DISPLAY DEVICE AND DISPLAY PANEL

Abstract: Display device with an illumination system, a wideband (cholesteric) reflector and a switchable electro-optical medium (liquid crystal cell) between the reflector and an analyser, in which various parameters (for example, twist angle, angle between the orientation plane and polarization plane as well as the thickness of the panel) have been optimized.

Description: Display device and display panel. The invention relates to a display device comprising an illumination system with a radiation source for supplying a radiation beam, and a display panel with an electro-optical display medium between two polarizers, which medium is switchable between two optical states. The invention also relates a display panel for such a display device. Such display devices, notably with a liquid crystalline medium as an electro-optical medium (LCD), are used, for example in video apparatus, monitors, but also in information and instrument panels (for example, motorcar dashboards). This type of display device is generally known but has the great drawback that, even when ideally absorbing polarizers are used, less than 50% and often only 25% of the light supplied by the radiation source can be used for forming an image, because a substantially 50% loss occurs in both polarizers due to absorption of the inappropriate direction of polarization. Further losses also occur in colour filters and other layers. There should be sufficient light for the viewer, both in a direct-vision device and in image projection devices. To compensate for the loss of light in the polarizers, high-power lamps must be used in both devices, which lamps consume much electrical power and thus have a relatively short lifetime, while they may also have to be cooled. If portable, direct- vision display devices are concerned, the batteries will have a short lifetime because they must feed a high-power lamp. An image projection device must be provided with a high-intensity radiation source. Since in both cases the light intensity incident on the polarizers is high, these polarizers are heated by absorption. Since the polarizers are in the vicinity of the display panel in this type of device, it may be necessary to provide a cooling system so that the display device becomes more complicated and more expensive and makes a troublesome noise. It is an object of the invention to provide a display device in which the above-mentioned drawbacks are mitigated as much as possible. To this end, a display device according to the invention is characterized in that the electro-optical display medium is birefringent in at least one of the two optical states and at least one polarizer is a reflecting polarizer, and in that polarization-converting means are present at a side facing away from the display panel. Polarization-converting means are understood to be means which convert the unwanted state of polarization into the desired state of polarization and also send it towards the display panel. A reflecting polarizer passes the radiation having the desired direction of polarization, whereas the radiation having the unwanted direction of polarization is reflected. If there are further means which convert the unwanted state of polarization at least partly into the desired state of polarization, the resulting radiation will also be passed at least partly to the electro-optical display medium. In this way substantially the full radiation intensity of the radiation beam is passed by the polarizer due to repeated reflection and conversion of the direction of polarization, so that a much larger part of the radiation intensity transmitted by the radiation source can be utilized for forming the image. An embodiment of the invention is characterized in that the reflecting polarizer is a cholesteric filter and the polarization-converting means comprise a reflector. Cholesteric filters have an optical layer of a liquid crystalline material with a cholesteric ordering. This means that the molecules of the material spontaneously order in solution to a spiral or helix structure with a pitch p. After such a solution has been provided as a thin, optically active layer between two parallel substrates, the helix-like structure is directed in such a way that the axis of the helix is transverse to the layer. The alignment of the helix can be improved by providing an orientation layer on the facing surfaces of the substrates. When an unpolarized radiation beam is incident on such a filter, radiation whose circular direction of polarization (levorotatory or dextrorotatory) corresponds to the direction of the molecular helix and whose wavelength corresponds to the pitch p of the helix will be reflected, whereas radiation having the opposite direction of polarization or a wavelength which is not adapted to the filter is passed. The reflection wave λ 0 of the cholesteric filter is defined by: X Q = -A. (n 0 + n^.p in which n 0 and n e are the ordinary and extraordinary refractive index of the material of the filter and p is the pitch of the molecular helix. The light having the direction of polarization which is unwanted for the polarizer is thus no longer absorbed but reflected by the cholesteric filter so that heating due to absorption is prevented. The pitch p of the molecular helix preferably varies across the layer thickness of the filter between a lower limit and an upper limit, such that the resultant bandwidth of the reflected light corresponds to a bandwidth which is required to activate the filter in the full visible range. Such cholesteric filters are known from EP-A-0,606,940 (PHN 