Optical: systems and elements – Holographic system or element – Using a hologram as an optical element
Reexamination Certificate
1999-07-20
2001-07-03
Spyrou, Cassandra (Department: 2872)
Optical: systems and elements
Holographic system or element
Using a hologram as an optical element
C359S015000, C359S019000, C359S493010, C359S494010, C359S506000, C349S005000
Reexamination Certificate
active
06256122
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a device for the elimination of the zero order beam emerging from a hologram illuminated in polarized light and more particularly to a video image projector equipped with such a device.
The functions of numerous conventional optical components, lenses, filters, etc., can today be produced by means of holograms which have various advantages, particularly those of having a planar shape which does not require much space and of being inexpensive to manufacture.
However, the functioning of a hologram, as that of any optics calling for diffraction of light, is a function of the wavelength of this light. When an incident beam of white light illuminates a hologram, this dependency is at the origin of the passage of the latter by a beam called “zero order” beam which is not diffracted and therefore not deflected, in contrast to the beam diffracted by the hologram. The presence of these two beams coming out of the hologram can have the effect of contaminating the beam of diffracted light with the white light of the nondiffracted beam. This is the case in particular when the hologram produces three beams of red, green ad blue light respectively, for example, for the projection of video images in color, the zero order beam, composed of white light, affects the purity of the colors.
In order to avoid this disadvantage, it is possible to manufacture the hologram in such a way that the usable diffracted beams diverge angularly to a greater extent from the axis of the zero order beam, or to use the diffracted beams a long distance from the hologram, where these beams are sufficiently separated spatially so that the objects which they illuminate cannot also receive the light of the zero order beam.
This solution unfortunately leads to very bulky assemblies, which are obviously not suitable in apparatuses such as, for example, the video image or television projectors intended for the general public, of which the space requirement must be as small as possible.
The present invention therefore aims to produce a device for elimination of the zero order beam emerging from a hologram illuminated in white light, which is both effective and has a small space requirement.
SUMMARY OF THE INVENTION
The present invention also aims to produce a video image projector equipped with such a device.
These aims of the invention are reached, as are others which will appear upon reading of the following description, with a device for elimination of the zero order beam emerging from a hologram illuminated in polarized light, this device being remarkable in that it includes a) a plate made of a birefringent material illuminated by said beam and the diffracted beam which emerges from the hologram, this plate being arranged and sized in such a way that one of the beams emerges from the plate without its plane of polarization having undergone appreciable rotation, while the plane of polarization of the other has undergone a rotation, of 90°, and b) a polarizer illuminated by the two beams which emerge from the birefringent plate, the plane of maximum transmittance of this polarizer being parallel to the plane of polarization of the beam which emerges from the birefringent plate from the beam diffracted by the hologram.
This plane of maximum transmittance is perpendicular to the plane of polarization of the other beam of zero order, which emerges from the birefringent plate, so that the zero order beam is extinguished by the polarizer.
Since the hologram the birefringent plate, and the polarizer can be very close to one another or even attached, one sees that the elimination of the zero order beam occurs in a very small volume.
The invention thus makes it possible to produce a video image projector with a small space requirement, the images being formed in an active matrix of liquid crystal cells each passed through by a polarized light beam emerging from a hologram. The projector is remarkable in that it includes a device, according to the invention, for elimination of the zero order beam emerging from said hologram.
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Dahmani Brahim
Korzinin Yuri
Sukhanov Vitaly I.
Corning Precision Lens
Jr. John Juba
Klee Maurice M.
Spyrou Cassandra
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