FR2580105A1 - Display cell with nematic liquid crystals of the reflective type with electrically-controlled birefringence making it possible to improve the illumination of the display - Google Patents

Abstract

The invention relates to a display cell with nematic liquid crystals of the reflective type with electrically-controlled birefringence making it possible to improve the illumination of the display. This display cell comprises a layer of liquid crystal 11 interposed between two glass plates 1, 3 held apart by a sealing joint 9 and between which are arranged electrodes 2, 7; it further comprises two delay plates 19, 21 deposited on either side of the glass plates 1, 3, a polariser 15 deposited on the lower delay plate 19, a reflecting background 17 deposited on this polariser 15, and a second polariser 14 situated in a plane above the plane formed by the upper delay plate 21, and inclined at an angle alpha with respect to the latter. Application to display cells with nematic liquid crystal of the reflective type with electrically-control birefringence.

Description

Nematic liquid crystal display cell type ref Electrically controlled birefringence active
improving the illumination of the display
The present invention relates to a nematic liquid crystal display cell of the reflective type with electrically controlled birefringence making it possible to improve the lighting of the display.

 The invention applies to nematic liquid crystal display cells with electrically controlled dual refraction, used for example as converters of electrical information into optical information, in particular in display systems for watches, calculators, measuring devices and in perinformatics.

 Nematic liquid crystal cells of the reflective type with electrically controlled birefringence are known. Such a cell is represented in FIG. 1. A nematic Liquid crystal cell of the reflective type with electrically controlled birefringence comprises two glass plates 1, 3 kept apart by a sealing joint 9, and between which is interposed a layer of Liquid crystal nematic 11.

On the internal faces of these plates 1, 3 are respectively deposited a metal type electrode 5 and transparent electrodes 7. A rectilinear polarizer 13 is located on the external face of the upper glass plate 3.

 The metal electrode 5 is optically reflective, it reflects the incident light 20 which is previously polarized by the polarizer 13 and then transmitted by the glass plate 3 and the transparent electrodes 7.

 When the cell is at rest, that is to say when no electric field is applied between the electrodes 5,7, the liquid crystal molecules 11 are oriented so as to be parallel to the same direction, perpendicular to the surfaces formed by the electrodes 5, 7, called the homeotropy direction.

When an appropriate electrical voltage is applied to the cell's electrodes 5, 7, the molecules
The Liquid crystal ones are all oriented in a non-direction forming an angle x with the direction of homeotropy.

The electrically controlled birefringence effect therefore consists in visualizing the rotation of the direction of orientation of the liquid crystal molecules under the effect of an electric field, by the variation of
The birefringence of cristaL Liquid induced by this rotation.

 Cells of this type have the disadvantage of offering a degraded contrast when they are observed obtique.ent and this as much as the angle of observation is important.

To overcome this drawback, the birefringence linked to the structure of these cells is compensated in a known manner, by associating with the polarizer 13 a delay blade disposed between the upper glass plate ′ 3 and
The polarizer 13, and by introducing a layer of liquid crystal 11 of determined thickness. The association of a delay plate, generally quarter wave, with a rectilinear polarizer makes it possible to obtain a quasi-circular polarization of the light polarized rectilinearly by the polarizer and vice versa.

 Thus a light wave polarized almost circularly after passing through the polarizer-delay blade assembly, passes through the liquid crystal layer for the first time, it is reflected by the metal electrode, then crosses the liquid crystal layer back in the opposite direction. .

 The thickness of the liquid crystal layer was chosen such that one observes, when the cell is not excited, a total extinction of the ionized waves polarized almost circularly passing through the liquid crystal layer before and after reflexion. This extinction takes place for light waves entering the cell both with a zero and non-zero angle of incidence, angle considered relative to the direction of homeotropy.

When the cell is excited, the quasi-circular polarization of the light wave passing through your liquid crystal layer is inverted when it passes through it before reflection, then it is again reversed during its passage, after reflection from
The metal electrode. In this case, the wave is transmitted by the delay-potariser blade assembly and emerges from the cell with a rectilinear polarization identical to that of the incident wave polarized by this polarizer.

The particular thickness of the liquid crystal layer can be determined by those skilled in the art. As an indication for a quarter wave plate making it possible to obtain a quasi-circular polarization, one can choose a liquid crystal in the family of the bases of
SCHIFF and make a layer of Liquid crystal with a thickness of the order of 5 μm, the optical anisotropy of which is equal to 0.2.

 The compensation of the birefringence obtained by the association of a delay blade with the poLarizer and by the realization of a layer of liquid crystal of determined thickness, makes it possible to obtain a high contrast in the visualization of a display in the cell.

