Smectic liquid crystal display cell

The invention provides a liquid crystal display cell in which particles having a preferential light absorption direction, particles which may be the molecules of a dichroic pigment, are inserted in the body of a layer of a mesomorphic material in a smectic state. The write-in can be made by a thermo-optical process. The molecular orientation of the smectic material controls the particles orientation, so that the variation in the light scattering coefficient commonly used with smectic layers are replaced by variations in the absorption coefficient, easier to use and provide a better contrast.

FIELD OF THE INVENTION 
This invention relates to liquid crystal cells and, more particularly, to 
cells using a liquid crystal in the smectic phase for display purposes. 
BACKGROUND OF THE INVENTION 
It is already known that particles having one or two preferential 
elongation directions can be oriented by dispersion in a medium formed by 
long molecules which are themselves capable of receiving a preferential 
orientation. Thus, linear polarisers in the form of plates have been 
obtained by applying a polymer film coating to fine filiform or 
lamelliform metallic particles or to elongated molecules of dichroic 
pigments, and subsequently drawing this film. The long molecules of the 
polymer, which are disposed parallel to the drawing direction, impose 
their orientation on the dispersed material which thus absorbs any 
luminous vibration parallel to its elongation direction. 
Mesomorphic materials also commonly called liquid crystals, which are also 
formed by long molecules, exhibit these same orienting properties with 
respect to particles dispersed within them. These long molecules arrange 
themselves over short distances to form microregions which, in the absence 
of external forces, are generally disorderly oriented relative to one 
another. By adequate wall treatments or by adding traces of a surfactant 
to the material, it is possible through the surface tension forces 
developed to obtain, between two supporting plates, thin layers in which 
these microregions are aligned in a single direction, the molecules having 
a uniform orientation which is generally parallel or perpendicular to the 
walls. 
One interesting feature of the constituent molecules of mesomorphic 
materials is their strong polarisability which enables them to be oriented 
under the action of a continuous or alternating electrical or magnetic 
field and to entrain in their movement the particles dispersed within the 
material. However, whereas in the smectic-phase materials, the orientation 
obtained persists when the field is no longer applied, the molecules of 
the materials in the nematic or cholesteric phase return to their initial 
orientation when the field is interrupted. 
These properties of the nematic or cholesteric phases have been utilised 
for forming optical modulators or data presentation systems which utilise 
the variations in absorption of a thin layer containing metallic particles 
or a dichroic pigment in suspension and subjected to the action of an 
electrical or magnetic field. In modulators of this type which use a 
nematic material, the quiescent state is either in disorderly orientation 
or is oriented in a direction parallel to the wall. In this latter case, 
the layers is illuminated with light polarised linearly in a direction 
parallel to the direction of alignment. Under the action of the field, the 
molecules arrange themselves into a homeotropic (i.e. perpendicular to the 
walls), orientation, causing the layer or those parts thereof which are 
subjected to the field to pass from the absorbent state to the transparent 
state. In modulators which use particles or pigments dispersed in a 
cholesteric material, the layer absorbs the natural light. Under the 
action of the field, the molecules right themselves perpendicularly to the 
walls, the helical structure is replaced by a homeotropic structure and 
the layer becomes transparent. 
It is also known that thin layers of mesomorphic materials in the smectic 
phase can be used for data presentation purposes. The walls are treated to 
promote a uniform orientation of the molecules. The layer, which is cooled 
slowly from the liquid isotropic phase to the smectic phase, adopts this 
uniform orientation and thus appears before recording as uniformly 
transparent. A light beam, generally infra-red, modulated in intensity by 
the information to be recorded and focussed at the level of the layer, 
scans the layer. Its maximum intensity is calculated in such a way that, 
at the point of impact, the power absorbed by the layer causes this latter 
to pass into the liquid isotropic phase. The sudden cooling which follows 
results in the formation at these points of disorderly oriented and 
therefore strongly scattering microregions, whereas the points subjected 
during scanning to the minimum intensity retain their initial state and 
remain transparent. The information thus recorded in the form of 
scattering dots on a transparent background can remain intact for several 
weeks. It is erased by restoring the uniform transparent state by fusion, 
followed by controlled cooling. The image obtained may be viewed directly 
or projected onto a screen by means of an auxiliary light source and a 
strioscopic system. 
