Optical: systems and elements – Diffraction – Using fourier transform spatial filtering
Reexamination Certificate
1999-05-05
2001-06-12
Spyrou, Cassandra (Department: 2872)
Optical: systems and elements
Diffraction
Using fourier transform spatial filtering
C359S567000, C359S571000, C349S095000
Reexamination Certificate
active
06246521
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The field of the invention is that of illumination devices and more specifically that of trichromatic illumination devices suitable for illuminating an LCD screen.
2. Discussion of the Background
At the present time, many display applications require the use of compact and lightweight illumination devices, allowing both small screens and very large screens (having an area of greater than 1 square meter) to be illuminated.
For some applications, a major requirement is to optimize the compactness, light efficiency and contrast parameters, especially in applications such as ultralight imagers for avionic helmet displays or else for large wall-mounted flat television screens typically 5 cm in depth for an image of 1 meter in diagonal, operating in direct-view mode.
The current solutions for helmet imagers are in fact based on the use of monochromatic CRT mini tubes; however, an essential requirement in these applications is to display trichromatic images, as long as it is possible simultaneously to satisfy the compactness-weight and brightness criteria. With the present techniques, the use of an LCD screen has a certain advantage with regard to compactness-weight, nevertheless new lighting solutions have to be envisaged in order to provide sufficient luminance allowing the user to display an image with sufficient contrast (typically greater than 5), particularly when he is moving around in bright ambient light (a pilot on a daytime mission).
Likewise, in large television screen applications, the LCD solutions currently employed are based on projection techniques which result in not insignificant depths, ranging from 20 to 40 cm, in the case of image formats of about 1 meter in diagonal. By way of example,
FIG. 1
illustrates an example of a display device using an image generator GI comprising especially a source and an active matrix, a projection optic OP and a deflecting mirror Mp which deflects the image towards the screen capital E, the whole system constituting a bulky device. One alternative would consist in using LCD screens in direct-view mode, based on a specific technology compatible with video-rate addressing, it being more difficult to address large pixels because of the large capacitances inherent in large electrooptic pixels. Large screens have the advantage of being able to dispense with a projection optic, but at the present time backlighting devices based on fluorescent tubes developed for computer applications do not have the luminance and contrast characteristics required for displaying a satisfactory video image in the mass market.
SUMMARY OF THE INVENTION
To alleviate these various drawbacks, the subject of the invention is a compact illumination device using novel anamorphic means to adapt the extent of a source to the illumination of a screen format, compatible with a compact display structure whose depth is small compared with the cross section of the illumination.
More specifically, the subject of the invention is a compact illumination device, comprising at least one source and collimating means, in order to deliver a lighting direction from the source along a so-called vertical direction Dy, characterized in that it comprises:
an optical component which anamorphoses the lighting in a direction approximately parallel to the so-called vertical direction;
an optical component which anamorphoses the lighting in a so-called horizontal direction approximately perpendicular to the so-called vertical direction so as to adapt the extent of the source to the illumination of a given rectangular format.
According to a variant, the optical component M
1
is a reflective component inclined at an angle &thgr;
1
with respect to the direction Dy.
This optical component M
1
may advantageously be reflective and consist of microprisms, each making an angle of approximately 45 with the direction D
y
. This optical component M
1
may also advantageously be a diffractive component consisting of index microstrata inclined at approximately 45° to the direction D
y
. In the case of trichromatic illumination intended for the lighting of a colour screen, the component M
1
may advantageously comprise the superposition of three holographic gratings, which reflect red, green and blue, respectively; these three gratings may be recorded in a single layer or in several layers of photosensitive materials. In addition, the index microstrata may give the grating a colour-splitting function, by dint of the angular dispersion of the red, green and blue colours. In order to produce this function, the component M
1
may comprise the superposition of at least two holographic gratings, the strata of which have a spacing and an orientation (close to 45° with respect to D
y
) which are matched to the dispersion in a given direction (close to the direction D
x
) of the red, green and blue colours. This is because two gratings may suffice insofar as a first grating causes extensive dispersion in the red and little in the green and a second grating causes extensive dispersion in the blue and little in the green, while together they cause sufficient dispersion in the red, the green and the blue.
According to a variant of the invention, the optical component M
2
is a component operating in reflection, inclined at an angle &thgr;
2
with respect to the direction D
x
approximately perpendicular to the directions D
y
and D
z
.
It may advantageously comprise microprisms, each making an angle of approximately 45° with the direction D
x
or comprise index microstrata inclined at approximately 45° to the direction D
x
.
When the component M
2
is a component of the diffractive holographic grating type, it may diffract the red, green and blue colours, this being so whether it operates in reflection or in transmission. When the component M
2
operates in reflection, it may be analogous to the holographic component M
1
and consist of at least two holographic gratings of a given strata orientation and spacing in order to cause dispersion of the light in different directions and thus provide a chromatic dispersion function.
When the component M
2
operates in transmission, the dispersion function is much more pronounced than in reflection and a single holographic grating may be enough to provide the angular chromatic dispersion.
This is why the subject of the invention is also a compact illumination device in which the optical component M
2
is a diffractive component, operating in transmission, which can be inclined at an angle &thgr;
2
with respect to the direction D
x
or parallel to the direction D
x
. The latter configuration may be particularly compact when the device is incorporated into a trichromatic display device and the component M
2
may consist of an array of holographic lenses juxtaposed on an active matrix. In this case, the component M
2
provides several functions:
anamorphosis in the direction D
x
;
colour splitting;
focusing onto the pixels of the active matrix.
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Delboulbe Anne
Huignard Jean-Pierre
Joubert Cecile
Loiseaux Brigitte
"Thomson-CSF"
Assaf Fayez
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Spyrou Cassandra
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