the black state in Fig. IB. The operation of the transfiective LCD device is based on light modulation in the active layer 110 including a liquid crystalline material. The active layer 110 is arranged between a front stack 130 of optical elements on a viewer side of the LCD, and a rear stack 120 of optical elements on the opposing side of the LCD. A backlight system 140 is arranged behind the rear stack 120 for emitting light for operating the device in the transmissive mode. The Figures display the optical configuration of the device only, additional elements such as color filters, front and rear substrate and pixel electrodes are generally provided in the LCD device, but not shown for clarity reasons. In the first embodiment, the rear optical stack 120 includes a first reflective linear polarizer 122, a partial mirror 126 and a first absorbing linear polarizer 124, arranged in this order between the backlight system 140 and the active layer 110. The first reflective linear polarizer 122 and first absorbing linear polarizer 124 have substantially parallel polarization axes. The front optical stack 130 essentially only includes a second absorbing polarizer 134 having its polarization axis arranged perpendicularly to those of the first absorbing polarizer 124 and first reflective polarizer 122. The active layer 110 is thus arranged between crossed linear polarizers. The optical mode of the active layer 110 is such that in the white state of the LCD device (Fig. 1A) the layer has half- wave retardation (R=λ/2), and in the black state of the LCD device (Fig. IB) the layer has zero retardation (R=0). Suitable optical modes for the . liquid crystalline active layer 110 include electrically compensated birefringence (ECB), optically compensated birefringence (OCB), twisted nematic with 90 degree twist angle (90TN), and in-plane switching (IPS). The absorbing polarizers 124, 134 are preferably coatable polarizers provided within the display cell. More preferably, a coatable polarizer from Optiva Inc. is used, based on dye molecules which form a lyotropic phase in aqueous solutions as a result of the formation of long columnar aggregates. When such lyotropic solution is applied to a substrate under shear, the columns will align parallel to the shear direction and, after evaporation of the solvent, a two dimensional crystal is foπned. The resulting crystalline film has a thickness below one micrometer and exhibits a highly dichroic absorption. The reflective polarizer 122 is preferably a wire-grid polarizer. A wire-grid polarizer comprises a grid of line-shaped conductors arranged parallel to each other on a transparent substrate. Such wire-grid polarizers are known in the art, for example from US 6,081 ,376, and can be obtained from Moxtek, USA. In order to linearly polarize light in the visible range, the wire-grid polarizer should have conductors being about 0,1 micrometers wide. The conductors are equally spaced at about 0,3 micrometers from each other. The wire grid polarizers transmits light having a polarization direction parallel to the conductors, and reflects light having a polarization direction perpendicular to the wires. The partial mirror 126 in the first embodiment comprises a reflector provided with holes 128 for passing light emitted by the backlight system 140. Other types of partial mirrors, such as a partially transmissive mirror having a reflectivity of for example 70 or 80 per cent, may also be used. It may further be advantageous to integrate the partial mirror and the first reflecting polarizer into a single element. The backlight system 140 comprises a lamp 142 and a light guide 144 which is provided with a reflector 146 on the backside facing away from the display. The reflector 146 may be a diffusive reflector or preferably a specular reflector. In the latter case, the light guide 144 should be as thin as possible, preferably about 100 micrometer. A specular reflector has the advantage that light efficiency of the transfiective LCD is further improved in the transmissive mode. Fig. 1 A shows a pixel 100 of the first embodiment in the white state. As an example of reflective operation, an ambient light source emits the ambient light ray 152 into the display device. The second absorbing polarizer 134 transmits a portion of the light ray 152 having a linear polarization in a first direction, towards the active layer 110. The first direction is, as an example, illustrated as being perpendicular to the drawing plane, and corresponds to the polarization axis of the second absorbing polarizer 134. The active layer 110 has half- wave retardation, and as a consequence the active layer 110 rotates the polarization direction of light ray 152 through 90 degrees, so that light ray 152 now has a linear polarization in a second direction perpendicular to the first direction. The second direction is illustrated as being parallel to the drawing plane, and perpendicular to the normal vector of surfaces within the display. Partial mirror 126 then reflects light ray 152 which subsequently passes the active layer 110 again. The active layer 110 rotates the polarization direction of light ray 152 through 90 degrees again, so that light ray 152 regains its original linear polarization direction, which is the first direction. As a result, second absorbing polarizer 134 transmits the reflected light ray 152 towards a viewer. As an example of transmissive operation, the lamp 142 of backlight system 140 emits the first backlight ray 154 and the second backlight ray 156. The rays travel through light guide 144 and are