EP1073932A1

EP1073932A1 - Liquid crystal display device with birefringence compensator

Info

Publication number: EP1073932A1
Application number: EP99914601A
Authority: EP
Inventors: Cécile Thomson-CSF Prop. Intell. JOUBERT; Jean-Claude Thomson-CSF Prop. Intell. LEHUREAU
Original assignee: Thomson CSF SA
Current assignee: Thales SA
Priority date: 1998-04-24
Filing date: 1999-04-16
Publication date: 2001-02-07
Also published as: WO1999056169A1; JP2002513168A; TW457390B; FR2778000B1; FR2778000A1; KR20010042963A

Abstract

The invention concerns a liquid crystal display device comprising a type II twisted nematic liquid crystal screen (1). The screen is provided on at least one of its main surfaces (10, 11) with a film (2) having biaxial birefringence with refractive index anisotropy and a film (3) having optical axis uniaxial birefringence slanting with respect to the liquid crystal screen. Preferably, the biaxial film (2) is a drawn plastic film and the uniaxial film (3) is a holographic grating. Moreover, a biaxial film (2, 2') and an uniaxial film (3, 3') are provided on each side of the screen. The assembly is placed between cross polarizers (4, 4'). The relative orientations of the different screen components are clearly defined. The invention is useful for increasing horizontally and vertically a liquid crystal screen viewing angles.

Description

LIQUID CRYSTAL DISPLAY DEVICE WITH BIREFRINGENCE COMPENSATOR

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). Manufacturers of LCD screens (Liquid Crystal Displays) 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.

The angle of view properties of a liquid crystal screen are evaluated from a conoscope, which gives the isocontrast curves as a function of the observation angle characterized by (θ, φ) with: θ = angle of the observer with the normal of the screen φ = angle of the projection of the direction of observation in the plane of the screen, identified with respect to the east-west axis (horizontal). In Figure 1 is shown the conoscope of an uncompensated TN liquid crystal cell.

Various methods have been proposed for the compensation of LCD screens:

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.

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. We characterize a birefringent medium by its ellipsoid of indices, surface characteristic of the index of propagation of a light wave of polarization and given direction: 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.

If the ellipsoid is of circular cross section, the middle is "uniaxial". For a uniaxial element, 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). If 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. If, on the contrary, the extraordinary index n _e is lower than the ordinary index n ₀ , 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.

Generally, 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.

Stretching a plain or two-dimensional PVA (Poly Vinyi Alcohol) plastic film makes it possible to obtain all the birefringences "in the plane"

(uniaxial type "plate" or "cigar", biaxial) that is to say with the axes of the ellipsoid of the indices contained in the yaws of the film or according to the normal. French patent application No. 94 13623 describes compensation using a stretched PVA type birefringent film of the POLATECHNO-LTD brand positioned on each side of the cell. The film is a biaxial whose axes of the indices ellipsoid are in the plane and normal to the substrate (Figure 4). One of the axes in the plane of the substrate corresponding to the axis passing from the corresponding polarizer. This configuration is shown in Figure 5. It is important to note that in Figure 5 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. For type I, the polarizer is parallel to the director of the liquid crystal molecule anchored on the surface located on the same side of the cell.

With this film, compensation on an essentially East-West display device was obtained in a type II configuration, as shown in FIG. 6 which presents the isocontrast curves of a TN liquid crystal cell compensated with two biaxial films, depending on the observation position expressed in (θ, φ). The horizontal viewing angle is very good (± 70 ° for isocontrast 50 but the vertical viewing angle is very limited (± 15 ° - isocontrast 50).

A more effective method is to compensate for the uniaxial “cigar” type birefringence of the liquid crystal (n _e > n ₀ ) with a “plate” birefringent film (n _e <n ₀ ). But then you have to have the effective compensation of a "plate" whose optical axis is inclined relative to the substrate plane. Patent application EP 0 645 829 describes in the patent a compensation film made up of polymerized discotic liquid crystal molecules (birefringence "plate") aligned obliquely. This film is commercially available.

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.

Note that for compensation to take place, the optical axis of the plates positioned on each side of the cell must be contained in the plane defined by the normal to the substrate and the rubbing direction on the corresponding side. Obtaining inclined plates is difficult and all the more difficult the higher the desired birefringence value. The solution described in EP 0 645 829 has an inclined attitude of about 90 nm of birefringence, which is a high value. Optimal compensation requires coupling on each side an "inclined attitude" and a "plane attitude", generally obtained by stretching a plastic film.

