Classifications

• • G—PHYSICS
  • G02—OPTICS
  • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
  • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
  • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
  • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
  • G02F1/133—Constructional arrangements; Operation of liquid crystal cells; Circuit arrangements
  • G02F1/1333—Constructional arrangements; Manufacturing methods
  • G02F1/1335—Structural association of cells with optical devices, e.g. polarisers or reflectors
  • G02F1/1336—Illuminating devices
  • G02F1/13362—Illuminating devices providing polarized light, e.g. by converting a polarisation component into another one
• • G—PHYSICS
  • G02—OPTICS
  • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
  • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
  • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
  • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
  • G02F1/133—Constructional arrangements; Operation of liquid crystal cells; Circuit arrangements
  • G02F1/1333—Constructional arrangements; Manufacturing methods
  • G02F1/1335—Structural association of cells with optical devices, e.g. polarisers or reflectors
  • G02F1/133528—Polarisers
  • G02F1/133536—Reflective polarizers
• • G—PHYSICS
  • G02—OPTICS
  • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
  • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
  • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
  • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
  • G02F1/133—Constructional arrangements; Operation of liquid crystal cells; Circuit arrangements
  • G02F1/1333—Constructional arrangements; Manufacturing methods
  • G02F1/1335—Structural association of cells with optical devices, e.g. polarisers or reflectors
  • G02F1/133528—Polarisers
  • G02F1/133543—Cholesteric polarisers
• • G—PHYSICS
  • G02—OPTICS
  • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
  • G02F2201/00—Constructional arrangements not provided for in groups G02F1/00 - G02F7/00
  • G02F2201/34—Constructional arrangements not provided for in groups G02F1/00 - G02F7/00 reflector
  • G02F2201/343—Constructional arrangements not provided for in groups G02F1/00 - G02F7/00 reflector cholesteric liquid crystal reflector

Definitions

• 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).
• LCD liquid crystalline medium
• 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.
• high-power lamps 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.
• 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.
• a display device 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.
• 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 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).
• FLCD ferro ⁇ electric liquid crystal display device
• SSFLCD surface-stabilized FLCD
• 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.
• Fig. 1 shows diagrammatically a display device according to the invention
• Fig. 2 shows diagrammatically the light path in such a device
• 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 ⁇ .
• 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.
• 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.
• 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.
• polarization-converting means in this case the metal mirror 8
• the radiation passed by such a cholesteric reflector is circularly polarized (see Fig. 2).
• the transmitted radiation intensity T is T»-i -iisin ( ⁇ 3) cos ( ⁇ 3) sin (2 ( ⁇ + ⁇ ) ) 2 ⁇
• ⁇ is defined by
• d denotes the thickness of the liquid crystal layer
• ⁇ n denotes the anisotropy in the refractive indices
• ⁇ 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.
• liquid crystal effects may be utilized in different manners.
• 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.
• the angle ⁇ in equation (4) is varied.
• a second possibility is to switch the liquid crystal cell between a birefringent and a non-birefringent state.
• a twisted-nematic effect may be used for this purpose.
• 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.
• various parameters for example, twist angle, angle between the orientation plane and the polarization plane, as well as the thickness of the panel

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• Physics & Mathematics (AREA)
• Nonlinear Science (AREA)
• Liquid Crystal (AREA)
• Mathematical Physics (AREA)
• Chemical & Material Sciences (AREA)
• Crystallography & Structural Chemistry (AREA)
• General Physics & Mathematics (AREA)
• Optics & Photonics (AREA)

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• 1995
  • 1995-10-23 CN CN95191686A patent/CN1141087A/zh active Pending
  • 1995-10-23 EP EP95933583A patent/EP0739498A1/de not_active Withdrawn
  • 1995-10-23 WO PCT/IB1995/000908 patent/WO1996015474A1/en not_active Application Discontinuation
  • 1995-10-23 JP JP8515871A patent/JPH09507926A/ja active Pending

Non-Patent Citations (1)

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