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
  • G02F1/133632—Birefringent elements, e.g. for optical compensation with refractive index ellipsoid inclined relative to the LC-layer surface
• • 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
  • G02F1/133634—Birefringent elements, e.g. for optical compensation the refractive index Nz perpendicular to the element surface being different from in-plane refractive indices Nx and Ny, e.g. biaxial or with normal optical axis
• • 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
  • G02F2202/00—Materials and properties
  • G02F2202/40—Materials having a particular birefringence, retardation

Definitions

• the invention relates to a birefringence compensation liquid crystal display device making it possible to significantly increase the viewing angle of the display device.
• Liquid crystal displays have experienced a great boom with the development of laptops using thin film transistor technology (TFT - Thin Film Transistor) and a helical liquid crystal cell TN (Twisted Nematic).
• TFT - Thin Film Transistor Thin Film Transistor
• TN Transmission Nematic
• Manufacturers of LCD screens are now looking to diversify and expand their market by positioning themselves, for example, in the niche of desktop computers. For this, a specific problem must be solved: the viewing angle of these screens. Indeed, due to the intrinsic birefringence of the liquid crystal, the contrast level drops as soon as the observer moves away from the normal on the screen, and for certain observation areas the gray levels are reversed. This phenomenon, tolerable for the portable screen application designed to be single-user, must absolutely be compensated when it comes to making screens of desktop computers.
• ⁇ angle of the observer with the normal of the screen
• ⁇ angle of the projection of the direction of observation in the plane of the screen, identified with respect to the east-west axis (horizontal).
• Figure 1 is shown the conoscope of an uncompensated TN liquid crystal cell.
• the first consists in modifying the structure of the cell by creating in each elementary cell (pixel) several domains in which the anchoring of the liquid crystal is different. We then obtain an averaging effect which significantly reduces the problem. This solution leads to greater complexity in the screen manufacturing process.
• Other types of liquid crystal cells where the alignment, the nature of the liquid crystal or the principle of addressing are different from the nematic liquid crystal in a TN (Twisted Nematic) helix have been tested.
• Some, such as In Plane Switching (IPS) cells have resulted in commercial products of equivalent quality to TN liquid crystal and with a very wide angle of view. However, these complex effects to implement are difficult to control.
• IPS In Plane Switching
• the third solution does not modify the structure of the cell and corrects the birefringence of the liquid crystal by adding one or more birefringent films optimized to compensate for the effect of the liquid crystal.
• the philosophy of compensation is as follows: the viewing angle problems of TN liquid crystal cells arise from the birefringent nature of liquid crystal, which transforms the polarization of a light wave differently depending on its angle of incidence. As the extinction between crossed polarizers is only possible if the output polarization is linear, black is only obtained for angles close to normal. We therefore seek to cancel this birefringence by the addition of films having a "reverse" birefringence.
• the axes of the reference frame of the ellipsoid constitute the own axes of the medium, and the length of these axes is equal to the propagation index of the light polarized along the corresponding axis.
• the middle is "uniaxial".
• the index along the axis of revolution or optical axis is called extraordinary index n e
• the index along the other two axes is the ordinary index n 0 (see Figures 2a and 2b).
• the extraordinary index n e is greater than the ordinary index
• the medium is said to be uniaxial positive
• the ellipsoid is elongated in the shape of a cigar ( Figure 2a).
• the extraordinary axis is the slow axis.
• the medium is said to be negative uniaxial, the ellipsoid is flattened in the shape of a cushion or plate ( Figure 2b).
• the extraordinary axis is the rapid axis. The difference between these two indices is very small, for example 0.1%, but this is enough to introduce very significant changes in polarization.
• the medium is biaxial if the ellipsoid is not of revolution, that is to say if there are three orthogonal proper axes with three different indices.
