WO2004003646A1

WO2004003646A1 - Transflective display

Info

Publication number: WO2004003646A1
Application number: PCT/IB2003/002869
Authority: WO
Inventors: Dirk J. Broer; Peter Van De Witte
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2002-07-01
Filing date: 2003-06-13
Publication date: 2004-01-08
Also published as: TW200419220A; AU2003244939A1

Abstract

A bright and highly efficient transflective display comprises, in succession, a transflective light valve (2), a transflector (4) and a backlight system (6). The backlight system and transflector cooperate to provide circularly polarized light with high brightness and efficiency. To achieve such high efficiency and brightness the backlight system comprises, in succession, a first quarter wave retarder (8), a linear-polarized light emitting light source (10), a second quarter wave retarder (12) and a light-reflecting surface (14) or a side-lit circular-polarized-light-emitting waveguide and a light-reflecting surface. The light source, wave retarders, if any, and the reflector cooperate to recycle the part of the light emitted by the light source which is reflected by the transflector. The efficiency of the transflector/backlight combination is substantially independent of the transmission of the transflector allowing the transmission of the transflector to be reduced and thus the brightness in reflection to be increased while substantially maintaining the same brightness and efficiency in transmission.

Description

Transflective display

The invention relates to a transflective display and a backlight system. Transflective displays are displays which are capable of operating in reflection and transmission. In reflection the picture information on display is typically made visible using ambient light whereas in transmission use is made of a backlight system, the backlight system being typically only used when ambient light levels are too low to afford comfortable viewing in the reflective mode alone. To obtain a display operable both in transmission and reflection, a transflective display comprises a transflector which is basically a semi- transparent mirror adapted to partially transmit and partially reflect light incident thereon. Relative to a pure reflective display, a transflective display operated in reflection is inefficient and less bright because the transflector allows a substantial part of the ambient light to be transmitted, such light being then lost for viewing the picture information on display. Relative to a purely transmissive display, a transflective display operated in transmission is inefficient and less bright, because the transflector only partially transmits the light emitted by the backlight. This inefficiency and lack of brightness is particularly a problem in color transflective displays as in such displays a lot of light is also absorbed in the color filter. Rendering the transflective display more bright in reflection by decreasing the transmission of the transflector requires a more powerful backlight system in order to maintain the brightness when the transflective display is operated in transmission. EP patent application 877282 attempts to improve the brightness and efficiency of a transflective display. The display disclosed therein comprises a liquid crystal panel, a transflector arranged behind the panel, and a backlight assembly arranged behind the transflector. In order to increase the efficiency of the display, EP 877282 suggests to use a reflective polarizer in combination with a back light assembly having light absorbing capability. However, reflective polarizers are relatively complicated and thus expensive optical components. Moreover, a backlight assembly having light absorbing capability has the disadvantage that light emitted by the backlight is partially absorbed which reduces the efficiency when the display is used in transmission. It is an object of the invention, inter alia, to provide a transflective display which is efficient and bright when the transflective display is operated in either transmissive or reflection mode or both.

This object is achieved by means of a transflective display comprising, in succession: a transflective light valve;

- a transflector; and

- a backlight system;

- the backlight system comprising, in succession: - a first quarter wave retarder; a transparent linear-polarized-light-emitting light source adapted to emit linear polarized light;

- a second quarter wave retarder; and

- a light-reflecting surface. The backlight system and transflector cooperate to provide circularly polarized light to the transflective light valve in a highly efficient manner thus providing a transflective display device which is very efficient in transmission. The high efficiency is achieved by recycling the light reflected off the transflector using the quarter wave retarders and the light- reflecting surface of the backlight system. Since none of the components of the backlight system absorb light, recycling allows, at least theoretically if all components are ideal, all the light from the linear-polarized-light-emitting light source to be provided to the transflective light valve. Moreover, the transmission of the transflector determines only the average number of cycles light is subjected to before being transmitted by the transflector. Therefore, the efficiency of the transflector backlight system is substantially independent of the transmission of the transflector. Accordingly, the transmission of transflector may be decreased to improve the brightness in reflection while substantially maintaining the brightness in transmission. Furthermore, in reflection the transflective display is in principle as efficient and bright as a pure reflective device because the backlight system does not absorb or change the polarization of ambient light incident thereon. In order to improve the contrast of the transflective display in transmission, a preferred embodiment of the transflective device is one wherein the linear-polarized-light- emitting light source is adapted to emit linear polarized light preferentially to the transflector side of the backlight system. Light from the linear-polarized-light-emitting light source which is emitted towards the light-reflecting surface reaches the transflector via the light reflecting surface, and when presented to the transflector, has a handedness with is complementary to that of the handedness of the light which is directly directed towards the transflector. Because, generally, a transflective light valve is adapted to function as a light valve with respect to only one polarization component, the other component being typically transmitted for both the open and closed state of the light valve, the light presented at the transflector with the complementary handedness reduces the contrast of the transflective display. The loss of contrast is prevented or at least reduced if the linear-polarized-light-emitting light source emits preferentially to the side of the transflector. Means for achieving such preferential outcoupling are well known in the art, one such means being a relief structured outcoupling surface.

