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Description  |
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BACKGROUND OF THE INVENTION
This invention relates to display devices for displaying a singe image or a
series of images represented by an electrical signal commonly known as a
video signal.
Such a device may be used in a television system, particularly for
projecting televised images onto a large screen.
It is known that a cathode ray tube may be used for displaying an image,
but such a tube wastes a large volume of space for displaying such a flat
image.
Accordingly, numerous devices have been proposed to replace cathode ray
tubes, but the results have always been disappointing. In some cases, the
structure is complicated and brilliance is poor, as in the case of plasma
screens such as that described by Yoshifumi Amano in the Journal "IEEE
Transactions on Electron Devices", Vol. ED-22, No. 1, January, 1975. In
other cases, the response time is slow thereby precluding use in a
television system, as in the case of the numerous liquid-crystal devices
with which experiments have hitherto been conducted.
French Pat. No. 2,275,087 granted to the Applicants herein on Feb. 13, 1976
on the basis of an application filed on June 14, 1974, describes a display
arrangement which utilizes a thermo-electric effect in a liquid crystal
layer, the image being recorded by scanning the layer with an infrared
beam produced by a laser and deflected by electro-optical or
acousto-optical deflectors.
This light source and these deflectors are both unwieldy to use and
expensive to produce. However, the most serious disadvantage is that the
scan used has to be slow enough to allow a local dot-by-dot phase change
of the liquid crystal layer in response to the heat supplied by the
infrared beam. A thermal phenomenon used in this manner is too slow to
enable the arrangement to be utilized in a television system.
SUMMARY OF THE INVENTION
The object of the present invention is to increase the speed with which the
image is recorded in a layer of material showing a thermo-electric effect
of the type in question using elements enabling this image to be recorded
line-by-line, so that the duration of a line is available for
simultaneously recording all the dots composing that line.
In accordance with the present invention, there is provided a display
device for displaying images represented by a video signal comprising
successive portions, said device comprising:
a thermo-electric recordable layer presenting an erasure temperature;
means for heating above said temperature said layer according to a
plurality of lines; said lines being heated temporarily one after the
other and each line being heated simultaneously along its length;
means for applying to said layer along all said lines after the heating of
one of said lines and until said one line cools under said erasure
temperature, an electrical field representing one of said successive
portions of the video signal; whereby said one portion is writen into said
one line.
BRIEF DESCRIPTION OF THE DRAWINGS
For a better understanding of the invention, and to show how the same may
be carried into effect, reference will be made to the ensuing description
and to the attached figures among which:
FIG. 1 illustrates a sectional view of a display cell comprising 4 display
points;
FIG. 2 illustrates a diagrammatic plan of a display device comprising
600.times.800 display points and control means;
FIG. 3 illustrates the block diagram of the control means 205 shown in FIG.
2;
FIG. 4 illustrates the block diagram of the control means 206 shown in FIG.
2;
FIG. 5 illustrates the diagram of a first embodiment of a system for
projecting the image displayed on the device of FIG. 2;
FIG. 6 illustrates a partial section view of a second embodiment of the
display of FIG. 2;
FIG. 7 illustrates the diagram of a second embodiment of a system for
projecting the image displayed on the device of FIG. 2.
FIG. 8 illustrates a partial sectional view of a third embodiment of the
display shown in FIG. 2.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 shows a very simple display cell which, in the interest of clarity,
is limited to two lines and two columns enabling an image composed of four
dots arranged in a square to be displayed.
A thin layer of liquid crystal 101 is inserted between a layer of silicon
dioxide (SiO.sub.2) 102 obtained by oxidizing a first substrate 103 of
silicon, and a second substrate 104 of sapphire. It is held in position by
shims (not shown).
Conductor lines 105 and 106, made e.g. of aluminum, are deposited on the
layer of silicon in contact with the liquid crystal. They serve as heating
resistances when they are fed through the connections L.sub.1 and L.sub.2
brought to an electrical potential in relation to a common earth
connection M and enable the temperature of the liquid crystal to be
locally increased along the entire line thus fed.
The liquid crystal thus heated is preferably of a type having a smectic A
phase and a nematic phase. Good results have been obtained with compounds
of the cyanobiphenyl group used either on their own or in admixture, and
in particular with cyanooctyl biphenyl (COB in short) of which the
structure is represented by the following formula:
##STR1##
COB is characterised by a smectic-nematic transition at 32.5.degree. C. and
by a nematic-isotropic transition at 40.5.degree. C.