14.345) in the name of the Applicant. An example of an electro-optical display medium which is birefringent in one of the two optical states is, for example a twisted-nematic display cell (TN or STN). A preferred embodiment of a display device according to the invention is characterized in that the display medium is switchable between two birefringent optical states. An example of an electro-optical display medium which is birefringent in the two optical states is, for example a uniaxially birefringent medium whose principal axis rotates in the plane of the display panel under the influence of an electric field, or a ferro¬ electric liquid crystal display device (FLCD), preferably a surface-stabilized FLCD (SSFLCD). A panel having a twist angle of at least 10 degrees and at most 60 degrees is then preferably chosen for the display device. An angle of at least 10 degrees and at most 60 degrees is then chosen between the direction of orientation of the liquid crystal material at the side of the reflecting polarizer and the polarization plane at the side of the linear polarizer. A display panel according to the invention is characterized in that the electro-optical display medium is birefringent in at least one of the two optical states and at least the polarizer is a reflecting polarizer, and in that polarization-converting means are present at a side facing away from the display panel. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. In the drawings: Fig. 1 shows diagrammatically a display device according to the invention, and Fig. 2 shows diagrammatically the light path in such a device, while Figs. 3-5 show transmission/voltage characteristics of some display devices according to the invention. Fig. 1 is a diagrammatic cross-section of a part of a display device 1 with a display panel 2 which comprises, in this example, a twisted-nematic liquid crystal material 3 which is present between two substrates (not shown) of, for example glass, and is provided with electrodes. The device further comprises orientation layers (not shown) which orient the liquid crystal material on the inner walls of the substrates in such a way that the cell has a twist angle φ. The device further comprises an illumination system 6 which is shown in the Figure as a simple backlight illumination with one or more radiation sources 7, as well as a diffusor 8 of a diffuse and depolarizing material (for example, a transparent plate having a surface structure) and a mirror 9 at the side of the radiation source(s) 7 remote from the display panel 2. At the side of the illumination system 6, the display panel 2 has a reflecting polarizer 4, in this example a cholesteric filter. Radiation coming from the radiation source 7 and having the desired state of polarization will be passed to the liquid crystal material 3, whereas radiation having the complementary state of polarization will be reflected. The reflected radiation is subsequently converted by polarization-converting means, in this case the metal mirror 8, into radiation having the state of polarization which can be passed by the polarizer 4. Radiation which is not passed by this polarizer 4 is reflected between the mirror 8 and the polarizer 4 until it has the suitable state of polarization to be passed towards the liquid crystal material 3. Radiation having a direction of polarization which is unsuitable for the polarizer 4 is therefore no longer absorbed in the polarizer but each time depolarized and then partly transmitted and partly reflected so as to change its state of polarization, so that, apart from reflection losses, all radiation from the source 7 will eventually reach the liquid crystal material 3. Such an effect can be eminently achieved by means of a cholesteric filter having a pitch continuously varying across the layer thickness, as described in EP-A-0,606,940. The radiation passed by such a cholesteric reflector is circularly polarized (see Fig. 2). For such a configuration, the transmitted radiation intensity T is T»-i -iisin (ø3) cos (φ3) sin (2 (Ω+ø) ) 2 β +-flsin 2 (φβ) cos (2 (Ω+ ) ) (l) 0 2 In this formula, β is defined by β-Jϊ oT (2) in which "TJΓ (3) In the definition of a. d denotes the thickness of the liquid crystal layer, Δn denotes the anisotropy in the refractive indices and λ denotes the wavelength of the light used. 0 is the angle between the direction of orientation at the side of the reflecting polarizer and the polarization plane of the linear polarizer. When the pass direction of the polarizer is rotated through an angle of τ/2, the transmission changes from T to 1-T. The same change is achieved by inverting the twist sense of the cholesteric reflector (left- or right-handed). For φ = 0, the transmission is: In this case, T = 1 or T = 0 for sin(2Ω) = ± 1 and dΔn = λ/4. In such a configuration, liquid crystal effects may be utilized in different manners. First, the liquid crystal cell may be used as a birefringent medium, in which the principal axis rotates in the plane of the cell under the influence of the (driving) electric field. The associated transmission is described by means of equation (4). Such a medium may be realised, for example by means of a ferro-electric liquid crystal, particularly a surface-stabilized ferro-electric liquid crystal (surface-stabilized FLCD or SSFLCD), or with the electroclinic effect which occurs in the smectic A phase. In this case, the angle Ω in equation (4) is varied. Maximum and minimum transmission (T = 1 and T = 0) are then achieved for d.Δn = ± λ/4, ± 3λ/4, ± 5λ/4, ... etc. In the most commonly used materials this leads to fairly thin cells (hence difficult to manufacture) at d.