eventually outcoupled from the light guide 144 so as to travel towards the active layer 110. It is noted that the path of light rays within the light guide 144 is not indicated in the Figures. The first reflecting polarizer 122 transmits a portion of the first backlight ray 154 having a linear polarization in the second direction and reflects a portion of the first backlight ray 154 having a linear polarziation in the first direction (not shown). The second direction corresponds to a polarization axis of the first reflecting polarizer 122. The light ray 154 passes through hole 128 in partial mirror 126. The first absorbing polarizer 134 also transmits light ray 154, as a consequence of the polarization axes of first reflective polarizer 122 and first absorbing polarizer 134 being parallel, that is, both corresponding to the second direction. Again, the active layer 110 rotates the polarization direction of light ray 154 through 90 degrees, so that light ray 154 has a linear polarization in the first direction after passing the active layer 110. Subsequently, the second absorbing polarizer 134 transmits light ray 154 towards the viewer. As is clear from the above, in this case the reflected ambient light ray 152 and first backlight ray 154 reach the viewer and thus, the transfiective LCD pixel 100 is observed as a white pixel. The first reflecting polarizer 122 also transmits a portion of the second backlight ray 156 having linear polarization in the second direction, however partial mirror 126 reflects light ray 156 back towards the backlight system 140. The second backlight ray 156 returns into the light guide 144 and can thus be outcoupled again at a different position. This process is referred to as 'backlight recycling' Thus, light that is emitted by the backlight system 140 and that does not directly enter the active layer 110, or that has an unsuitable direction of linear polarization, can easily be returned to the backlight system 140 and contribute to the brightness in the same pixel or in other pixels of the LCD device. The light efficiency of the transmissive mode of the transfiective LCD is increased. This allows the use of a less bright backlight reducing power consumption, or alternatively a brighter display. Alternatively, a partial mirror can be used that has a higher reflectivity rate that in conventional transfiective LCD designs, without compromising display performance in the transmissive mode. For instance, for a partial mirror comprising a reflector provided with holes, the hole dimensions may be chosen smaller than in conventional transfiective LCD devices with such partial mirror. In Fig. IB, the same LCD pixel 100 is shown in its black state. The active layer 110 now has zero retardation (R=0) and as a result, the active layer 110 substantially does not change the direction of polarization of a passing light ray. Because of this, ambient light ray 153 still has a linear polarization in the first direction after passing the active layer 110. The first absorbing polarizer 124 now absorbs ambient light ray 153. Similarly, the first backlight ray 155 still has a linear polarization in the second direction after passing the active layer 110, and the second absorbing polarizer 134 now absorbs the first backlight ray 155. Thus, substantially no light rays travel towards the viewer of the LCD device, who observes the pixel 100 as black. The second backlight ray 157 is still returned to the light guide 144 by reflection off partial mirror 126 and can be recycled by the backlight system 140. This ray can thus contribute to the brightness of an other pixel of the display device, either a pixel in the white state or an intermediate state corresponding to a greyscale level. A pixel 200 of the second embodiment is shown in its black state in Fig. 2. It is largely similar to the pixel 100 of the first embodiment, in particular with respect to the active layer 210, rear stack 220 of optical elements and backlight system 240. The second embodiment incorporates a second reflecting linear polarizer 232 in the front stack 230 of optical elements. This second reflecting linear polarizer 232 is arranged between the second absorbing linear polarizer 234 and the active layer 210, and has its polarization axis substantially in parallel with that of the second absorbing polarizer 234. The second reflecting polarizer 232 is preferably a wire-grid polarizer. After passing the active layer 210 having zero retardation, the backlight ray 255 has linear polarization in the second direction, which is perpendicular to the polarization axis of the second reflecting polarizer 232 and the second absorbing polarizer 234. The second reflecting polarizer 232 thus reflects the backlight ray 255, which travels back to the backlight system 240. This ray can thus further contribute to the brightness of an other pixel of the display, either a pixel in the white state or an intermediate state corresponding to a greyscale level. In the second embodiment, the absorbing polarizer of the rear optical stack 220 absorbs the ambient light ray 253, similar to the first embodiment. However, the backlight ray 255 emitted into a black pixel 200 can be recycled, whereas in the first embodiment, such backlight ray would be absorbed by the absorbing polarizer in the front optical stack. As a result, the light efficiency of the transmissive mode is further increased. The light efficiency of conventional transfiective LCD designs and the embodiments of the present invention can be calculated to be as follows: reflective mode transmissive mode

