FR2543720A1 - Liquid-crystal cell having variable light absorption and high contrast - Google Patents

Abstract

Cell marked with graphic symbols: numbers, drawings or the like, 5, 6, 7, by the shape of the electrodes, in which shape the volume delimited by the non-treated internal surfaces is filled with a black dichroic solution of cholesteric liquid crystals, the solution being made from a nematic solution by the addition of a small percentage of an initiating cholesteric solution to which is added a black dichroic solution.

Description

The invention relates to a liquid crystal cell having a

  variable light absorption on surfaces determined by shape

of flat electrodes.

  Molecules of cholesteric liquid crystals

  are organized in helical structures

  In the state of rest, these structures have any orientations according to a perfectly random distribution, in other words, the structure

  of matter is perfectly isotropic.

  This solution introduced in the volume

  delimited by two glass plates with

  these interior compared, do not lose this distribution

  It becomes ordered when applying

  that a continuous voltage between the two

  glass plates via the electrodes.

  Subject to an electric field, molecules become

in the field direction.

  It is known, moreover, that the properties

  Dichroic tees of a material allow it to absorb the light rays differently according to their polarization. The general idea of the invention consists

  to combine these two properties to obtain an absorp-

  variable luminous weight, externally adjustable in a certain proportion from the level of

electric field.

  According to the invention, a cholesteric-dichroic solution of liquid crystals is produced using a black dichroic solution and a positive cholesteric solution obtained from a nematic solution to which is added a small percentage of initiating cholesteric liquid crystals which

  transform the nematic solution into cholesterol solution

  The final cholesteric black dichroic solution is introduced into the bounded interior volume

  by two parallel glass plates with interior faces

  untreated rests having on one side of a plate a continuous and homogeneous electrode and on the opposite side of the other plate electrodes cut into single or multiple surfaces according to contours determined by the shapes, graphics,

  signs: letters, objects, signs to appear.

  A cholesteric-black dichroic solution is used to obtain, in the state of rest, a sufficiently disordered structure to constitute a black opaque screen and which remains perfectly black.

  even when illuminated by indirect light from behind.

  In the presence of an electric field, the cell becomes transparent or at least translucent at the locations covered by the electrodes due to the alignment of molecules and absorption

bright became very weak.

  The applications targeted by the invention are multiple They concern for the most part the display of all the information not only in alpha-numeric form, but also in the most appropriate graphic symbolic form, comic strip, graphic representations, cartoons Indeed , it is possible to envisage a structured structure forming a matrix of elementary points which allows, by composition of points,

  to obtain the most diverse forms and signs.

  Other applications are in the

field of toys and games.

  The cell according to the invention offers several simultaneous advantages the internal faces of the glass plates can remain raw. The surfaces can be dispensed with as in a previous embodiment in order to force the

  molecules to orient themselves in the same direction

  tion. The contrast is important o o

  comfortable viewing for the reader.

  The display speed can be high.

  The manufacturing process is simple

  and economical, easy to automate.

  We avoid the use of a polarizer and

3 2543720

  correlatively the difficulty of having to align the polarizer and the crystalline direction. Other technical characteristics and

  O advantages of the liquid crystal cell to absorb

  light according to the invention will emerge from

  the following description given as an example

  non-limiting with reference to the accompanying drawing in which

  Figures 1a and 1b are schematic diagrams

  in section illustrating the orientations of molecules without and with

an electric field.

  Figure 2 is a perspective view

  of the cell under electrical tension.

  Figures 3a and 3b are diagrammatic views of

  general cross-sectional and planar sketches of an excitation variant

by the same tension.

  FIG. 4 is a sectional detail view at the level of a group of elementary points of a point application

controlled by matrix network.

  FIG. 5 is a detailed view in

2) 5 plan of the same variant.

  The variable light absorption cell according to the invention is formed of two vortex plates 1 and 2 untreated, coated electrodes in thin conductive layers For one, in principle the back plate, the layer 3 is continuous and homogeneous For the other, 4, it is present only in specific places and according to the desired forms, for example,, 6, 7, graphics, patterns, alpha-numeric signs

  which will allow the display of information, resulting in

  They are connected by an electrical connection

  to an external electric control.

  For the front plate, the localized conductive zones are for example formed from a

X 4 2543720

  conductive thin layer on which the

  non-conductive surfaces are removed by

  chemical action, the action of an acid, or any other mechanism.

  Of course, it is also possible to deposit the conductive layer only in discrete surfaces defined by the graphic requirements of the display

  for example using the technique of caches.

  The walls of the cell coated with their electrodes of appropriate shapes and surfaces, the interior volume is filled with a solution S of liquid crystals, a so-called positive solution because

  it reacts to a positive voltage.

  This solution has the following particularities: A positive solution of crystals is produced

  cholesteric liquids from a nematological solution

  addition of a small percentage of cholesteric initiator crystals, the percentage between

1 and 10 ', preferably 5%.

  A solution of liquid crystals with dichroic properties is added in a small percentage to this solution of cholesteric liquid crystals.

in the dark.

  The dichroic properties of this solution in the dark give the liquid crystal solution a very marked absorption of light in the absence of an electric field.

this is greatly the contrast.

  In the presence of an electric field, this absorption drops to a low level. Thus, the zone under each electrode diffuses the light during the application of an electric field whereas the neighboring surfaces remain perfectly black, whatever the polarization The contrast gets better again if the plate is lit.

  in indirect light from behind.

