Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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BACKGROUND OF THE INVENTION
The present invention relates to devices for
5 projecting colour pictures onto a screen which make
it possible on the basis of a small display cell
to obtain an image or picture of desired size which
can be observed with the naked eye under good
conditions. It also relates to display systems
10 incorporating such a device.
It is known to produce devices based on
liquid crystals making it possible to display mis-
cellaneous information, particularly television
pictures. However, the size of such devices is in
15 most cases due to the thermal phenomena used therein.
Such a device is for example described in Canadian
Patent Application 302,627 filed by the Applicant
Company on May 4, 1978 and entitled "Picture display
device and television system using such a device".
In this known device, television pictures
analysed in the form of lines forming a field are
entered in a smectic liquid crystal layer using a
mixed thermal - electrical effect. Each line is
successively entered in the form of a more or less
25 diffusive structure along the line. The effect used
makes it possible to realise devices of small size
making it possible to project the picture entered
this way in the device onto a screen. Generally,
projection takes place by means of a so-called
30 schlieren optics making it possible to convert
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variations in the diffusive power of the liquid crystal
layer into variations in the brightness of the screen.
The aforementioned Canadian Patent Application
also describes a variant making it possible to
display a colour picture. For this purpose, the
initial number of lines is multiplied by three by
allocating a triplet of contiguous lines to each
picture analysis line. A filtering layer formed from
alternating red, green and blue lines superimposed on
the aforementioned lines makes it possible to obtain
the picture with the desired colours as a result of
additive synthesis.
The image entered in this visual display cell
can be observed in colour either at short distance
or projected onto a screen. In the case where the
picture is projected onto a screen, it is also
possible to use three separate devices, each corres-
ponding to a coloured primary and whereby the pictures
are projected in superimposed manner onto the same
screen.
In both cases, these devices are more complica-
ted than a black and white device and in the case of
projections using three devices, setting is also
very difficult.
BRIEF SUMMARY OF THB INVBNTION
In order to obtain a colour projection device
operating without complicated settings and at a
cost which is substantially the same as that of a
black and white device, the present invention proposes
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a device for projecting colour pictures onto a screen,
of the type incorporating a visual display cell making
it possible to display pictures in the form of a
plurality of fields formed from interlaced parallel
lines, a polychromatic light source making it possible
to illuminate the said cell, and means making it
possible to project the picture from the cell onto
the screen, wherein the lines of the cell are grouped
into line systems, each line of a line system making
it possible to display in accordance with a grey
scale pa~ng from black to white the infor~ation
corresponding to a given separate colour and wherein
the device also comprises means outside the display
cell and placed on the optical path between the light
source and the screen and optically connected to the
cell, making it possible to filter the light passing
by each line of the cell to obtain the gi~7en corres-
ponding colour.
BRIEF DESCRIPTIO~ OF THE DRAWINGS
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The invention is described in greater detail
hereinafter relative to non-limitative embodiments
and the attached drawings, wherein show:
Fig l a first embodiment of a device according to the
invention.
Fig 2 a variant of the device of Fig l.
Fig 3 a second embodiment of a device according to the
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
According to a first embodiment of a projecting
device according to the invent~n shown in Fig l, the
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picture to be projected is entered in a liquid
crystal display device lOl shown in a greatly
simplified manner and having a system of lines
forming a field seen in the drawing by the end of
the lines. This devîce has therefore three times
as many lines as there are lines in the field
according to which the picture is analysed. By an
electronic processing of the video signal representing
the picture, it is possible to separate the three
components corresponding to the red, green and blue
signals. The picture is then displayed on device lOl
in the form of triplets of contiguous lines, each
corresponding to a line of different colour as in the
device which has a screen formed from coloured filter-
ing lines incorporated into the device. However, thepresent device lOl does not have filtering lines
and the displayed in picture is consequently a black
and white picture.
For the purpose of illumi~ting the screen,
a light source 102 emits a beam of light which is
collected by a first lens 103 which, in the present
embodiment, focuses it in the vicinity of the surface
of deviating mirror 104. This mirror directs the
light beam towards an objective 105 making it possible
to illuminate cell lOl by a substantially parallel
and homogeneous light beam.