In order to recycle the light emitted by the backlight system and reflected off the transflector into light of the appropriate handedness, the light-reflecting surface is adapted to invert the handedness of any light incident thereon. As is well known, such inversion of handedness may be achieved using a reflecting surface which is - at least partially - specularly reflective. For the purpose of efficiency, specular reflectivity should be as high as possible, say 80 % or more, on the other hand for obtaining a wide viewing angle and/or to suppress undesirable mirror effects it may be preferred to have some diffuse reflection as well.

In order to, inter alia, obtain a very flat transflective display, a side-lit (edge- lit) linear-polarized-light-emitting waveguide may be used as opposed to a linear-polarized- light-emitting light source in which the lamp(s) are arranged in the light path between reflector and transflector. The transflector which is partly transmissive and partly reflective preferably has a transmission of 20 % or less to optimize viewing using ambient light. Preferably, the transflector has a reflecting surface facing the backlight system, the reflecting surface being a silver or a silver alloy surface. Silver and silver alloy surfaces generally have a higher reflectivity than aluminum. Since as a result of recycling the light is incident on the transflector a number of times it is beneficial to use such a highly efficient reflecting surface. Alternatively, a highly efficient transflector is obtained if the transflector comprises a dielectric mirror adapted to partially reflect and partially transmit light incident thereon.

High brightness and efficiency is of particular importance in color transflective displays, as such displays comprise color filters which typically absorb a substantial part of the light incident thereon. Since the displays of the present invention provide such a high efficiency and brightness the invention is of particular importance for displays including such color filters. In order to be available in both reflective and transmissive mode, the color filter is typically arranged on the viewing side of the transflector.

The invention also relates to a backlight system per se, in particular a backlight system for use in a transflective display in accordance with the invention. In accordance with the invention, the backlight system comprises, in succession: a first quarter wave retarder; - a transparent linear-polarized-light-emitting light source adapted to emit linear polarized light;

- a second quarter wave retarder; and

- a light-reflecting surface.

The invention also relates to a transflective display comprising, in succession: - a transflective light valve; a transflector; and a backlight system;

- the backlight system comprising, in succession:

- a transparent side-lit circular-polarized-light-emitting waveguide; and - a light-reflecting surface.

A transflective display having the same functionality as the transflective display comprising the backlight system with the linear-polarized light source is obtained if the backlight system comprises a transparent side-lit circular-polarized-light-emitting waveguide. Such a backlight system comprises less parts and is therefore of a simpler design and more economical to manufacture.

Like the transflective display comprising the linear-polarized light emitting backlight, the circularly-polarized-light-emitting light source is preferably adapted to emit polarized light preferentially to the transflector side of the backlight system and/or is combined with a transflector which has a transmission of 20 % or less and/or combined with a transflector which has a reflecting surface facing the backlight system which is a surface of a silver or a silver alloy layer and/or is combined with a transflector which comprises a dielectric mirror adapted to partially reflect and partially transmit light incident thereon and/or is used in a transflective display which also comprises a color filter. A side-lit circular-polarized-light emitting waveguide may be obtained by a combination of a transparent waveguiding substrate and a cholesteric layer (that is a cholesterically ordered layer) of which the pitch is selected such that, for waveguided light having a wavelength in the visible range of the spectrum incident on the cholesteric layer, a circularly polarized component of a first handedness is selectively reflected and a circularly polarized component of a second handedness, complementary to the first, is selectively transmitted.