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 various objects and characteristics of the invention will appear in the description which follows given by way of example and in the appended figures which represent:

- Figure 1, the conoscope of a helical nematic liquid crystal cell (type II);

- Figures 2a and 2b, the ellipsoids of the indices of a uniaxial medium, cigar-shaped for Figure 2a and plate-shaped for Figure 2b;

- Figure 3, a liquid crystal cell provided with compensation means;

- Figure 4, the ellipsoid of the indices of a biaxial film;

- Figure 5, a liquid crystal cell provided with biaxial plastic films known in the art;

- Figure 6, the conoscope of a nematic liquid crystal cell in a helix compensated with a biaxial film 80 nm;

- Figure 7, the conoscope of a TN liquid crystal cell compensated with a biaxial film 80 nm and an inclined attitude of -40 nm;

- Figure 8, a simplified embodiment of a display device according to the invention;

- Figure 9, an exploded view of the device according to the invention;

- Figures 10a and 10b, a holographic film with a network of layers usable in the device of Figure 9;

- Figures 11 a to 11 c, conoscopes of display devices according to the invention.

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 + θ:

• optical axis oriented in the direction of positive θ for a dish type obliquity;

• optical axis oriented in the direction of negative θ for a cigar-like 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.

The presence of 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:

- a liquid crystal screen (or cell) 1; - two biaxial films 2 and 2 ′ having an anisotropy of refractive indices joined to each side of the liquid crystal screen;

- two films 3 and 3 ′ having a uniaxial birefringence with an optical axis oblique to the plane of the liquid crystal screen. The two films 3 and 3 'are joined to the films 2, 2'; - two crossed polarizers 4 and 4 'arranged on either side of the films 3 and 3'.

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;

- either a biaxial birefringence film and two uniaxial birefringence films; - either two biaxial birefringence films and one uniaxial birefringence film. The basic idea of the invention is therefore to associate with the liquid crystal screen at least one biaxial birefringence film and a uniaxial birefringence film. We will now describe in detail the nature of the various components of an embodiment of the invention and their relative orientations with respect to each other.

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. According to a configuration designated type II in the art, 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. Similarly, 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 θ 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 °;

- and that the projection of the optical axis of this film on the yaw of the film makes an angle between 90 ° and 135 ° (for example 125 °) with the West-East direction. 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;

- and that the projection of the optical axis of this film 3 'on the film plane makes an angle between 45 ° and 90 ° with the West-East direction.

According to a preferred embodiment, 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. In such a film, the optical axis is normal to the plane of the index strata.

According to this exemplary embodiment, the optical axis of the film makes an angle θ 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 direction of these waves corresponds to that of the normal to the plane of the strata to be obtained. The wavelength of these waves is less than the range of wavelengths of use of the display device. According to an exemplary embodiment, 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

• Optical path difference in the film plane (n _e - n ₀ ) xd = 80 nm

• Optical path difference in the film section (n _z - n ₀ ) xd = - 128 nm

• Biaxiality report

ⁿ e - ⁿ o

• The axis of highest index is perpendicular to the direction of rubbing (to a few degrees depending on the configuration)

Holographic film

• Overall path difference n _e - n ₀ = - 40 nm obtained with a 25 μm thick film

• Tilt angle - φ = 90 ° North-South

- θ ₀ = 33 ° (in the direction of the middle molecule)

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. By comparing it with the compensation conoscope with biaxial film alone, we observe for example for an isocontrast of 50, a significant widening of the vertical viewing angle (+30, -40 °). We will now describe exemplary embodiments and the corresponding conoscopes making it possible to highlight the advantage of the invention. 10

In all the cases below the 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.

- 135 ° polarizer

- A first biaxial film at 140 °

- TN cell in type II configuration - A second biaxial film at 40 °

- 45 ° analyzer

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 φ made by the projection of the optical axis of the holographic film on the film plane with the West-East direction.

Configuration # 1

- Polarizer 4: 135 ° - Holographic film 3: φ = 90 °

- Biaxial plastic film 2: 132 °

- TN cubicle in type II configuration

- 2 'biaxial plastic film: 48 °

- 3 'holographic film: φ' = 90 ° - 4 'polarizer: 45 °

In this device, the projection of the optical axis on the plane of the holographic film is therefore oriented North-South. The conoscope of this device is shown in Figure 11 a. 11

Configuration # 2

- Polarizer 4: 135 °

- Holographic film 3: φ = 70 ° - Biaxial plastic film 2: 135 °

- TN cubicle in type II configuration

- 2 'biaxial plastic film: 45 °

- 3 'holographic film: φ' = 110 °

- 4 'polarizer: 45 °

The conoscope of this device is shown in Figure 11b.