  Molecules activated by the electric field

  that align and allow the solution to become

  transparent or at least milky in appearance.

_ 5 2 2543720

  It should be observed at this level that the inner faces of the walls do not have to be treated to obtain the desired effect.

  is an important step forward in relation to the technical

  than earlier that the walls had to be treated mechanically streaking, grooving to constrain the molecules to be oriented in

the same direction.

  We can also increase the possibilities

  display by providing an additional conductive layer

  used as a reference for tension and wearing

  one or the other, or both at the same

  In this case, the excitation consists in canceling the voltage between the display electrodes by

the application of the same voltage.

  One of the embodiments of this variant is illustrated in FIGS. 3a and 3b. In this embodiment, the rear plate A is coated.

  a homogeneous and conductive continuous layer 9.

  The front plate B has a conductive coating with discontinuities constituting the bottom Conductive display areas It is lying on the free areas of the bottom and surrounded by a gap 12 non-conductive They are connected by a connection

  suitable for external electrical control

  pool. The operation is as follows In a first phase, the continuous layer 9 is brought to a voltage V, the bottom 10 and the zones 11 are at zero potential * everything is transparent In a second phase, 9 is brought to a voltage V, 10 is at zero potential and It at voltage V In this case, the figures appear black against a background

white or milky in appearance.

  Conversely, if 10 is brought to the voltage V and It to the zero potential, the figures appear

  white (transparent or milky) on a black background.

  We can play on the white alternation-

  black to constitute a composition of static figures

b 2543720

on the move.

  Another variant of application according to the general idea of the invention is shown in FIGS. 4 and 5 EL. It consists in forming on a substrate 13, for example a silicon wafer, an integrated circuit 14 of the MOS type covered with a network. 15 of conductive lines in crossed arrangement according to lines

  16 and columns 17 to form a matrix Well understood

  of, points of contact and suitable isolations

  are provided to selectively control each

  elementary MOS tor 18 on its drain 19 and its gate In this configuration, the network of lines

  can be connected to gates 20 and the colon network

  The source 21 is connected to the adjacent zone of the liquid crystal layer 22 by a conductive bridge 23 passing through the insulating layer 24. A glass wall 25 coated on its inner face with a thin layer conductive layer

26 covers the whole.

  The conductive layer 26 constitutes the first

  energizing electrode The second electro-

  de is formed by the conducting bridge 23 which makes contact with the source of the elementary transistor

Corresponding MOS.

  The space between the elementary zone delimited by the base of the bridge 23 and the electrode 26 constitutes the elementary point 27 of liquid crystals

  susceptible of optical modifications.

  This elemental point 27 is placed in

  the drain circuit of the adjacent elementary transistor.

  It becomes transparent when the transistor is powered and controlled on its gate The elementary point 27 of liquid crystal is then energized between

the corresponding electrodes.

  The presence of voltage on the electrodes or electrode portions facing either side of the elementary point 27 results from the voltage control of the line 16 and the column 17 concerned at the intersection of which is located the elementary point 27. The same applies to all the points lying on the level of the intersection

a row and a column.

  By appropriate matrix control, it is possible to perform a dot display which leads to the composition of all the known graphic signs,

  letters, numbers This display may be static

that, but also in motion.

  Protection can not be limited to the variants described only, it extends on the contrary to all direct modifications, addition of inoperative elements, substitution by materials,

equivalent materials or means.

S 2543720

Claims (10)

  1 cell, presentation of information with variable light absorption and high contrast,   consisting of two glass plates (1) and (2) delimiting   both an interior volume filled with a liquid crystal solution characterized in that the internal faces facing the glass plates are left raw and coated, for one, a homogeneous conductive layer (3) and for the other , conductive areas (5), (6) and (7) of desired shape and extent   for the purposes of display and that the   liquid crystal is composed of crystals   cholesteric liquids plus a small percentage   a black dichroic solution.   Cell according to claim 1, characterized   in that the solution of cholesteric liquid crystals   is made from a nematic solution by addition of a small percentage of cholesteric crystals.   Cell according to claim 1, characterized   in that one of the plates has its patterns, graphics, letters, numbers and other signs to be displayed by electrodes (11) insulated from the conductive bottom.   4 cell according to claims 1 and   3 characterized in that the electrodes (11) are   formed on an insulating background and separated from the   neighbor (10) through an isotermal peripheral gap   lant (12), the electrodes being electrically connected to an external control.   Cell according to claim 1, characterized   characterized in that the rear glass plate is replaced by a silicon substrate (13) coated with an integrated circuit (14) in connection with a matrix network of conductive lines and columns delineating liquid crystal elemental points (27). of the above solution, each point being electrically connected to the drain-source circuit of the elementary integrated transistor (18) concerned and to an electrode commune (26).   Cell according to claim 5, characterized   in that the lines (16) are electrically   at the gates (20) of the elementary transistors (18) and in that the -columns (17) are connected   electrically to the drains (19) or vice versa.   Cell according to claim 5, characterized   in that the network of rows and columns is coated with a layer of insulation (24) delimiting with   the glass wall (25) covered with a conductive layer   (26) a volume (22) filled with the solution of liquid crystal.   Cell according to claims 5,   6 and 7 characterized in that the second electrode   for energizing the point (27) of crys-   liquid level is a conductive bridge (23) providing the electrical connection between the crystal layer   the liquid sources (22) and the source (21) of the adjacent re. _ 9 -