By thermostatically controlling the display cell to a few degrees below the
smectic-nematic transition, the increase in temperature produced by the
heating effect of the resistance line enables the liquid crystal along
that line to be brought into the isotropic phase in a very short time.
When the voltage applied to the line used is cut off, the heat is
essentially dissipated through the substrate 103 which is a good heat
conductor. By contrast, the layer 102 of SiO.sub.2, which is a poor heat
conductor tends to limit the heat dissipation rate. If, therefore, the
response time of the system is required to be short, the layer 102 has to
be thin to ensure rapid cooling. However, this approach is limited because
this dissipation effect occurs also in evidence during heating, which
means that as the layer 102 is made thinner the heating power has to be
increased at the risk of melting the heating lines.
Good results have been obtained in a more complex cell than that
illustrated in FIG. 1 and comprising 600 lines and 800 columns by using
aluminium lines with a thickness of 1 .mu.m for a width of 15 .mu.m and a
length of 2 cm spaced 5 .mu.m apart from one another and deposited onto a
layer of SiO.sub.2 having a thickness of 3 .mu.m. These lines had a
resistance of approximately 50 ohms and enabled the nematic-isotropic
transition of a 15 .mu.m thick layer of COB to be obtained in less than 60
.mu.s under a feed voltage of 25 V. The return to the smectic state also
took place in less than 60 .mu.s.
When the liquid crystal cools as quickly as this, it assumes so-called
conic focal structure which gives it a diffusing appearance opposite to
that (completely transparent) of the liquid phase and the normal
homeotropic phase. Accordingly, by applying an electrical field
(corresponding to a voltage) perpendicular to the liquid crystal layer
during its cooling, a more or less diffusing structure, depending on the
value of that field, is obtained. Thus, in this cell, the crystal is
completely diffusing for a voltage of 10 V and completely transparent for
a voltage of 20 V, the transparency percentage varying substantially
linearly with the voltage.
An effect such as this, which provides for recording under the action of
the electrical field during the cooling of a material, is known as a
thermo-optical effect. It is not specific to liquid crystals, particularly
of the COB-type, and occurs in particular in lithium niobate doped with
iron, in which the recording is reflected in a variation in index. It
would also be possible to start with a diffusing liquid crystal and to
return it to a more or less diffusing state after heating.
Transparent conductor lines 107 and 108, for example of tin oxide
(SnO.sub.2), are deposited on that surface of the substrate 104 which is
in contact with the layer 101 and perpendicularly of the lines 105 and
106. In the example described, the width and the spacing of these columns
are identical with those of the lines to obtain substantially square
elementary recording dots at the intersection points of the lines and
columns. The thickness of the layer of SnO.sub.2 is minimal to obtain a
high degree of transparency, although its value is by no means critical
because the current delivered is substantially zero.
By applying a d.c. or a.c. potential to one or both columns, an electrical
field is developed between them and the lines of which one end is earthed.
The value of this field is constant only along aline which is not supplied
with heating current because, in the opposite case, the line potential
decreases from the side corresponding to the connection L.sub.1 or L.sub.2
to the side which is connected to earth. However, this is unimportant
because the recording phenomenon only occurs on cooling, i.e. when the
line is not supplied with heating current and thus has a constant
potential.
Accordingly, four dots of which the power of diffusion may be continuously
controlled from a zero value to a maximum value, may be recorded in the
liquid crystal layer 101 respectively at the intersection points of the
lines 105, 106 with the columns 107, 108.
It is pointed out that recording is of necessity carried out line by line
because, when a line heats the crystal, it erases the preceding recording
both at the intersection with one column and at the intersection with the
other column, so that the dots corresponding to these two intersections
have to be re-recorded. Conversely, if the two lines were to heat
simultaneously, the same column would simultaneously record the same
information at its intersections with the lines.
The cell illustrated in FIG. 1 is essentially experimental. Using the
results obtained from this cell, it has been possible to construct the
display device illustrated diagrammatically in FIG. 2 which comprises 600
Lines LA.sub.1 to LA.sub.600 on a first substrate 203 of silicon and 800
columns CA.sub.1 to CA.sub.800 on a second substrate 204 of sapphire. With
the shim 201, these substrates delimit a layer of liquid crystal. These
values (600 and 800) have been selected to allow the display of a
television program under the European Standards of 625 lines and 25 frames
per second, allowing for the time in the lines and frame returns.