• Liquid crystals are positive uniaxial media, the optical axis corresponding to the director of the liquid crystal molecule.
• birefringent films are positioned between the polarizers and the cell substrates.
• Figure 3 provides an example cell with a film on each side of the cell.
• the polarizer is oriented 90 ° from the friction direction of the liquid crystal molecule located on the same side of the cell. This configuration is called type II.
• the polarizer is parallel to the director of the liquid crystal molecule anchored on the surface located on the same side of the cell.
• FIG. 6 presents the isocontrast curves of a TN liquid crystal cell compensated with two biaxial films, depending on the observation position expressed in ( ⁇ , ⁇ ).
• the horizontal viewing angle is very good ( ⁇ 70 ° for isocontrast 50 but the vertical viewing angle is very limited ( ⁇ 15 ° - isocontrast 50).
• Another method for obtaining an inclined plate is to use a holographic network whose fringes are inclined relative to the substrate. If the pitch of the fringes is sufficiently low compared to the illumination wavelength, the hologram works in “birefringence form” and is equivalent to a uniaxial medium of the attitude type whose optical axis is coincident with the normal to plan of the fringes. Such a correction method is described in French patent application No. 2,754,609.
• a “oblique” dielectric layer evaporation process also makes it possible to obtain, by the concept of “birefringence form”, thin films of inclined optical axis of biaxial type with a dominant “cigar”.
• US Patent 5,504,603 describes compensation architectures including an inclined positive biaxial and various combinations of birefringent films "in the plane". The object of the invention is to improve the viewing angle both horizontally and vertically.
• the invention therefore relates to a liquid crystal display device comprising a liquid crystal screen provided on at least one of its main faces with means for correcting the birefringence of the liquid crystal, characterized in that the correction means comprise a film having a biaxial birefringence with anisotropy of refractive indices as well as a film having a uniaxial birefringence of optical axis oblique to the plane of the liquid crystal screen.
• the invention therefore provides for combining a biaxial film and a uniaxial film of oblique optical axis to allow a correction of angle of view horizontally and vertically.
• the invention also provides the relative orientations of the various components of the device.
• the invention consists in combining in a type II configuration of a TN cell, a biaxial stretched plastic film and a uniaxial birefringence film inclined in a particular and unconventional orientation by compared to that usually recommended in known corrections.
• the compensator is therefore hybrid because it consists of two different technology elements: a stretched plastic film having a birefringence called "in the plane" and an oblique birefringence which can be positive or negative.
• the orientation of the optical axis is according to an optimized angle dont whose sign is given by the direction of the middle molecule when the cell is energized (black state for a TN between crossed polarizers). For example, for a median molecule oriented along + ⁇ :
• optical axis oriented in the direction of positive ⁇ for a dish type obliquity
• Plate-type obliquity can be obtained by holography or with a discotic-type polymerized liquid crystal.
• the cigar type obliquity can be obtained by oblique evaporation or with a nematic type polymerized liquid crystal.
• this inclined birefringence has the effect of improving the vertical viewing angle of the cell conoscope with biaxial film in the plane, while maintaining its good initial horizontal viewing angle.
• the advantage of such a structure lies in the fact that a low inclined birefringence, typically of the order of 40 nm, allows good compensation to be obtained (see embodiment) when it is coupled with a biaxial commercial film 80 nm behind in the plane. Recall that the inclined birefringence, difficult to achieve, is all the more accessible technologically as it is weak.
• Figure 8 shows in a simplified manner a display device according to the invention.
• This device includes:
• liquid crystal screen or cell 1
• biaxial films 2 and 2 ′ having an anisotropy of refractive indices joined to each side of the liquid crystal screen
• the device of Figure 8 is a preferred embodiment. However, one could only provide: - either a biaxial 2 or 2 'birefringence film and a uniaxial 3 or 3' birefringence film arranged on each side or on the same side of the liquid crystal screen;
• FIG. 9 represents an exploded view of such an exemplary embodiment.
