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
  • G02F1/13363—Birefringent elements, e.g. for optical compensation
• • 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
  • G02F1/137—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 characterised by the electro-optical or magneto-optical effect, e.g. field-induced phase transition, orientation effect, guest-host interaction or dynamic scattering
  • G02F1/139—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 characterised by the electro-optical or magneto-optical effect, e.g. field-induced phase transition, orientation effect, guest-host interaction or dynamic scattering based on orientation effects in which the liquid crystal remains transparent
  • G02F1/1396—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 characterised by the electro-optical or magneto-optical effect, e.g. field-induced phase transition, orientation effect, guest-host interaction or dynamic scattering based on orientation effects in which the liquid crystal remains transparent the liquid crystal being selectively controlled between a twisted state and a non-twisted state, e.g. TN-LC cell
• • 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
  • G02F1/133531—Polarisers characterised by the arrangement of polariser or analyser axes
• • 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
  • G02F1/136—Liquid crystal cells structurally associated with a semi-conducting layer or substrate, e.g. cells forming part of an integrated circuit
  • G02F1/1362—Active matrix addressed cells
• • 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
  • G02F2413/00—Indexing scheme related to G02F1/13363, i.e. to birefringent elements, e.g. for optical compensation, characterised by the number, position, orientation or value of the compensation plates
  • G02F2413/02—Number of plates being 2