Configuration # 3

- Polarizer 4: 135 °

- Holographic film 3: φ = 55 °

- Biaxial plastic film 2: 135 °

- TN cubicle in type II configuration

- 2 'biaxial plastic film: 45 ° - 3' holographic film: φ '= 125 °

- 4 'polarizer: 45 °

The conoscope of this device is shown in Figure 11c.

Claims

12 CLAIMS

1. Liquid crystal display device comprising a liquid crystal screen (1) provided on at least one of its main faces (10, 1 1) with means for correcting the birefringence of the liquid crystal, characterized in that the means for correction comprises a film (2) having a biaxial birefringence with anisotropy of refractive indices as well as a film (3) having a uniaxial birefringence with an optical axis oblique to the plane of the liquid crystal screen.

2. Device according to claim 1, characterized in that the film (2) having a biaxial birefringence is a plastic film stretched in a plane parallel to the plane of the liquid crystal screen.

3. Device according to claim 2, characterized in that the plastic film has its largest index axis and a lower index axis located in the plane of the film.

4. Device according to claim 1, characterized in that the film (3) having an oblique birefringence is a holographic film comprising a network of layers in volume.

5. Device according to claim 4, characterized in that the pitch of the network of strata is smaller than the lowest wavelength of a range of wavelengths illuminating the screen.

6. Device according to one of claims 1 to 5, characterized in that a first face (10) of the liquid crystal screen (1) being intended to be located on the side of an observer, the plastic film ( 2) is attached to this first face and in that the film (3) having an oblique birefringence is attached to the second face (1 1) opposite the first face.

7. Device according to one of claims 1 to 5, characterized in that a first face (10) of the liquid crystal screen being intended to be located on the side of an observer, the film (3) having a oblique birefringence is attached to this first face and in that the plastic film (2) is attached to the second face (1 1) opposite the first face.

8. Device according to one of claims 6 or 7, characterized in that an oblique birefringence film having a birefringence network is also attached to the plastic film (2).

9. Device according to one of claims 6 or 7, characterized in that a stretched plastic film having an anisotropic birefringence 13

refractive indices is also attached to the film (3) with oblique birefringence.

10. Device according to one of claims 1 to 5, characterized in that a first face (10) of the liquid crystal screen is intended to be located on the side of an observer and in that the plastic film and the oblique birefringence film (3) are attached to this first face (10).

11. Device according to one of claims 1 to 5, characterized in that a first face (10) of the liquid crystal screen is intended to be located on the side of an observer and in that the plastic film and the oblique birefringence film are attached to a second face opposite this first face.

12. Device according to any one of the preceding claims, characterized in that the liquid crystal of the liquid crystal screen is a nematic liquid crystal in a helix and in that it comprises crossed polarizers arranged on either side of the liquid crystal display.

13. Device according to claim 12, characterized in that the liquid crystal screen is produced in a type II configuration and in that the polarizers enclose the entire liquid crystal screen device / correction means.

14. Device according to claim 13, characterized in that each stretched plastic film is oriented so that its major axis of birefringence is substantially parallel to the direction of the polarizer located on the same side as said film relative to the liquid crystal screen .

15. Device according to claim 14, characterized in that said parallelism is within 10 °.

16. Device according to claim 1, characterized in that each holographic film (3) is oriented so that the normal to the plane of the strata of the holographic film makes an angle between 25 ° and 60 ° with the normal to the plane of the liquid crystal display.

17. Device according to claim 16, characterized in that the liquid crystal screen is oriented so that the directions of friction of anchoring of the liquid crystal molecules on the confining faces of the liquid crystal form an angle of 45 ° with an average horizontal direction of observation, the direction of friction of a first face making an angle of -45 ° relative to the horizontal direction and the direction of friction 14

of a second face making an angle of + 45 ° with respect to the horizontal direction and in that the projection of the normal to the plane of the strata of the holographic film associated with the first face on the plane of the liquid crystal screen makes an angle (φ) between 90 ° and ± 45 ° with the horizontal direction.

18. Device according to claim 17, characterized in that the projection on the plane of the first face of the normal to the plane of the strata of the holographic film associated with the first face makes an angle between 45 ° and 90 ° with the horizontal direction and in that the projection onto the plane of the second face of the normal to the plane of the strata of the holographic film associated with the second face makes an angle between 90 ° and 135 ° with the horizontal direction.