In an U.S. Pat. No. 4,040,047 entitled "Erasable thermo-optic storage 
display of a transmitted image", HARENG et al described a data 
presentation system which also uses a thermo-optical recording process in 
a thin layer of a material in the smectic phase. In this system, a light 
beam of constant intensity scans the layer for successively causing the 
temporary fusion of each dot. The signal corresponding to the information 
to be recorded is applied in synchronism with the scan between two 
electrodes surrounding the thin layer. The various dots recrystallise into 
a structure which is the less disorderly, the more intense the field thus 
applied during cooling, and the recording is made in the form of more or 
less diffusing dots on a transparent background. Projection is carried out 
by means of a strioscopic system. Erasure is obtained by applying a 
voltage pulse which is considerably higher than the maximum values of the 
recording voltage, the field thus created restoring the layer to a 
uniformly oriented and therefore uniformly transparent state. 
In a copending patent application filed on Dec. 13, 1975 under the Ser. No. 
643,866, now abandoned, and entitled "Thermo-optic smectic liquid crystal 
storage display", L. THYRANT described another data presentation system 
which also uses a thermo-optical recording in a thin layer in the smectic 
phase. A treatment of the walls promotes the orientation of the layer in a 
first direction (for example parallel), whilst electrodes surrounding the 
layer enable it to be subjected to an electrical field which imposes on it 
a second orientation (for example homeotropic) perpendicular to the first. 
An initial voltage pulse imposes this second orientation on the layer and 
thus renders it transparent. The luminous power applied by a global 
projection of the image or by a modulated scanning beam enables the layer 
which remains in the smectic phase to be locally heated to the vicinity of 
the smectic phase/nematic phase transition point. During the subsequent 
cooling step, the layer readopts the first orientation imposed by the 
walls at the heated points. The connecting zones between regions of 
parallel and perpendicular orientation, which are microscopically 
disordered, are diffusing. The process only requires a relatively weak 
luminous intensity because the dots to be recorded do not change phase, 
but can only be used for recording half-tone images. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to improve the operation and to 
simplify the conditions of use of cells which use a mesomorphic material 
in the smectic phase by dispersing in this material particles having a 
coefficient of absorption variable according to the orientation of the 
luminous vibration passing through them. By acting on the orientation of 
the smectic layer, the orientation of the dispersed particles is 
controlled which thus modulates the coefficient of absorption of the cell. 
It is a further object of the invention, to retain the advantages of cells 
which use a smectic phase for recording images and, in particular, their 
property of being memory devices which it is possible to erase, whilst at 
the same time obviating some of their disadvantages. This object is 
accomplished by recording the image in the form of a variation in the 
coefficient of absorption rather than diffusion in such a way that its 
projection no longer requires the intervention of strioscopic systems, but 
instead is carried out by means of entirely conventional systems which are 
much more luminous. In addition, the light diffused by the smectic-phase 
layers is diffused at a relatively closed angle which limits the viewing 
angle at which the presentation may be directly viewed. The association of 
dispersed particles eliminates this problem. 
It is still a further object of the invention to strongly lower the 
residual absorption which persists when the particles are dispersed in 
layers where the molecules are aligned parallel to the direction of 
propagation of the incident light. This result is achieved by substituting 
a smectic material for a nematic or cholesteric material, an important 
characteristic of the smectic-phase layers with respect to the dispersed 
particles, which distinguishes them in this respect from layers in the 
nematic or cholesteric phase, being the excellent quality of the alignment 
induced in the dispersed particles.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The drawing shows a layer 1 of a mesomorphic material in the smectic phase 
containing dispersed particles. This layer 1 is enclosed between two 
plates of glass 21 and 22 which are kept at the requisite distance apart 
by means of two wedges 212 and 221. Parallel heating bands are arranged on 
the plate 21. These heating bands, which are formed by transparent 
resistive elements, such as 30, may be selectively or simultaneously 
heated by connecting them to a voltage source 3. These resistive elements 
are coated with a transparent insulating layer 40 which supports a 
continuous transparent electrode 50. Two transparent electrodes 51 and 52 
are also arranged on the plate 22. A generator 5 enables a control voltage 
to be applied between the electrode 50 and one or other of the electrodes 
51 and 52. A coating 60 lines the interface between the electrodes 50, 51 
and 52 and the layer 1. The mesomorphic material forming the layer 1 may 
be a diphenyl, a Schiff's base, a diphenyl ester or any other mesomorphic 
material having a smectic phase which, in the case of the FIGURE, is 
selected for its sensitivity to the electrical field. The thickness of the 
layer may be selected between approximately ten and a few tens of 
micrometers. 
The dispersed particles used are either filiform or lamelliform metallic 
particles or molecules of dichroic organic pigments. For example, 
particularly interesting results may be obtained by using as pigment 
4-(N-pyrolidinophenyl)-azo-5-nitrothiazole corresponding to the formula: 
##STR1## 
dissolved in a proportion of around 1% in 4,4'-cyano-octyldiphenyl 
corresponding to the formula: 
##STR2## 
The resistive elements 30 and the electrodes 50, 51 and 52 are formed in 
known manner by deposits of tin or indium oxide or of a mixture of these 
two oxides. The insulating layer 40 is a layer of silica. 
