Optical: systems and elements – Holographic system or element – Using a hologram as an optical element
Patent
1996-02-16
1999-08-31
Spyrou, Cassandra
Optical: systems and elements
Holographic system or element
Using a hologram as an optical element
349 5, 349 8, 349104, 349105, G02B 532, G02F 11335
Patent
active
059461144
DESCRIPTION:
BRIEF SUMMARY
The invention relates to an optical filtering device and its application to a liquid crystal projector, making it possible in particular to carry out chromatic correction.
Television is presently moving towards the presentation of images of large size and high resolution (HDTV format). The liquid crystal projector constitutes one of the solutions currently being developed to satisfy these needs.
These devices operate on the same principle as a slide projector in which the slide has been replaced by a liquid crystal valve. They generally comprise three cells, one for each of the primary colors (red, green, blue) originating from the same source of white light. The objective being to produce a good quality image, the principal criteria for evaluating the image will be the following:
The obtaining of trichromatic coordinates which are as saturated as possible will make it possible to increase the number of tones restored in the image and thus to approach the calorimetric content encoded during filming. This problem constitutes one of the important parameters to be taken into account when defining the architecture of liquid crystal projectors and choosing lighting sources.
One of the principal limitations of liquid crystal projection devices is imposed by the illumination device. This limitation originates in part from the narrow choice of white sources which are compatible with this application. In effect, the white sources must at one and the same time satisfy criteria of high luminous efficacy, stability of their colorimetry, long lifetime, as well as imperatives of cost. At present, arc lamps of metal halide type appear to constitute the best compromise in respect of these criteria.
These arc lamps, developed more especially for cinema projection, exhibit excellent colorimetry for this application, since its chromatic x/y coordinates are very close to those of the reference "WHITE" of the television standard (denoted D65 such that x=0.313; y=0.329).
Nevertheless, in order to be able to be used in a liquid crystal projector it is necessary to carry out chromatic separation of the emission from the source. This function is performed with the aid of dichroic mirrors (DM) whose typical spectral transmission curves are given in FIG. 2. Their characteristics will be chosen so as to obtain the lighting of the liquid crystal cell by primary colors which are as close as possible to the TV signal coding standard.
The notable differences in the spectral characteristics of the radiation emitted by metal halide lamps (FIG. 3) make it necessary to specify the set of dichroic mirrors to be used for each type of lamp. One of the difficulties to be solved originates from the correct filtering of the following spectral bands: line close to 570 nm.
In fact the low-pass or high-pass dichroic functions which satisfy this application require high accuracy in regard to:
The standard accuracy for carrying out these functions, typically 15 nm, will enable acceptable colorimetry to be obtained only at the price of a loss of flux by underdimensioning these dichroic mirrors. The production of filters meeting the required specifications (accuracy of about 5 nm in the cutoff frequency) would entail, on the one hand, high cost and, on the other hand, a poorer luminous balance, their transmission being poorer.
SUMMARY OF THE INVENTION
The invention therefore relates to an optical filtering device comprising a dichroic separator situated on the path of a beam to be filtered, characterized in that it includes a holographic filtering device also situated on the path of the beam to be filtered.
The invention also relates to a liquid crystal projector applying the device, characterized in that it includes: wavelengths of the range of colors; wavelengths; corresponding to wavelength ranges of different primary colors; for modulating each beam of a primary wavelength;
The various subjects and characteristics of the invention will emerge more clearly from the description which follows and from the attached figures which represent:
BRIEF DES
REFERENCES:
patent: 4752130 (1988-06-01), George
patent: 5181054 (1993-01-01), Nicolas et al.
patent: 5229874 (1993-07-01), Lehureau et al.
patent: 5245449 (1993-09-01), Ooi et al.
patent: 5247343 (1993-09-01), Burch
patent: 5272496 (1993-12-01), Nicolas et al.
patent: 5347380 (1994-09-01), Lehureau
patent: 5392181 (1995-02-01), Lehureau et al.
patent: 5410421 (1995-04-01), Huignard et al.
patent: 5539544 (1996-07-01), Paih et al.
patent: 5546200 (1996-08-01), Nicolas et al.
Patent Abstracts of Japan, vol. 12, No. 411 (P-779), Oct. 31, 1988, JP-A-63-147135, Jun. 20, 1988.
Huignard Jean-Pierre
Joubert Cecile
Loiseaux Brigitte
"Thomson-CSF"
Chang Audrey
Spyrou Cassandra
LandOfFree
Optical filtering device and application to a liquid crystal pro does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Optical filtering device and application to a liquid crystal pro, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Optical filtering device and application to a liquid crystal pro will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2427736