 A second type of nematic liquid crystal display cell is also known, using the birefringence effect. FIG. 2 schematically represents such a cell, of the "helical nematic" type in reflective mode.

This cell includes a layer of crystal
nematic liquid 11 interposed between two facing glass plates 1, 3 and kept apart by a sealing joint 9. On the internal faces of the plates 1, 3 are arranged transparent electrodes 2, 7.

Two crossed polarizers 13, 15 are arranged on either side of the two glass plates 1, 3. A metallic layer 17, in particular of aluninium, is deposited on the rear face of the cell, on
The polarizer 15; this layer 17 constitutes the reflective bottom of the cell. The different elements: glass plates 1, 3; pots 15, 13 and reflective bottom 17 are glued to each other.

 In this type of cell, your liquid crystal layer It is arranged so that, in the absence of an electric field, the molecules which compose it have a disposition parallel to the internal surface of the plates and a helical structure while in presence of an electric field, The molecules are all parallel to the direction of homeotropy.

 The liquid crystal cells of the second type which have just been described with reference to FIG. 2 are among the known cells, those whose structure most closely resembles that of the cells of the present invention. It is for this reason that their existence has been recalled here, but it should however be noted that they use crystals of the "nematic propeller" type which are not compatible with the improvement brought by the invention. .

 To avoid depolarization of an incident light wave during its reflection in a cell operating in reflective mode, in a known manner as an optical reflector, a thin metallic layer deposited under vacuum and having undergone annealing, giving this layer a particle size. of the order of 1 to 1.5 ddm.

Such a layer makes it possible to diffuse incident light by its matt appearance and to keep the polarization of the incident wave.

 The object of the invention is to produce a nematic liquid crystal display cell in reflective mode with electrically controlled birefringence, making it possible to improve the luminance of this type of cell.

 The subject of the invention is a nematic liquid crystal display cell of the reflective type, with electrically controlled birefringence, comprising a layer of liquid crystal interposed between two glass plates held apart by a sealing joint and between which are deposited electrodes.

This cell is characterized in that it further comprises two delay plates deposited on either side of the glass plates, a polarizer deposited on the lower delay plate of La ceLluLe, a reflective bottom deposited on this polarizer and a second polarizer located in a plane above the plane formed by the upper delay plate of the cell and inclined at an angle with respect thereto.

 According to a preferred embodiment of a display cell according to the invention, the angle is between 200 and 300.

Other characteristics and advantages of the invention will emerge more clearly from the description which follows, given purely by way of illustration and without limitation, with reference to the appended FIG. 3 which follows FIGS. 1 and 2, in which:
FIG. 1, already described, schematically represents a nematic liquid crystal display cell with electrically controlled birefringence,
FIG. 2, already described, schematically represents a liquid crystal display cell of the "helical nematic" type,
- Your Figure 3 shows schematically a liquid crystal display cell according to the invention.

 In Figure 3 is shown schematically a liquid crystal display cell according to the invention. This cell comprises a layer of Liquid crystal 11 interposed between two glass plates 1, 3 held apart by a sealing joint 9. On the internal face of these glass plates 1, 3 are arranged transparent electrodes 2, 7 and on their external face, delay blades 19, 21. In the rest of the text, we will call the upper delay blade The blade closest to an observer looking at the cell, and the lower delay blade, the one farthest from the latter . A polarizer 15 is deposited on the external face of the lower delay plate 19 of the cell and a reflective bottom 17 is deposited on this polarizer 15. A second polarizer 14 is arranged in a plane above the plane formed by the plate at upper delay 21 of the cell and inclined at an angle a with respect to this. The polarizer 14 is the element of the cell closest to the observer. The different elements, glass plates 1, 3, delay plate. 19, 21, polarizers 15 and reflective bottom 17 are glued on each other.

On the other hand, the polarizer 14 is not in contact with these elements, it is kept in its inclined plane thanks to a protective glass placed above it; the polarizer 14 is glued to the protective glass, itself secured to a box containing the cell.

 The delay plates 19, 21 used are generally quarter-wave plates which polarize a rectilinear wave passing through them quasi-circularly and the polarizers 14, 15 are rectilinear polarizers. The delay blades 19, 21 and the polarizers 14, 15 are arranged so that the pairs of polarizer 14 delay blade 21 and polarizer 15-delay blade 19 are complementary, that is to say crossed.

 Thus, it is possible, for example, to have the polarizing couple 14-delay blade 21 so as to obtain a left quasi-circular polarization of an incident wave passing through this couple and to arrange the potentiometric cutter 15-delay blade 19 so as to obtain a right quasi-circular polarization of an incident wave passing through you.