Accordingly, these standards correspond to a line duration of 64 .mu.s
which is compatible with the numerical values quoted above and which allow
heating and cooling times per line of less than 60.mu.. The line duration
in US standard is substantially the same and a device with 500 lines and
660 columns should be convenient.
A television video signal is applied by the connection VS to a sampling
circuit 205. This circuit samples the video signal and memorizes the
samples thus obtained under the control of a clock signal which arrives by
the connection HC and a line synchronization signal which is applied to
the sampling circuit by the connection HL. In this way, 800 samples of the
video signal taken over the duration of one scanning line are available at
the 800 connections C.sub.1 to C.sub.800 every 64 .mu.s and for a duration
of 64 .mu.s, so that this scanning line begins at C.sub.1 and ends at
C.sub.800. These connections are connected to the columns CA supported by
the substrate 204.
The circuit 205 is formed on the substrate 204 itself by integration using
the known, so-called SOS method. During integration by this method, both
the connections C.sub.1 to C.sub.800 and the display columns CA.sub.1 to
CA.sub.800 are formed by evaporation. This solves the problem posed by the
interconnections which is known to be a serious problem in all matrix
access systems. This is because, once the production masks have been
formed, the connections as a whole are formed in a single, simple and
inexpensive operation incorporated in the production process of an
integrated circuit. The dimensions of the circuit are not prohibitive by
virtue of the value of the devices (cathode ray tubes for example) which
the display device according to the invention enables to be replaced.
Accordingly, to record the sampled and memorized lines, a line, for example
LA.sub.j, is heated for the period during which the corresponding video
signal arrives in the circuit 205, after which the voltages applied to
heat that line is cut off and the sampled voltages are applied to the
columns CA in the period during which the video signal corresponding to
the line LA.sub.j+1 arrives in the circuit 205. During this same period,
this line LA.sub.j+1 is heated to prepare it recording, and so on for a
non-interlaced image.
To obtain this successive heating sequence of the lines, they are fed by
means of the circuit 206 which is itself formed on the substrate 203,
preferably by integration. In one step of this integration process, there
are formed the connections L.sub.1 to L.sub.600 and the lines LA.sub.1 to
LA.sub.600 which are themselves only an extension of these connections
because they can be made of the same material, for example aluminum.
It has been possible to obtain a line heating rate of less than 10 .mu.s by
increasing the power applied to the lines and a cooling rate of less than
50 .mu.s by reducing the thickness of the layer of SiO.sub.2. Accordingly,
the complete recording cycle may be carried out over the duration of one
line. In an other embodiment a line is recording during the heating of the
following one.
The circuit 206 receives clock signals at the frequency of the succession
of the lines in the video signal through the connection HL, clock signals
at the frequency of the succession of the fields through the connection HT
and a feed voltage for heating the lines LA through the connection VT. The
other feed connections for the circuits 205 and 206 have not been shown,
nor has the common earth at the right-hand end of the lines.
FIG. 3 illustrates one example of embodiment of the circuit 205.
The clock signals HC arrive at a counter 301. For obtaining a horizontal
definition substantially equal to the vertical definition, a rate equal to
880 times the rate of the line synchronization signals has been selected
for the clock signals HC, the fraction of the scanning line lost for
synchronization being estimated at 10%. Accordingly, the counter 301
counts at the frequency of the signals HC and, for synchronization, it is
symmetrically reset to zero by the line synchronization signal arriving at
the connection HL. Assuming for example that the lost part of the video
signal is situated halfway in front of and behind the line synchronization
pulse, it can be seen that the 800 samples to be taken from this video
signal are situated between the states 40 and 839 of the counter. Each of
these states is decoded and successively gives a logic signal on the
connections SC.sub.40 to SC.sub.839.
These connections are respectively connected to 800 sample and hold
circuits EA.sub.1 to EA.sub.800 which permanently receive the video signal
VS. Thus, as the counter 301 progresses from 40 to 839, the sample and
hold circuits EA sample the video signal in turn under the control of the
counter and memorize the samples thus taken.
The outputs of the circuits EA are respectively connected to the inputs of
800 other sample and hold circuits EB.sub.1 to EB.sub.800 which
simultaneously receive in parallel at their sampling control input the
line synchronization signal through the connection HL. Thus, when the
counter 301 is restarted, the samples contained in the circuits EA are
transferred to the circuits EB which memorize them for the duration of the
following line, the circuits EA then becoming available again. The outputs
of the circuits EB are precisely the connections C.sub.1 to C.sub.800 at
which are thus present for the duration of one line the samples of the
video signal which were taken during the preceding line.