These and other aspects of the invention will be apparent from and elucidated with reference to the drawings and the embodiments described hereinafter.

In the drawings:

Fig. 1 shows, schematically, a cross-sectional view of an embodiment of a transflective display in accordance with the invention; Fig. 2 shows, schematically, a trace of a light ray emitted by the backlight system as it propagates through the transflective display of Fig. 1 in an ON state;

Fig. 3 shows, schematically, a trace of a light ray emitted by the backlight system as it propagates through the transflective display of Fig. 1 in an OFF state;

Fig. 4 shows, schematically, a trace of an ambient light ray as it propagates through the transflective display of Fig. 1 in an ON state; and

Fig. 5 shows, schematically, a trace of an ambient light ray passing through the transflective display of Fig. 1 in an OFF state.

Fig. 1 shows, schematically, a cross-sectional view of an embodiment of a transflective display in accordance with the invention. The transflective display 1 comprises, in succession, a transflective light valve 2, a transflector 4 and a backlight system 6. The light valve 2 is a light valve adapted to be, in cooperation with the transflector 4, reversibly electrically switchable between an open state, also referred to as an ON state and a closed state, also referred to as an OFF state, for ambient light and/or light emitted by the backlight system 6. In the present embodiment, but other options are available, the transflective light valve 2 comprises a linear (dichroic) polarizer 16, a quarter wave retarder 18 and a liquid crystal (LC) cell 20. The LC effect used in the LC cell 20 is such that the LC cell 20 and the quarter wave retarder 18 have the combined effect of providing, in the ON state, 0 (zero) or a half wave retardation (or any multiple thereof) and, in the OFF state, a quarter wave retardation.

The backlight system 6 comprises, in succession, a first quarter wave retarder 8, a linear-polarized-light-emitting waveguide 10 comprising a lamp 11, a second quarter wave retarder 12 and a light-reflecting surface 14. The linear-polarized- light-emitting light source 10 is adapted to emit p-polarized light. More particularly, in order to improve contrast in transmission mode, the linear-polarized-light-emitting light source 10 preferentially emit light to the transflector side of the backlight system 6.

Fig. 2 shows, schematically, a trace of a light ray emitted by the backlight system as it propagates through the transflective display of Fig. 1 in an ON state.

In transmissive operation, that is with the backlight system 6 switched on, a light ray 31 emitted by the linear-polarized-light-emitting light source 10 towards the transflector 4 is linearly polarized (in the present embodiment the light source emits p- polarized light but evidently s-polarized light would have been equally possible) and is incident on the first quarter wave retarder 8 which converts the p-polarized light ray 31 into a right-handed (RH) circularly polarized light ray 31 (by using a retarder having an opposite polarity, left-handed circularly polarized light could have been obtained). The RH polarized light ray 31 is then incident on the transflector 4. Being partly transmissive and partly reflective, the transflector 4 splits the light ray 31 into a transmitted light ray 33 and a reflected light ray 35. Since the reflection off the transflector 4 is arranged to be specular, the handedness is inverted upon reflection and the reflected light ray 35 is left-handed (LH) circularly polarized. For the purpose of clarity, the reflected light ray 35 is drawn displaced relative to the light ray 31. In passing the quarter wave retarder 8 the reflected RH light ray 35 becomes s-polarized, is then transmitted by the transparent linear-polarized-light-emitting light source 10 and converted into an LH circularly polarized light ray 35 by the quarter wave retarder 12. The LH light ray 35 is then reflected by the light reflecting surface 14 to form a light ray 37. Because the light-reflecting surface 14 is -at least partially - specularly reflective, the light ray 37 becomes right-handed circularly polarized. The RH polarized light ray 37 becomes p-polarized by the quarter wave retarder 12, and transmitted unaffected by the transparent linear-polarized-light-emitting light source 10. The light ray 37 is now in the same position as the light ray 31 initially was and it is therefore evident that the same light path is traced over and over again to harvest more and more light of the correct polarization to present to the transflective LC cell. The net result of this recycling process is that in theory all the light the backlight system emits towards the transflector is ultimately transmitted by the transflector and the efficiency is, at least theoretically, that is not considering any non- idealities of the various components involved, 100 %. Moreover, the light rays 31 and 37 are circularly polarized, more particular in this embodiment right-handed circularly polarized, therefore further polarization means to bring the light into a condition in which it can be suitably used by the transflective light valve 2 is not necessary. Moreover, the transmission of the transflector only affects the average number of times a light ray is recycled before being transmitted and does not substantially affect the efficiency of the combination of transflector 4 and backlight system 6. Consequently, the transmission of the transflector 4 may be decreased to increase the brightness of the transflective display in reflective mode, while maintaining substantially the same brightness and efficiency in transmission.