The control voltage may also be applied between the electrodes 51 and 52 
which, in that case, are spaced sufficiently apart from one another to 
create an electrical field parallel to the plane of the layer 1. In this 
case, the electrode 50 may be omitted. If the cell is intended for 
examination by reflection, the electrode 50 for example may be formed by a 
reflecting metallic layer. 
The coating 60 may be obtained by a deposit of silane which promotes the 
homeotropic orientation of the mesomorphic material. Other types of 
coating may be used according to the nature of the mesomorphic material 
and the required orientation thereof, for example a deposit of silica made 
at grazing incidence to promote the parallel orientation. Traces of 
various surfactants, of which the composition is well known in the field 
of liquid crystals, may also be dissolved in the mesomorphic material to 
promote a particular orientation of its constituent molecules relative to 
the walls. 
The resistive elements are used for locally or globally increasing the 
temperature of the layer 1 either to keep the material in the smectic 
phase if the thermal range of existence of this phase is above the ambient 
temperature, or to increase the temperature in this same phase or to 
obtain the conversion of the smectic phase into the liquid isotropic 
phase. In one known technique, each band may be replaced by a row of 
discrete elements to form a matrix fed by a series of conductors arranged 
in lines and columns. By contrast, if the entire layer is to be maintained 
or increased in temperature, the resistive bands may be replaced by a 
uniform layer. In this case, it is also possible by a suitable electrical 
arrangement to use the layer 50 both as an electrode and as a heating 
element. It is also possible in accordance with the invention to use a 
luminous beam for heating the layer 1. In this latter case, the heating of 
the layer 1 is facilitated by the presence of the electrodes 50, 51 and 52 
of tin or indium oxide which, by absorbing the infra-red part of the 
luminous radiation, transmit the heat thus released to the layer 1. 
In the FIGURE, the molecules of the mesomorphic material have been 
symbolised by rodlets and the dispersed particles by longer, thicker 
rodlets. In the region 10, situated opposite the electrode 51, a voltage 
pulse applied to the electrode 51 has reorganised the thin smectic layer 
into a uniform homeotropic orientation, thus controlling the orientation 
of the dispersed particles which align themselves with their direction of 
elongation perpendicular to the walls. The part 10 of the layer is thus 
uniformly transparent to the light travelling parallel to the arrow 7. 
By contrast, in the region 11 which has been brought into the liquid phase 
and suddenly cooled, the mesomorphic molecules have reorganised themselves 
into microregions disorderly oriented relative to one another. The 
dispersed particles oriented by the various microregions have all the 
possible orientations relative to the direction of propagation 7 of the 
incident light which is thus absorbed, irrespectively of its polarisation 
direction. The region 11 thus appears opaque both in natural light and in 
polarised light. 
By way of a first example of application, the cell according to the 
invention may be used in the image reproduction system according to the 
above-mentioned U.S. Pat. No. 4,040,047 of HARENG et al to replace the 
cell provided with a thin smectic layer. In this case, the electrodes 51 
and 52 are replaced by a single electrode occupying the entire surface of 
the plate 22. The heating electrode 30 is also a single electrode, 
occupies the entire surface of the plate 21 and serves to keep the thin 
layer of liquid crystal in the temperature range where it is in the 
smectic phase. In the non-recorded parts of the cell, the smectic 
material, which is in homeotropic orientation, orients the particles 
perpendicularly to the walls (which is the case in the region 10 of the 
accompanying FIGURE). These parts are thus transparent. The recorded parts 
have a structure which is closer to the completely disordered structure 
described by the region 11 in the accompanying FIGURE, the lower the value 
of the videosignal applied during cooling. They appear more absorbent, the 
more disordered the structure. The cell may be illuminated with natural 
light. The projection of the image thus recorded no longer requires a 
strioscopic system. 
By way of a second example of application, the cell according to the 
invention, modified as described in the above example, may replace the 
smectic liquid crystal cell of the image reproduction apparatus with a 
memory which is the subject of the above-mentioned patent application Ser. 
No. 643,866 of THYRANT, now abandoned. The recorded and non-recorded dots 
which are distinguished by orientations perpendicular to one another of 
the smectic-phase material thus appear as absorbent (dots oriented 
parallel to the walls) or transparent (homeotropically oriented dots) with 
respect to light polarised linearly in the direction of the molecules in 
parallel orientation. There is thus no need for the image to be half-toned 
before recording or for a strioscopic system to be used for projecting the 
recorded image.