PRIOR ART : conventional transfiective LCD 3,3 % 0,3 % conventional transfiective LCD

(dual cell-gap design) 3,3 % 0,7 %

INVENTION: first embodiment (diffusive reflector 146) 2,5 % 2,4 % first embodiment (specular reflector 146) 2,5 % 4,9 % second embodiment (diffusive reflector 246) 2,0 % 2,7 % second embodiment (specular reflector 246) 2,0 % 5,5 %

The above calculations assumes the efficiency of a linear polarizer to be 44% for unpolarized light and 88% for light being linearly polarized and having its polarization vector in parallel with the polarization axis of the polarizer. These percentages are assumed the same for an absorbing polarizer and a reflecting polarizer. The reflectivity of the partial mirror is chosen to be 70%. The calculations are for a color transfiective LCD incorporating a color filter with a light efficiency of 30%. Furthermore, it is assumed that 8% of light reflected off a reflecting polarized will be lost, and that 20% of recycled light is lost by re-entering and leaving the backlight system. A diffusive reflector is assumed to fully depolarize incident light. In the second embodiment, it is assumed that 50% of the display pixels are white and 50% of the display pixels are black. From the above table, it is clear that the transfiective LCD designs according to the invention achieve a large improvement in transmissive mode light efficiency, while maintaining reflective mode light efficiency rather well. In summary, a transfiective LCD device comprises a liquid crystal display cell including an active layer. The device is simultaneously operable in the reflective mode and in the transmissive mode. For this purpose, a rear stack of optical elements comprises a partial mirror for reflecting ambient light and passing light originating from the backlight system. The partial mirror is arranged in between a reflective linear polarizer which is preferably a wire-grid polarizer, and an absorbing linear polarizer which is preferably a coatable absorbing polarizer provided inside the display cell. This configuration of a transfiective LCD has a particularly high light efficiency in the transmissive mode, whereas the light efficiency in the reflective mode is hardly affected.

Claims

CLAIMS:

1. A transfiective liquid crystal display (LCD) device, comprising: a liquid crystal display cell including an active layer (110); a backlight system (140) for backlighting of said display cell, and a first arrangement (120) of optical elements between said active layer and said backlight system, the first arrangement including a first absorbing linear polarizer (122), a partial mirror (126) for reflecting ambient light and passing light originating from the backlight system, and a first reflecting linear polarizer

(124), in this order as seen from the active layer (110).

2. The transfiective LCD device of Claim 1, comprising a second arrangement (130) of optical elements between said active layer (110) and a viewing surface of the LCD device, the second arrangement including a second reflecting linear polarizer (134) and a second absorbing linear polarizer (132), in this order as seen from the active layer (110).

3. The transfiective LCD device of Claim 1 or 2, wherein the absorbing polarizer and the reflective polarizer in one or both arrangements have substantially parallel polarization axes.

4. The transfiective LCD device of Claim 1 or 2, wherein one or both of the reflective polarizers is a wire-grid polarizer.

5. The transfiective LCD device of Claim 1 or 2, wherein one or both of the absorbing polarizers is a coatable polarizer.

6. The transfiective LCD device of Claim 1 or 2, wherein one or more of the optical elements in the first and second arrangements are provided inside the liquid crystal display cell.

7. The transfiective LCD device of Claim 1 or 2, wherein the backlight system

(140) comprises a light guide (144) with a specular reflector for reflecting light between the backlight system and the first reflective linear polarizer.