Definitions

• the present invention relates to liquid crystal displays and active matrix.
• Such displays conventionally comprise a cell having a thin layer of helical nematic liquid crystals trapped between two transparent plates having two facing faces carrying electrodes for creating an electric field connected to a control circuit and comprising two polarizers framing the cell.
• the facing faces in contact with the thin layer are treated, in general by depositing a coating and brushing, so as to impose an orientation on the molecules of the liquid crystals along these faces.
• the thickness of the layer and the treatment are generally provided in such a way that the twist through the cell or "twist" is 90 ° and that the corresponding spatial delay ⁇ , defined as the product of the thickness . of the layer by the birefringence ⁇ n of the liquid crystal, that is to say about 0.4 u-m. This choice is consistent with the model that the orientation of the polarization vector is guided by the helical arrangement of the liquid crystals.
• a display thus formed and with two crossed polarizers, one of which is perpendicular or parallel to the direction of the molecules along the nearest orientation wall, is qualified as "normally white", in the sense that it is transparent when no electric field is applied to the liquid crystal layer. On the contrary, the display becomes opaque when an electric field of sufficient value is applied to the layer.
• Such a display has a contrast (ratio between transparency in the idle state, that is to say in the absence of an electric field, and transparency when an electric field is applied) satisfactory when the light reaching the eye of the observer crosses the display substantially perpendicular to the layer.
• a contrast ratio between transparency in the idle state, that is to say in the absence of an electric field, and transparency when an electric field is applied
• it presents, in oblique incidence, light leaks which reduce the contrast and distort the colors.
• the delays introduced by such sheets must be optimized taking into account that the presence of these sheets has an unfavorable effect on transparency at rest, in oblique mink.
• the present invention aims to provide a display that meets the requirements of practice better than those previously known, in particular in that it makes it possible to obtain a contrast comparable to that of a conventional display and improves lateral vision while not putting in work that simple means.
• the liquid crystal layer does not have to have a 90 ° twist for it to act as a half-wave retarder.
• Document EP-A-0 448 173 discloses a liquid crystal display in which the twist can go down to 40 °.
• the transmission T of the cell at rest can s to write :
• T i sin k ⁇ .sin ( ⁇ + ⁇ / 2) / k + cos k ⁇ .cos ( ⁇ + ⁇ / 2) I + sin 2 k ⁇ .cos 2 ( ⁇ -2P 1 + ⁇ / 2) .u 2 / (l + u 2 ) (1)
• Equation (1) makes it possible to make a choice of ⁇ leading to the best possible transparency at rest and favoring the contrast.
• the polarizer advantageously makes the angle at rest of the crystals in the boundary layer an angle which is of the order of (2 ⁇ + ⁇ ) / 4.
• liquid crystal cells are not ideal, in the sense that the change in orientation is not uniformly helical and that there is a pre-tilt or "pre-tilt" along the walls.
• ⁇ is consequently carried out by using not the formula (1) above, but a process involving first of all a simulation of the orientation profile in the liquid crystals as a function of the applied voltage.
• software usable for this purpose including DIMOS software, available from Autronic-Melchers GmbH, Düsseldorf, Germany. This software is used with representative values for the constants of the liquid crystals and in particular the constants corresponding to the ZLI-3771 liquid crystals, supplied by the company Merck, Darmstadt, Germany. The software was used on the assumption of a pre-tilt angle on the faces of 3 °, which can be considered as representative.
• the value found for the optimized thickness of the liquid crystal layer must be considered as a "virtual" value, since the para è- being significant optics is in fact the delay given by the liquid crystals, which is the product of the thickness d. of the liquid crystal layer and the birefringence of liquid crystals.
• the birefringence of the liquid crystals used is that of the ZLI-3771, which is equal to 0.1067 for a wavelength of 550 nm.
• Y is the perceived luminance, taking into account the spectral distribution of the light source and the sensitivity curve of the eye.
• the maximum transmission cannot exceed 50%.
• the transmission coefficient which is greater in the "black” state for the 30 ° twist configuration, would normally lead to significantly lower contrast values than with the 90 ° twist configuration and in the normally "white” state .
• the invention makes it possible to arrive at an architecture which provides a maximum contrast of the same order as those obtained with the configuration at 90 ° of twist.
• Two reasons can be mentioned to explain the increased transmission in the "black" state of the configuration at 30 ° torsion.
• the transmission curve as a function of the voltage becomes less steep. This implies that at a given voltage (above the threshold voltage) the transmission coefficient, and therefore the luminance, is all the more important the smaller the angle of torsion.
• transmission in the "black” state is essentially due to the residual delay of the boundary layers of liquid crystals. For the 90 ° twist configuration and normally "white” state, the effects of these layers compensate each other.
• the device takes these effects into account by the presence of at least one delay sheet with positive birefringence, placed between the liquid crystal layer and one of the polarizers, the optical axis of the sheet being parallel to the wall of the cell.
• the sheet is placed so that its optical axis is perpendicular to the orientation of the liquid crystal molecules along the nearest wall.
• two sheets will be used, each placed on one side of the liquid crystal layer.
• a very small spatial delay introduced by each sheet is sufficient, typically less than or equal to one fifth of the spatial delay introduced by the layer of liquid crystals. Owing to their short delay, the sheets practically do not affect the optical quality in the "white” state, both in incident light normal to the cell, and in oblique incidence. On the other hand, these sheets make it possible to obtain a transmission coefficient in the "black" state at normal incidence which is practically as low as with a twist angle of 90 °.
• the delay of each sheet will generally be less than 50 nm and often 20 nm.
• the spatial delay values due to the liquid crystal layer which will be chosen to implement the invention range from approximately 280 nm, for a twist angle of 0 °, up to approximately 400 nm, for an angle 80 ° twist.
• the implementation of the invention brings in all cases an improvement from the point of view of the contrast in lateral vision, due to the maintenance of a low transmission coefficient in the "black” state.
• the improvement is all the more important as the angle of torsion approaches zero. This does not result in an annoying degradation of the transmission coefficient in the "white” state, for a suitable choice of the spatial delay given by the liquid crystal layer.
• FIG. 1 shows a possible constitution of a display according to the invention
• FIG. 1A is a diagram showing the orientations P-L and P2 of the input and output polarizers
• FIG. 2 is a diagram showing an optimal zone for choosing the spatial delay ⁇ for given values of the angle of twist ⁇ , on the one hand without a positive birefringence compensation sheet and on the other hand with such sheets;
• FIGS. 3 and 4 are diagrams showing the variation curves of the transmission coefficient, in polar coordinates, for a display whose layer has a twist angle of 30 °, with and without sheets with positive birefringence;
• the display the basic structure of which is shown in FIG. 1, comprises a layer 10 of nematic liquid crystals, having a thickness of a few ⁇ m.
• the layer 10 is trapped between two transparent plates 12 and 14, generally made of glass.
• the interval between the plates is kept constant by spacer braces 15.
• the crystals are oriented by coatings 16, provided so that the angle of twist or "twist "is less than 90 °. As a rule, the angle will not exceed 85 °. It is possible to go down to 0 °.
• the plates 12 and 14 carry electrodes intended to constitute elementary capacitors each corresponding to a pixel.
• the internal surface of the plate 12 can carry control electrodes constituting an array 18 and each connected to a thin film transistor.
• the transistors are distributed in rows and columns and controlled by a circuit not shown, by the intermediary of a connector 30.
• the other transparent plate 14 carries a counter electrode 20 which is usually a thin film of indium oxide -tin.
• the liquid crystal layer is separated from the plates by brushed orientation layers 16.
• colored filters 25 are interposed between one of the glass plates 14 for example, and the liquid crystal layer.
• the cell thus formed is placed between two crossed polarizers 26 and 28.
• two sheets with positive birefringence 34 and 36, with an optical axis parallel to the plates, are each placed between one of the plates 12 and 14 and the polarizer which is closest thereto.
• FIG. 3 shows the variation of the transmission coefficient in the "black” state for a conventional liquid crystal display with a twist angle of 90 ° (dashed curve ) and for a display optimized at 30 ° torsion angle, with delay sheets.
• the gain obtained in terms of transmission coefficient in the "black” state is found in terms of "iso-contrast.
• FIG. 4 is a comparison of the black states between the transmittance in the case of example 2 (dashed curve) and in the case of sheets whose optical axes are placed perpendicular each to the direction of the molecules at the limit (example 1 ).
• the optimization procedure implemented differs from the previous ones.
• the virtual thickness of the layer of liquid crystal is chosen.
• the delay of the sheets is determined such that the transmission coefficient in the "black" state is less than 0.05%.
• Results obtained are given in the table below.
• the first table corresponds to the optimized value.
• the other tables show the possibility of keeping favorable results with modified values in the domain defined by FIG. 2.
• FIG. 5 shows the transmission curves in the "black” state, for an obliquity of 60 °, on the one hand for the configuration with torsion of 90 ° (in dashes) and on the other hand for the configuration with torsion of 0 ° optimized according to the example (in solid lines). It shows a reduced transmission in a ratio at least equal to two for almost all of the angles.
• Liquid crystal layer 2.6 ⁇ m -> Layer delay: 277.4 nm
• FIG. 7 shows the angular gain in iso ⁇ contrast for a reduction in the delay.
• the curves in dashes correspond to a layer delay of 213.4 nm, and the curves in solid lines with optimized values (retad of the layer of 282.9 nm and of each sheet of 15.7 nm). However, this gain in iso-contrast is offset by a loss of transmission in the "white" state.

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

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Citations (2)

• 1994
  • 1994-12-15 FR FR9415120A patent/FR2728358A1/fr active Granted
• 1995
  • 1995-12-13 WO PCT/FR1995/001661 patent/WO1996018931A1/fr active Application Filing

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