 In addition, a layer of crystal is used which has a determined thickness which allows, as we have seen previously, to compensate for the birefringence of the liquid crystal.

 The reflective bottom 17 comprises a metallic layer which has been annealed so that it makes it possible to keep the polarization of a polarized wave straightly reflecting on it and to diffuse Light.

 The remainder of the description makes it possible to understand the operation of a cell in accordance with the invention.

 The inclination of the polarizer 14 makes it possible to allow the light waves 20 arriving on the cell to enter the cell with a non-nu angle of incidence through the rear and lateral sides of the cell without passing through the polarizer 14; these waves are not polarized at the input of the cell by the polarizer 14 and are therefore not attenuated by crossing this. On the other hand, the light waves 20 entering the cell through the polarizer 14 are polarized. straight and are attenuated compared to non-polarized waves.

 When the cell is not excited, an incident wave 20 penetrating into it by the polarizer 14 is rectilinearly polarized by it and then almost circularly by the delay plate 21. This quasi-circular wave then crosses the crystal layer liquid It then the delay plate 19 and the polarizer 15 in which it is absorbed. This absorption is due to the complementarity of the potentiating couple 15-delay blade 19 with the first polarizing couple 14-laze delay 21.

 When an incident wave 20 enters the unexcited cell without passing through the polarizer 14, it is rectilinearly polarized by the polarizer 15 after having passed through the delay plates 19, 21 and the liquid crystal layer 11. This polarized wave is titrating reflected by the reflective bottom 17 while retaining its polarization. It then crosses in the opposite direction The polarizing couple 15-delay blade 19 which transforms you into a quasi-circular wave then the layer of Liquid crystal 11 and finally The delay blade 21 and the potentiometer 14 in which it is absorbed. The absorption of wave 20 is also due to the complementarity of the two delay potarizer-blade pairs.

 Thus, whatever the incidence of the light wave 20 entering the unexcited cell, it will be absorbed whether or not it crosses the polarizer 14, due to the particular thickness of the liquid crystal layer 11 and of the complementarity of the two delay polarizer-plate pairs. The cell will therefore remain extinct.

 On the other hand, when the cell is excited, an incident wave 20 penetrating into it by the polarizer 14 is transformed into a quasi-circular wave by the delay plate 21. This wave passes for the first time through the layer of Liquid crystal in which its polarization reverses then The polarizer 15-delay blade couple 19 before and after reflection on the reflective background 17. This wave is not absorbed by the polarizer 15 due to the inversion of polarity suffered by the latter in the liquid crystal, before its first passage in the polarizer 15. During its second passage in the liquid crystal layer 11, the polarization of the wave reverses again and therefore crosses the delay blade 21 and the polarizer 14 without being absorbed by it.

 When an incident wave 20 enters the excited cell without passing through the polarizer 14, it is rectilinearly polarized by the polarizer 15 and then reflected by the reflective bottom 17 while retaining its polarization. It then crosses the polarizing pair 15-delay blade 19 which polarizes it almost circularly. During its second passage in the liquid crystal layer 11, the polarization of the wave is reversed, it therefore passes through the delay plate 21 and the polarizer 14 without it being absorbed by the latter.

 Thus, whatever the incidence of the light wave 20 entering the excited cell by crossing or not crossing the polarizer 14, due to the particular thickness of the liquid crystal layer II and the complementarity of the two pairs of polarizers - delay blades, this wave will emerge from the polarizer 14 with minimal absorption. In addition, the incident waves not polarized by the polarizer 14 which have not undergone the crossing thereof before reflection on the reflective background 17, are less attenuated, they thereby strengthen the luminance of the cell and therefore the contrasts.

 The use of a polarizer 14 inclined at an angle relative to the whole of the cell makes it possible to improve the illumination of the display in the cell, of the order of 2X. The angle of inclination depends on the supposed reading position of the observer relative to the cell. it is generally in the range of 200 to 30 ".

Claims (2)

 1. Electric liquid crystal display birefringence reflective type liquid crystal cell comprising a layer of liquid crystal (11) interposed between two glass plates (1, 3) held apart by a sealing joint (9) and between which electrodes (2, 7) are deposited, characterized in that it further comprises two delay blades (19, 21) deposited on either side of the glass plates (1, 3), a polarizer ( 15) deposited on the lower delay plate (19), a reflective bottom (17) deposited on this polarizer (15), and a second polarizer (14) located in a plane above the plane formed by the upper delay blade (21), and inclined at an angle to it.  2. Display cell according to claim 1, characterized in that The angle a is between 20- and 304.