The RH polarized light transmitted by the transflector 4, indicated by light rays 33 and 37, is incident on the LC cell 20. The LC cell 20 being configured such that the LC cell 20 and the quarter wave retarder 18 have the combined effect of providing, in the ON state, 0 (zero) or a half wave retardation (or any multiple thereof), the light transmitted by the transflector 4 is transmitted by the LC cell 20 and quarter wave retarder 18 without a change in polarization. The right-handed circularly polarized light consists of equal amounts of s- polarized and p-polarized light, the s-polarized component being absorbed by the polarizer 16 to produce a p-polarized light ray which can be presented to a viewer. Accordingly, in the ON state the transflective display 1 appears bright. Fig. 3 shows, schematically, a trace of a light ray emitted by the backlight system through the transflective display of Fig. 1 in an OFF state.

The optical path of the light ray 31 is identical to that shown in Fig. 2. The LC cell 20 now being brought in a state in which the combined effect of the LC cell 20 and quarter wave retarder 18 is a quarter wave retardation, the right-handed circularly polarized light rays 33 and 37 provided by the backlight system 6 are converted into s-polarized light. The s-polarized light is absorbed in the dichroic polarizer 16. No light is able to reach the viewer and the transflective display 1 appears dark.

Fig. 4 shows, schematically, a trace of an ambient light ray as it propagates through the transflective display of Fig. 1 in an ON state. When the transflective display 1 is operated in reflection, an ambient unpolarized light ray 41 is converted into a p-polarized light ray by the dichroic polarizer 16. The LC cell 20 being configured such that the LC cell 20 and the quarter wave retarder 18 have the combined effect of providing, in the ON state, 0 (zero) or a half wave retardation (or any multiple thereof), the light transmitted by the polarizer 16 is transmitted by the LC cell 20 and quarter wave retarder 18 without its polarization being changed, a p-polarized light ray 41 is incident on the transflector 4. Specular reflection does not change the polarization of linearly polarized light so the reflected light ray 43 is also p-polarized. The polarization of the light ray 43 being unaffected by the LC cell 20 and retarder 18 combination, p-polarized light is incident on the dichroic polarizer 16 and the polarization being aligned with the transmission axis of the dichroic polarizer 16, the light ray 43 is able to reach a viewer.

The light ray 45 transmitted by the transflector 4 leaves it p-polarized and is then successively converted into a RH circularly polarized light ray 45 by the quarter wave retarder 8, transmitted unaffected by the linear-polarized-light-emitting light source 10 without the polarization being changed, converted into p-polarized light ray by the quarter wave retarder 12. The polarization of linear polarized light does not change upon reflection off a specularly reflecting surface, therefore the reflected light ray 47 is p-polarized. In a manner analogous to light ray 45, the light ray 47 is, after having passed in succession, the quarter wave retarder 12, the linear-polarized-light-emitting light source 10, the quarter wave retarder 8 an the transflector 4, presented to the LC cell 20 as a p-polarized light ray. It is evident from Fig. 3 that any light ray reflected off the transflector 4 towards the light reflecting surface 14 is ultimately available to the viewer.

In short, Fig. 3 demonstrates that, at least theoretically, neither the backlight system 6 nor the transflector 4 adversely affects the brightness of the transflective display 1 in reflection, the brightness obtained, at least theoretically if all components perform ideal, equaling that of a pure reflective display.

In reflection with the transflective display 1 in the OFF state, an unpolarized ambient light ray 51 becomes p-polarized by the dichroic polarizer 16 and, as the combined effect of the LC cell 20 and the quarter wave retarder 18 in the OFF state is to provide a quarter wave retardation, the light ray 53 emerges as a left-handed circularly polarized light ray 53. As a specularly reflecting surface changes the handedness of light incident thereon upon reflection, reflection off the transflector 4 produces a right-handed light ray 53. As explained with reference to Fig. 3, in the off state, right-hand circularly polarized light is converted into s-polarized light by the LC cell 20 and the retarder 18 which is then absorbed in the dichroic polarizer 16 of the transflective light valve 2.

