Television – Holographic – Color tv
Reexamination Certificate
1999-10-01
2001-11-27
Britton, Howard (Department: 2713)
Television
Holographic
Color tv
C348S751000, C348S757000, C359S015000
Reexamination Certificate
active
06323896
ABSTRACT:
The present invention relates to a holographic device for formation of at least one light beam with predetermined spectral composition, and more particularly to such a device designed for the projection of video images displayed on a matrix screen with liquid crystal cells.
From the publication entitled “Compact Spatio-Chromatic Single-LCD Projection Architecture” of Loiseaux et al. (publication S7-4 of the conference Asia Display 95 which is held at Hamamatsu (Japan) in 1995), such a device is known, which has a white light source, and a volume phase hologram is illuminated by said source. This hologram is recorded in such a way as to disperse the light continuously from the red to the blue. The resulting spread beam illuminates a grid of cylindrical microlenses with a variable incidence which depends on the wavelength. In the focal plane of each microlens, the spectrum of the incident light is spread spatially over three columns of cells facing it. The dispersing power of the hologram, the focal distance, and the grid pitch of the microlenses are chosen so that the chromatic contents of the light illuminating each of the three columns of cells facing each microlens corresponds to the red, green, and blue parts of the spectrum, respectively. The cells are liquid crystal cells forming part of a matrix screen of such cells. Three cells are associated with each image element, or pixel, of an image displayed on this screen. They control the transmission, or the lack of transmission, of the light received from the hologram to an objective that projects an enlarged image of the image displayed on the screen onto an opaque or translucent observation surface. Such a device makes it possible to increase the fraction of the light coming from the source that is received by the cells of the liquid crystal screen, and therefore the luminance of the image projected onto the observation surface, in comparison with that obtained with video image projection devices of conventional design, which have matrices of light filters attached to the liquid crystal screen.
The device described in the aforementioned publication can only be used with liquid crystal screens in which the cells corresponding to red, green and blue components of a pixel of the image are aligned perpendicular to the direction of alignment of the cylindrical microlenses of the grid. This device therefore cannot be used when the cells of such a triplet are arranged at the vertices of a triangle, for example, as is ordinarily the case.
Also known from International Patent Application No. WO 92/09915 is a device for lighting a trichromatic liquid crystal screen for the projection of an image displayed on this screen. This device has three assemblies of prisms and holograms. The holograms are illuminated at an incidence of 45° for which their angular selectivity is high. Each of the assemblies is associated with one of the monochromatic components of the image, and a system of mirrors illuminates the liquid crystal screen with the radiation coming from the three assemblies. The light processed by these three assemblies comes from a single source which must emit a beam with low divergence. This source must then be a point source, which implies a short lifetime and a limited power. Furthermore, the light passing through the three assemblies also passes through numerous surfaces and therefore undergoes losses due to stray reflections on the holograms and on the faces of the prisms. Finally, the assembly described is bulky and therefore difficult to integrate in a video image projection device intended for the greater public.
The present invention therefore aims to produce a holographic device for formation of at least one light beam with predetermined spectral composition, designed particularly to make possible the projection of trichromatic images displayed on a matrix screen with liquid crystal cells, such that this device does not have the disadvantages of the devices of the prior art described above. To do this, this device must be able to use effectively the light energy of a real white light source, which is not a point source, and must be designed in such a way as to minimize the light losses by reflection or diffusion while having a small space requirement, making it easy to integrate in a video image projector.
The present invention also aims to produce such a device which is compatible with a matrix screen with liquid crystal cells in which the cells corresponding to a pixel of the image which is displayed are not aligned and are arranged at the vertices of a triangle, for example.
These aims of the invention, as well as others which will appear upon reading of the description which follows, are accomplished with a holographic device for formation of at least one light beam with predetermined spectral composition. This device is remarkable in the fact that it includes a) a flat mirror formed by adjacent areas which are alternately transparent and reflecting, b) at least identical first and second reflection holograms, each delivering at least one diffracted beam with said spectral composition, c) a light source for illuminating said holograms at a roughly normal incidence by the intermediary of said mirror. Said holograms are arranged symmetrically with respect to the plane of said mirror, in such a way as to be illuminated by the intermediary of the transparent and reflecting areas, respectively, of the mirror and in such a way that the beams diffracted by the first and second holograms are reflected and transmitted, respectively, by the reflecting and transparent areas, respectively, of the mirror, in order to be combined into a continuous light beam with said spectral composition, formed at the outlet of the device.
As will be seen further on, this arrangement allows one to take advantage of the low angular selectivity of the reflection holograms, particularly at the normal incidence of the radiation illuminating said hologram. This low selectivity allows one to use more effectively the quantity of light coming from the source used by the holograms, in particular when such source is extended, that is to say, not a point source.
According to one embodiment of the device according to the invention, a part of the beam emitted by the source, transmitted or reflected by the mirror, and illuminating the hologram, is diffracted by the hologram by an angle such that it ensures the reflection or transmission, respectively, of the diffracted beam by the areas of the mirror adjacent to those that transmitted or reflected said part, respectively.
According to a first variant, each of the holograms forms at least two diffracted beams, each of which has a predetermined spectral composition. The angles of diffraction of the two beams are such that the traces of the central rays of these beams in the plane of the mirror are on the axis of the same area of the mirror.
According to a second variant, each of the holograms also forms a third diffracted beam with a predetermined spectral composition. The angle of diffraction of this third beam is such that the trace of its central ray in the plane of the mirror is on the axis of the area of the mirror closest to that of the same nature (reflecting or transparent, respectively) containing the traces of the central rays of the first two beams.
The device according to the invention can be applied to a projector of video images displayed on a matrix screen with liquid crystal cells attached to a grid of optical microlenses arranged in order to focus each at least one beam of radiation with predetermined spectral composition on a corresponding cell of the screen. The projector has a holographic device according to the invention for illuminating the lenses of the grid with at least one light beam with predetermined spectral composition.
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patent: 4715683 (1987-12-01), Gregory et al.
patent: 4807978 (1989-02-01), Grinberg et al.
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patent: 5172222 (1992-12-01), Plantier et al.
patent: 5267060 (1993-11-01), Colton
pat
Dahmani Brahim
Galpern Alexander
Sukhanov Vitaly
Britton Howard
Corning Precision Lens
Klee Maurice M.
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