Television – Video display – Projection device
Patent
1994-03-22
1996-06-11
Metjahic, Safet
Television
Video display
Projection device
348750, 348759, 348751, 348761, H04N 931
Patent
active
055260632
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to image projectors, in particular video projectors, using several light beams of different colours to form the image to be projected. It relates especially to means for improving the luminous efficiency of such projectors.
In regard to colour image projection, greater and greater emphasis is being placed on image projection devices which implement techniques of spatial modulation of light. These techniques use electro-optical valves or screens for spatial modulation of light, termed "modulation screens" in the remainder of the description. Spatial modulation of light is to be understood to mean that a beam is projected globally onto the whole surface of an image to be projected, as opposed to temporal, or time, modulation systems in which a slender pencil of light (corresponding to an image point only rather than to the whole image) is modulated temporally at the same time as it scans the surface of a projection screen.
A spatial modulation screen usually consists of a screen of the LCD type (Liquid Crystal Display) comprising a matrix layout of liquid crystal cells, each cell corresponding to an elementary image area or pixel.
The polychrome image can be obtained either with the aid of three modulation screens each forming a monochrome image of one of the three primary colours red, green and blue, or from a single three-colour modulating screen, that is to say a modulating screen furnished with filters coloured according to the three primary colours.
In the case of projectors using a single three-colour modulation screen, at least three neighbouring elementary image areas constitute a three-colour pixel. Each of these three elementary areas corresponds to one of the primary colours, red, green and blue depending on the colour of the filter assigned thereto.
The solutions using a single three-colour modulation screen are well known, and they have the advantage of being particularly simple. On the other hand, they lead to a loss of brightness by a factor of at least three in each of the three chromatic components, this loss being imposed through the spatial distribution of the colour filters.
In the case of projectors using three modulating screens each forming a monochrome image, each modulating screen spatially modulates a beam of light coloured in accordance with a primary colour, and the polychrome image is obtained by superimposing the three monochrome images on a projection screen.
The images formed by the three modulating screens are projected onto the projection screen with the aid of a single objective which is common to the three monochrome images, or else with the aid of three objectives. The coloured beams of the various primary colours, are obtained from a single beam of white light produced by a light source termed the "white lamp", behind which are placed colour filters. Generally the white lamps used are of the arc lamp type, and more especially of the halogen type which exhibits a good luminous yield.
2. Description of the Prior Art
FIG. 1 shows diagrammatically a conventional colour image projector of the type with three modulating screens as defined above.
The projector includes a source of white light SL formed by an arc lamp 1 arranged at the focus of a reflector 2. This arrangement gives rise to a beam of white light FB of small divergence, propagating towards a modulating assembly with three modulation screens ER, EG, EB each intended to form an image having one of the primary colours; this assembly has reflecting mirrors MR1, MR2 and dichroic mirrors MD1 and MD4 with the aid of which the beam of white light FB is split into three coloured beams FPR, FPG, FPB whose spectral band corresponds to one of the primary colours red, green and blue. The dichroic mirrors are wavelength-selective mirrors, that is to say they transmit a certain spectral range and reflect the complementary range.
The three coloured beams each pass through a modulating screen ER, EG, EB by which they are modulated. They are later superimp
REFERENCES:
patent: 3507984 (1970-04-01), Stavis
patent: 4145712 (1979-03-01), Spooner et al.
patent: 4150396 (1979-04-01), Hareng
patent: 4229072 (1980-10-01), Torok
patent: 4882617 (1989-11-01), Vriens
patent: 5148285 (1992-09-01), Nakane
patent: 5255082 (1993-10-01), Tamada
patent: 5317348 (1994-05-01), Knize
Schedule A: List of related cases by Assignee.
Huignard Jean-Pierre
Joubert Cecile
Loiseaux Brigitte
Puech Claude
"Thomson-CSF"
Hsia Sherrie
Metjahic Safet
Plottel Roland
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