FIG. 4 illustrates an embodiment of the circuit 206 in the case where the
recorded image is made up of two interlaced frames of 312 and a half
lines.
The line synchronization signals arrive at a counter 401 through the
connection HL. This counter also receives the frame synchronization
signals through the connection HT which enables the counter to be reset to
zero at the beginning of a frame. The counter 401 therefore counts at the
frequency of the signals HL. Assuming for example that 12.5 lines are lost
for the frame synchronization, of which 5 complete lines are lost at the
beginning and 7 complete lines at the end of a frame, the half line
required for interlacing being situated at the end of an odd frame and at
the beginning of an even frame, it can be seen that the 300 lines of a
frame which are to be recorded correspond to the states 5 to 304 of the
counter. Each of these states is decoded and successively gives a logic
signal at the 300 connections SL.sub.5 to SL.sub.304.
Each of these connections leads to one pair of AND-gates of the plurality
of gates ET.sub.1 to ET.sub.600. The output of each these gates is
connected to the base of an NPN transistor. The collectors of these
transistors are connected in parallel to the connection VT (power supply
for heating the lines), whilst their emitters are respectively connected
to the connections L.sub.1 to L.sub.600.
In order to select the even/odd frames, the frame synchronization signal is
applied by the connection HT to a bistable circuit 402 which changes state
at the frequency of these signals and of which one of the outputs is
connected in parallel to the inputs of the AND-gates of even index and the
other to the inputs of the AND-gates of odd index. Thus, the counter will
successively open the 300 odd-index gates which themselves will unblock
the 300 corresponding transistors for successively heating the 300 odd
lines, after which this same counter will successively open the 300
even-index gates for successively heating the 300 even lines. To ensure
that interlacing is correct, the trigger circuit 402 is set into the state
enabling the odd gates to be opened at the beginning of the odd frames. To
this end, the line and frame synchronization pulses are applied by the
connections HL and HT to an AND gate 403 which only opens when these
pulses are synchronous, which is only the case at the beginning of an odd
frame. The output of this gate is connected to a setting input of the
bistable circuit 402.
A display device of the type in question is not luminous per se and can
only be used under external illumination. In the example illustrated, this
external illumination is obtained by reflection on the metallic layer
which forms the lines, this reflection being more or less diffuse
according to the state of the liquid crystal. If the lines did not have a
sufficient reflective power (due to the use of another material for
example) it is possible to use a dielectric mirror deposited on these
lines.
If a material other than a liquid crystal, such as the lithium niobate
mentioned above, is used, the modifications to the optical properties
introduced by the recording are not always directly visible so that it is
advisable to make them show up. In the case of a modification to the index
by birefraction (as happens with lithium niobate), it is possible for
example to examine the device through a circular polarizer. Although the
dimensions are small, by virtue of the use of a thermal phenomenon, they
still allow close direct vision. One interesting application consists in
incorporating this device in a headset useable in particular by an
aircraft pilot.
In order to obtain an image of greater size, it is possible to use a
projection system. This projection system has to be adapted to the
diffusion phenomenon used so as to obtain a satisfactory contrast. Thus,
it is possible to use a Schlieren lens, as in the device known by the name
of Eidophore. However, it is equally possible to use a lens having a small
numerical aperture which in a way functions oppositely to the Schlieren
lens, i.e. by eliminating as many of the diffused rays as possible and by
transmitting only the directly reflected rays.
A projection system such as this is diagrammatically illustrated in FIG. 5.
A light source 501 emitting a beam condensed by a condenser 502
illuminates a display cell 503 according to the ivention at a relatively
small angle .theta. relative to the normal Z of the cell. The light
reflected by the cell is divided, according to the recorded image, into a
beam directly reflected at the same angle .theta. symmetrically in
relation to the incident beam and a part diffused into the right-hand
half-space. A projection lens 504, of which the aperture is just
sufficient to collect the reflected beam, forms an enlarged image of the
cell 503 on a projection screen 505. The respective inclinations and
positions of the cell 503, the lens 504 and the screen 505 are selected in
such a way that this image is not deformed, this in accordance with a
method commonly used in cinematographic projection where the projector is
rarely situated in the axis of the screen. Since the lens has a small
aperture, it has a considerable field depth and minimal aberration, which
facilitates its positioning relative to the cell and the screen.