The light ray 55 transmitted by the transflector 4 remains LH polarized, is then s-polarized by quarter wave retarder 8, transmitted unaffected that is without changing polarization by the linear-polarized-light-emitting light source 10, converted to LH polarized light by quarter wave retarder 12, converted into a light ray 55 which is RH polarized after the specular light-reflecting surface 14, which light ray 55 is, in succession, converted to p- polarized light by quarter wave retarder 12, transmitted without changing polarization by the linear-polarized-light-emitting light source 10, converted to RH polarized light by the quarter wave retarder 8 and transmitted by the transflector 4 without changing polarization to produce a RH polarized light ray which as is explained above is completed absorbed by the transflective valve 2. The light ray after reflection of the RH light ray 55 off the transflector 4 is again LH polarized. In summary, the transflective display 1 appears dark, more particular dark to the extent a purely reflective display would be which demonstrates that neither the transflector 4 nor the backlight system 6 has an adverse affect on the contrast of the transflective display operated in reflection.

In the display of Fig. 1, the LC cell 20 is configured to provide, in combination with the quarter wave retarder 18, zero or a half wave retardation or a multiple thereof in the ON state and a quarter wave retardation in the OFF state. The quarter wave retarder 18 providing a quarter wave retardation, the LC cell should provide a quarter wave retardation (positive or negative relative to the retarder 18) in the ON state and zero retardation in the OFF state. An example of a cell providing such states is the well known electrically controlled birefringence (ECB) cell which comprises a uniaxially oriented nematic LC layer disposed between transparent electrodes, the direction of uniaxial orientation being set at angle of 45° with the transmission axis of the dichroic polarizer 16. As an alternative to the ECB cell, use can be made of switchable half wave retarder combined with a non-switchable quarter wave retarder or a ferro-electric LC cell. The type of transflective light valve 2 is not critical. Any transflective light valve which, in cooperation with the transflector, is adapted to switch, in reflection, between a light and dark state using unpolarized ambient light and in transmission between light and dark using circularly polarized light may be used. For example, a (super) twisted nematic cell may also be used. In particular, a color super twisted nematic cell having 240° twist having a 830 nm retardation layer combined with a twisted retarder layer (known in the art as such) may be used. In case of a thin-film-transistor active matrix LCD, a 60° twist TN cell may be suitably used.

The transflector 4 may be used as a component separate from the transflective light valve 2 as shown in Fig. 2 or may be integrated with the transflective light valve 2 by using electrodes adapted to partially transmit and partially reflect light incident thereon. The operation of the present embodiment has been explained in terms binary ON and OFF states, however this is not essential gray levels are readily implemented in manners conventional in the art. The transflective light valve 2 may comprise a single valve or a plurality of light valves. In particular the light valve may comprise a plurality of independently addressable light valves. The plurality of light valves may be configured to form a segmented display cell or arranged in a matrix, to obtain a passive matrix display or an active matrix display in which each of the individually addressable light valves is driven by for example a thin film transistor or a thin film diode. Very advantageously the plurality of light valves may be combined with a color filter to obtain a multi-color or full-color transflective display 1.

Because generally a color filter absorbs a large part of the light incident thereon (for example a color filter comprising red green and blue colored regions absorbs typically one third of the incident light) efficiency and brightness is of particular importance in color transflective displays. In order to use the color filter both in reflection and transmission the color filter is typically provided on the viewing side of the transflector 4.