Δn = ± λ/4; an optimum choice in connection with the technology and optical properties to be used (discoloration with thicker cells) appears to be d.Δn = ± 3λ/4. A second possibility is to switch the liquid crystal cell between a birefringent and a non-birefringent state. For example, a twisted-nematic effect may be used for this purpose. However, the configuration of Fig. 2 then has a transmission of approximately -A in the (light-transmissive) non-birefringent state of the liquid crystal cell (Δn = 0). The most favourable results, both as regards brightness (or psychometric lightness) and manufacturing possibilities were achieved with cells in which the effective birefringence of the cells switches between two birefringent states, the one state leading to a substantially complete extinction and the other state leading to a substantially full transmission. To this end both the angles Ω and φ in equation (4) are varied. The angle φ varies between, for example 10 and 60°. At larger angles, a full modulation of the transmission is substantially impossible. Example I: Fig. 3 shows the psychometric lightness L as a function of the voltage V across the cell for a configuration with φ = 0 and Ω = τcl4. Example II: Fig. 4 shows the transmission/voltage curve for a configuration with φ = τ/9 and Ω = 2τ/9. It is apparent therefrom that with an increasing drive voltage across the cell, the transmission decreases from approximately 100 to substantially zero. Example III: Fig. 5 shows the transmission/voltage curve for a configuration with φ = τ/6 and Ω = τ/6. It is apparent therefrom that with an increasing drive voltage across the cell, the transmission decreases from approximately 100 to substantially zero. Other combinations of φ and Ω are of course alternatively possible, while also birefringent media other than twisted-nematic liquid crystal cells can be used. More generally, the invention relates to a display device having an illumination system, a wideband (cholesteric) reflector and a switchable electro-optical medium (liquid crystal cell) between the reflector and an analyser, in which various parameters (for example, twist angle, angle between the orientation plane and the polarization plane, as well as the thickness of the panel) have been optimized.

Claims: CLAIMS: 1. A display device comprising an illumination system with a radiation source for supplying a radiation beam, and a display panel with an electro-optical display medium between two polarizers, which medium is switchable between two optical states, characterized in that the electro-optical display medium is birefringent in at least one of the two optical states and at least one polarizer is a reflecting polarizer, and in that polarization- converting means are present at a side facing away from the display panel. 2. A display device as claimed in Claim 1, characterized in that the reflecting polarizer is a cholesteric filter and the polarization-converting means comprise a reflector. 3. A display device as claimed in Claim 1 or 2, characterized in that the other polarizer is a linear polarizer. 4. A display device as claimed in Claim 1, 2 or 3, characterized in that the display medium is switchable between two birefringent optical states. 5. A display device as claimed in Claim 4, characterized in that the device comprises a twisted-nematic liquid crystal material and has a twist angle of at least 10 degrees and at most 60 degrees. 6. A display device as claimed in Claim 4, characterized in that the angle between the orientation direction of the liquid crystal material at the side of the reflecting polarizer and the polarizer plane of the linear polarizer is at least 10 degrees and at most 60 degrees. 7. A display device as claimed in Claim 4, characterized in that the display panel comprises a ferro-electric liquid crystal or an electroclinic material. 8. A display panel with an electro-optical display medium between two polarizers, which medium is switchable between two optical states, characterized in that the electro-optical display medium is birefringent in at least one of the two optical states and at least the polarizer is a reflecting polarizer, and in that polarization-converting means are present at a side facing away from the display panel. 9. A display panel as claimed in Claim 8, characterized in that the reflecting polarizer is a cholesteric filter and the polarization-converting means comprise a reflector. 10. A display panel as claimed in Claim 8 or 9, characterized in that the other polarizer is a linear polarizer. 11. A display panel as claimed in Claim 8, 9 or 10, characterized in that the display medium is switchable between two birefringent optical states. 12. A display panel as claimed in Claim 10 or 11, characterized in that the device panel comprises a twisted-nematic liquid crystal material and has a twist angle of at least 10 degrees and at most 60 degrees. 13. A display panel as claimed in Claim 12, characterized in that the angle between the orientation direction of the liquid crystal material at the side of the reflecting polarizer and the polarizer plane of the linear polarizer is at least 10 degrees and at most 60 degrees. 14. A display panel as claimed in Claim 11, characterized in that the display panel comprises a ferro-electric liquid crystal or an electroclinic material.

Inventors: VAN HAAREN JOHANNES ALBERTUS M, VAN HAAREN, JOHANNES, ALBERTUS, MATTHIJS, MARIA
Applicants: PHILIPS ELECTRONICS NV, PHILIPS NORDEN AB, PHILIPS ELECTRONICS N.V.
Application year: 1995
Main classification: G02F1
IPC codes: G02F1