The light is reflected by display device
101 towards objective 105, this taking place in
specular manner by reflection on the heating electrodes
when the liquid crystal is not diffusive and in accordance
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with a l~be of variable shape as a function of the
liquid crystal used and which will be considered
as su~stantially Lambertian for the purposes of
the description.
Objectives 105 collects the light reflected
by display device 101 and forms therefrom a picture
on a slide 106.
In their passage between objective 105 and
slide 106, the rays reemitted by device 101 in a
specular manner pass through a focusing point
located in the centre of a diaphragm 107. This point
substantially corresponds to the image of the light
source 102 given by optical elements 103, 104, 101
and 105.
Diaphragm 107 enables it to eliminate the
light rays from the retrodiffusion lobe corresponding
to the point where the liquid crystal is diffusive.
In this way, the desired schlieren optics are
produced for the purpose of obtaining a good contrast.
Slide 106 on which is formed the image of
device 101 forms a coloured filter constituted by
bandscorresponding to the lines of device 101. Thus,
the image of each line of device 101 is superimposed
on one of the bands of slide 106. These bands are
coloured red, g~en and blue and are grouped in such
a way as to form triplets corresponding to the
triplets de~ined hereinbefore in the display device
101. T.hus, the light reemitted in black and white
by one line of a triplet of device 101 is filtered
by the corresponding band of slide 106, which colours
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it in accordance with the corresponding primary.
The image of device 101 formed in the plane of slide
106 is thus seen in colours by transparency through
said slides. A projection objective 104 then makes
5` it possible to project onto a screen 109 the image
coloured by slide 106.
It is easy to obtain such a slide 106 by
photographing a large dimension coloured image
reproducing the necess~y filter bands. The design
of such a large size image presents no difficulties~
It can for example be photographed by using a re-
production caption stand, so that there is a negligible
distortion. If it is desired to use a simpler optical
device for forming this slide, the distortions intro-
duced can be accepted and they are then compensatedduring the design of the large scale image.
Slide 106 can also be located on the lighting
beam of device 101. For this purpose, it is necessary
that the optics illuminating device 101 form on the
surface of the latt~r an image of slide 106 in such
a way that the lines of the device are illuminated
by the corresponding bands.
A variant using this positioning of the filter-
ing slide, as well as a device 101 for use by trans-
parency is partly shown in Fig 2.
Slide 106 is illuminated by a source 102 bymeans of a parallel light beam obtained via a lens
203. An objective 205 forms the image of slide 105
on the rear face of the display device 101 in such
a way that the image of the coloured bands of the slide
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coincides with the display lines of the device.
Thus, these display lines are illuminated
by the light of colour corresponding to that which
it is necessary to display and it is possible to
directly form the image of the other face of the
device 101 on a screen. The optical device necessary
for forming this image comprises for example a pro-
jection objective and a diaphragm for forming
schlieren optics.
According to another variant shown in Fig
3, the system is identical to that of Fig 1 from
light source 102 to diaphragm 107 inclusive. However,
instead of forming an intermediate image on a slide
and then projecting it onto a white screen, the
image is directly projected by means of a projection
objective 308 onto a specially coloured screen 309.
This screen is coloured in a series of red, green
and blue bands forming triplets in much the same
way as for slide 106. These bands are located on the
screen in such a way that the images of the display
lines of device 101 are formed thereon. In this way,
each band illuminated in white light only reflects
the colour corresponding to the line used for illuminat-
ing it. Thus, the image is coloured directly on the
screen. In another variant where the cell 101 is
positioned closer to objective 105, the latter makes
it possible to directly project the image onto the
screen.
These devices for projecting coloured images
or pictures onto a screen can be used in all display
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systems where it is desired to have a large picture
from a small display cell. It is in particular pointed
out that screen 309 in Fig 3 can be observed both
by transparency and by reflection. When observing by
transparency, the complete device can be placed in
a case providing the equivalent of a display system
equipped with a cathode tube. The visual analogy with
the system of so-called autoconvergent tubes with
aligned cathodes shows that the image or picture
quality of this system is very good.
Finally, in the case where screen 309 is
transparent and is observed from behind, the complete
device of Fig 3 is equivalent to that part of the
device of Fig 1 limited to the system terminated by
the slide.
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