The transflector 4 is partially transmissive and partially reflective for light incident thereon. Transflectors are known per se and such known transflectors may be suitably used in the transflective display of the present invention. For example, the transflector may be a transparent substrate, such as glass or a transparent synthetic resin, provided with on one side or on two opposite sides with a light reflective layer, such as a metal layer. The transmission of the metal layer and thus the transflector may be tuned by varying the thickness of the metal layer. Alternatively the metal layer may cover the substrate in accordance with a pattern and the percent of area so covered with metal varied to tune the transmission. Theoretically, the transmission of the transflector 4 should be as low as possible because, as explained above with reference to Figs. 2 and 3, the back light system 6 ensures that, despite the limited transmission, all light emitted by the back light system 6 is ultimately presented to the transflective light valve 2 whereas in reflection more ambient light is available when the transmission is decreased. Therefore, the transmission may be for example less tan 50 %, but preferably it less than 20 %. On the other hand, because the components of the backlight system 6 as well as the transflector itself are not ideal and loss of light unavoidably occurs to some extent, for example by parasitic reflection at interfaces and absorption at light-reflection surfaces, it is in practice advantageous to limit the number of times the light emitted by the light source 10 is recycled, such limitation being achieved by the increasing transmission of the transflector 4. The metal layer may be made of any metal such as aluminum. However because the light emitted by the light source 10 typically reflects off the transflector 4 a number of times and each time some light is absorbed in the metal layer, preferably a metal surface of silver or a silver alloy is used. Silver and silver metal alloy layers are known for their low absorbance. The transflector 4 may also comprise a dielectric mirror such mirrors being well known in the art. Such dielectric stacks have the advantage of being substantially non-absorptive.

The backlight system 6 comprises a first and second quarter wave retarder 8 and 12. Such retarders are well known in the art. The handedness of the first and second quarter wave retarder can be selected independently of one another. The linear-polarized-light-emitting light waveguide 10 is adapted to emit linearly polarized light. Preferably, it preferentially emits light the transflector side of the backlight system 6. If the linear-polarized- light-emitting light source 10 of Fig. 1 emits a p- polarized light ray towards the light-reflecting surface 14 then, in the OFF state, such a light ray is ultimately presented to the polarizer 16 as p-polarized, since this direction coincides with the transmission axis of the polarizer 16, this light ray is able to reach the viewer which is desired as in the OFF state the display is to appear dark. The reduction in contrast can be counteracted by providing a linear-polarized-light-emitting light source 10 which preferentially emits light to the transflector side. The linear-polarized-light-emitting light source 10 is furthermore transparent in order to allow light reflected off the transflector 4 to reach the light-reflecting surface 14 and vice versa.

The linear-polarized-light-emitting light source 10 shown in Fig. 1 is a side-lit linear-polarized-light-emitting light source. This is preferred but not essential. A linear- polarized-light-emitting light of another suitable type is disclosed in EP 606939. The linear- polarized-light-emitting light source disclosed therein has a lamp which is arranged between a broad band cholesteric polarizer and a light reflector. A disadvantage of using such an arrangement in the backlight system of the present system is that the lamp and the light reflector obstruct the optical path of light reflected off the transflector 4 towards the light- reflective surface 14 and thus impedes the recycling process. Furthermore, a relatively thick display results. In the preferred side-lit linear-polarized-light-emitting light source 10 the lamp is arranged on a side of a waveguide and light is coupled in via an entry side face of the waveguide. The waveguide guides, by means of total internal reflection, the light down the waveguide. Along the waveguide light is coupled out at an exit surface, a major surface of the waveguide which faces the side to which the light is to be emitted. By including appropriate polarization-selection means light is coupled out polarization selectively. Such linear-polarized-light-emitting light source are known as such. See eg US 5,729,311, US 5,845,035, WO 01/53745 and WO 01/90637. P Particularly suitable embodiments are described in European patent applications having application numbers 01203674.5 (Applicant's reference PHNL010690EPP) and 01203666.1 (Applicant's reference

PHNL010683EPP). These documents also provide suitable means for making the waveguide 10 emit preferentially to one side, one such means being to provide the waveguide a relief structured outcoupling surface by providing outcoupling elements which protrude from the waveguide or by using a grooved outcoupling surface. The light-reflecting surface 14 is adapted to invert the handedness if circularly polarized light is incident thereon. As is well known in the art, specularly reflective metal surfaces provide such inversion. A convenient low cost metal of choice is aluminum, but since aluminum has a relatively high absorption, silver or silver alloy layers surfaces may be preferred since such surfaces show less absorption. In view of efficiency and brightness specular reflectivity is to be as high as possible. However, in order obtain afford comfortable viewing under a range of viewing angles it may be preferable to have a certain degree of diffuse reflectivity say 10 % or less.

Although the components in Fig. 1 are drawn spaced from one another this is by no means not essential. Components may be laminated together to form an integrated laminate where appropriate. Furthermore, components may be integrated where appropriate to save space.