• the orientation of the device is relative to the East-West and North-South directions indicated at the bottom of the figure.
• the liquid crystal screen 1 comprises a helical nematic liquid crystal. This liquid crystal is sandwiched between two glass slides whose faces 11 and 11 'in contact with the liquid crystal have been treated by friction so as to determine the orientation of the molecules in contact with these faces and their inclination (tilt) with respect to at the plane of the faces.
• the direction of friction of the face 11 is directed at -45 ° from the west-east direction.
• the direction of friction of the face 11 ' is directed at + 45 ° in the same direction.
• the polarizer 4 associated with the face 11 is oriented at 90 ° from the direction of friction of this face. It is the same for the polarizer 4 'which is oriented at 90 ° from the direction of friction of the face 11'.
• the two polarizers are therefore oriented 90 ° from each other, possibly within a few degrees.
• the biaxial birefringence film 2 is oriented in such a way that its largest index axis is oriented perpendicular to the direction of friction of the face 11.
• the biaxial birefringence film 2 ' is oriented in such a way that its largest index axis is oriented perpendicular to the direction of friction of the face 11 '.
• the uniaxial film 3 is oriented so that:
• the optical axis of this film makes an angle ⁇ 0 between 25 ° and 60 ° with the normal to the plane of the film (the plane of the liquid crystal screen). For example, this angle is 35 °;
• the 3 'uniaxial film is similarly oriented so that:
• the optical axis of this film makes an angle between 25 ° and 60 ° with the normal to the plane of this film;
• the biaxial film is a stretched plastic film.
• the uniaxial film is a holographic film in which a network of layers has been recorded in volume.
• Figures 10a and 10b show such a network and its registration process.
• the optical axis is normal to the plane of the index strata.
• the optical axis of the film makes an angle ⁇ 0 of 35 ° with the normal to the plane of the film and the optical axis of the film projects along the axis Oy on the plane of the film.
• FIG. 10a represents a recording mode of the network of strata according to which two counterpropagative waves interfere in the film.
• the device of the invention uses a biaxial film in the POLATECHNO brand plane and a film of the type
• Inclined "plate” produced by holography, the characteristics of which are as follows: POLATECHNO Film
• the axis of highest index is perpendicular to the direction of rubbing (to a few degrees depending on the configuration)
• the inclined attitude is positioned between the biaxial and the polarizer, on either side of the cell.
• the conoscope corresponding to this compensation is given in FIG. 7.
• a slight uncrossing of the polarizers (a few degrees) is necessary to optimize the structure.
• liquid crystal cell configuration is a TN liquid crystal cell in type II configuration described above.
• a first device only provides biaxial stretched plastic films (POLATECHNO type) placed on either side of the liquid crystal screen and corresponds to a device known in the art.
• POLATECHNO type biaxial stretched plastic films
• the conoscope obtained with such a device is of the type shown in FIG. 6.
• the three device configurations which follow correspond to configurations according to the invention comprising on each side of the liquid crystal cell a biaxial plastic film and a uniaxial holographic film ( see Figure 9). These different configurations essentially differ from each other by the angle ⁇ made by the projection of the optical axis of the holographic film on the film plane with the West-East direction.

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

Priority Applications (3)

Applications Claiming Priority (2)

Publications (1)

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ID=9525654

Family Applications (1)

Country Status (6)

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Citations (5)

• 1998
  • 1998-04-24 FR FR9805181A patent/FR2778000B1/fr not_active Expired - Fee Related
• 1999
  • 1999-04-16 WO PCT/FR1999/000902 patent/WO1999056169A1/fr not_active Application Discontinuation
  • 1999-04-16 EP EP99914601A patent/EP1073932A1/fr not_active Withdrawn
  • 1999-04-16 KR KR1020007011791A patent/KR20010042963A/ko not_active Application Discontinuation
  • 1999-04-16 JP JP2000546271A patent/JP2002513168A/ja active Pending
  • 1999-04-21 TW TW088106383A patent/TW457390B/zh not_active IP Right Cessation

Patent Citations (5)

Non-Patent Citations (1)

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