It is pointed out that, in contrast to cathode ray tubes for example, the
image is permanently recorded except for the line which is to be recorded
during the period in which it is heated. Accordingly, this provides for a
good contrast and a memory effect which may be utilised in cases where it
is desired to freeze an image by stopping the arrival of the video signal.
In a liquid crystal cell, the smectic phase tends to be permanently
slightly diffusing unless means are used to enable the liquid crystal to
return spontaneously to a homogeneous and therefore completely transparent
orderly phase. In order to overcome this phenomenon, it is possible for
example to cover the surfaces of the substrates with a thin layer of a
surfactant adapted to the liquid crystal and enabling the crystal to be
slowly and spontaneously reoriented. At least one adequate surfactant is
known for each liquid crystal; the product known by the name of Silane is
very widely used and may be used in particular in cases where COB is used
as the liquid crystal, as described above. In this case, the memory effect
is lost under the effect of this spontaneous reorientation because the
recorded image is erased after a period of a few tens of milliseconds.
Accordingly, some of the contrast is lost in the event of use in a
television system because the image is partly erased from one frame to the
following frame, but the gain obtained is greater than the loss and the
balance is positive.
In a case such as this, the use of Schlieren lens is of greater advantage
because the spontaneous erasure of the image, in the case of a lens such
as that illustrated in FIG. 5, leads to a lightening which is more
unpleasant than the darkening introduced by the Schlieren lens.
FIG. 7 illustrates one example of embodiment of a projection system using a
lens of this type. Through a condenser 702 and a semitransparent mirror
706, a light source 701 illuminates a display cell 703 with a condensed
incident beam aligned with the normal Z to that cell. The reflected light
is divided into a beam which is directly and quasispecularly reflected in
the form of a beam F which is also aligned with Z, and a diffused part D.
A wide-aperture projection lens 704 collects at least some of this part D
and forms an enlarged image of the cell 703 on the projection screen 705.
An absorbing screen 707 preferably situated in front of the lens 704 and
close to the focussing spot of the beam F absorbs this beam so that the
image formed on the screen is formed solely by the diffused part D.
FIG. 6 illustrates a section through part of another embodiment of a
display device according to the invention. A liquid crystal layer 601 is
inserted between a transparent substrate 604, for example of glass, and a
substrate 603 of silicon. The substrate 603 carries an insulating layer
602 of silicon oxide (SiO.sub.2) obtained by oxidation and on which the
heating lines 605, seen in cross-section, are deposited. These lines are
embedded in a layer 609 of silicon oxide obtained by sputtering. The
columns 608, of which only one is visible in the FIG. in longitudinal
section, are deposited on this layer 609. This enables the control element
of the columns to be integrated in the substrate 603 and hence ordinary
glass to be used for the substrate 604. The sampled voltages of the video
signal are applied between these columns and a transparent
counterelectrode 610 (for example of Sn O.sub.2) at earth potential
deposited on the inner surface of the substrate 604. In this embodiment, a
dielectric mirror composed of bands 611 parallel to the lines 605 is
deposited on the columns 608. These bands respectively reflect three
colours additively synthesized from the colours--for example red, green
and blue. They each correspond to one heating line and together form a
regular permutation. Accordingly, it is possible by this device to
reproduce a colored image, although the means used for this purpose may
also be applied to the device shown in FIG. 1. This means may also assume
other forms and, for example, the bands 611 may be parallel to the columns
608 illustrated in FIG. 8 or, alternatively, may simply be filtering and
deposited on the substrate 604. Similarly, it is possible to carry out
other syntheses (for example binary) adapted to particular systems.
Another embodiment of the device shown in FIG. 2 consists in using a
substrate 204 of ordinary glass and in forming the control element 205
with thin-film transistors of the TFT-type.
To ensure that these devices function correctly, it is useful to control
the liquid crystal layer and, hence, the entire cell thermostatically to a
temperature slightly below (1 or 2.degree. C.) the smectic-nematic
transition temperature.
One effective way of doing this consists in depositing a transparent
electrode on the surface of the transparent substrate and in passing
through this electrode an electric current of which the value is adjusted
in dependence upon the temperature of the substrate, this temperature
being measured for example by a thermistor embedded in the substrate.
The devices thus described may be produced by the techniques used for the
production of integrated circuits. To be effectively used, they only have
to be fed with ordinary voltages and signals and illuminated by an
optionally ambient light source or placed in a simple projection
apparatus.
The invention covers any use of devices of the type in question for
displaying moving or still images analysed in accordance with a standard
which is adapted to the image to be displayed and which is not limited to
the official television standards used in the present description.
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