In the display described in Fig. 1 the backlight system may be replaced with a backlight system which comprises a transparent side-lit circular-polarized-light-emitting waveguide, transparent meaning in particular transparent for light incident on the major surfaces of the waveguide, such as the light which is reflected from the transflector and the light-reflecting surface.

A side-lit circular-polarized-light emitting waveguide may be obtained by a combination of a transparent waveguiding substrate and a cholesteric layer, the pitch of the cholesteric layer being selected such that visible waveguided light is selectively reflected at the interface of the waveguiding substrate and the cholesteric layer thus obtaining a reflected beam of circularly polarized light of a first handedness and a transmitted beam of circularly polarized light of a second handedness complementary to the first. Those skilled in the art will appreciate that, dependent on the specific design of the backlight, specifically on how the refractive indexes of the cholesteric layer and the waveguiding substrate are matched, either the beam having the first handedness or the beam having the second handedness can be selectively coupled out.

The cholesteric layers disclosed in EP 606 940 can be suitably used to obtain a cholesteric layer for use in the backlight provided the pitch is selected such that the cholesteric layer is optimized for selectively reflecting circularly polarized visible light incident at waveguiding angles instead of at normal or near-normal angles as in EP 606940. Pitch selection is done using the well-known relationship λ = n.p.cosθ wherein n is the average refractive index of the cholesteric material, p is the pitch and θ is the angle incidence measured from the normal. The pitch light incident at an off-normal angle encounters is larger than encountered by normally incident light. Therefore, if visible light is to be selectively reflected at non-normal angles of incidence, in particular at waveguiding angles, the pitch needs to be made smaller. An advantageous side-effect of making the pitch smaller is that at normal incidence the reflection wavelength shifts in or towards the UV range rendering the side-lit circular-polarized-light emitting waveguide more transmissive in the visible light range as a result of which reflections leading light of the wrong handedness being presented to the LC cell is suppressed.

In order to render the side-lit circular-polarized-light emitting waveguide even more transmissive for light incident at normal or near-normal angles the waveguiding substrates may be provided with discrete outcoupling elements in accordance with a desired pattern, such outcoupling elements as such being well known in the art, the outcoupling elements being formed from cholesterically ordered material. The area not covered by outcoupling elements is highly transmissive with respect to normal or near-normal incident light.

In order to obtain effective reflection at the interface of the cholesteric layer and the waveguiding substrate, the cholesteric layer or element is to have a minimum thickness or height corresponding to about 4 times the pitch of the cholesteric material from which the cholesteric layer or element is formed. Typically, the thickness is about 1 to 10 μm.

Claims

CLAIMS:

1. A transflective display comprising, in succession: a transflective light valve;

- a transflector; and

- a backlight system; - the backlight system comprising, in succession: a first quarter wave retarder;

- a transparent linear-polarized-light-emitting light source adapted to emit linear polarized light; a second quarter wave retarder; and - a light-reflecting surface.

2. A transflective display as claimed in claim 2, wherein the linear-polarized- light-emitting light source is adapted to emit linear polarized light preferentially to the transflector side of the backlight system.

3. A transflective display as claimed in claim 1 or 2, wherein the transparent linear-polarized-light-emitting light source is a side-lit linear-polarized-light-emitting waveguide.

4. A transflective display as claimed in claim 1, 2 or 3, wherein the transflector has a transmission of 20 % or less.

5. A transflective display as claimed in claim 1, 2, 3 or 4, wherein the transflector has a reflecting surface facing the backlight system, the reflecting surface being a silver or a silver alloy surface.

6. A transflective display as claimed in claim 1, 2, 3, 4 or 5, wherein the transflector comprises a dielectric mirror adapted to partially reflect and partially transmit light incident thereon.

A transflective display as claimed in claim 1, 2, 3, 4, 5 or 6, including a color filter.

8. A backlight system comprising, in succession: a first quarter wave retarder; a transparent linear-polarized-light-emitting light source adapted to emit linear polarized light; a second quarter wave retarder; and - a light-reflecting surface.

9. A transflective display comprising, in succession: a transflective light valve; a transflector; and a backlight system; - the backlight system comprising, in succession: a transparent side-lit circular-polarized-light-emitting waveguide; and - a light-reflecting surface.