Liquid crystal cells – elements and systems – Particular structure – Particular illumination
Reexamination Certificate
1995-02-02
2001-10-23
Parker, Kenneth (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Particular illumination
C349S098000, C349S115000, C349S176000
Reexamination Certificate
active
06307604
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a light source having a luminescent layer, whose light is guided through an optical filter.
Such a light source for use in a colour projection arrangement with three monochromatic cathodoluminescence light sources is known from DE-OS 38 36 955.
A multilayer interference filter, which concentrates the light radiation in the forward direction as a light collection filter, is arranged between a front substrate and a layer of luminescent material. Interference filters are difficult to manufacture. A significant problem is that the thickness of the individual layers should be maintained constant, which is an absolute requirement for a satisfactory operation.
OBJECTS AND SUMMARY OF THE INVENTION
It is an object of the invention to provide a light source with a filter which can be easily manufactured and produces low light losses.
This object is solved in that the optical filter comprises at least one cholesteric crystal layer (cholesteric filter).
Known cholesteric filters (cf. Philips Research Bulletin on Materials 1991, page 15, or EP-A 0 154 953) are transmissive to light which is circularly polarized in a given direction of rotation (for example, laevorotatory) within a selectable wavelength range, whereas the circularly polarized light is reflected in the opposite direction of rotation (for example, dextrorotatory). If the filter is used as a polarization filter, either the transmitted or the reflected light can be utilized, hence at most half the unpolarized entrance light. In the combination according to the invention of a luminescent narrow-band light source with a cholesteric polarisation filter, the light reflected by the cholesteric layer is depolarized by the luminescent layer and reflected back to the cholesteric layer, so that approximately half the quantity of this light passes through the cholesteric polarization filter as, for example laevorotatory polarised light. The rest of the light which is reflected back is again reflected towards the luminescent layer from which it is again depolarized and reflected back to the cholesteric layer. This process may be repeated many times in an analogue manner, so that, while allowing for unavoidable absorption losses, approximately 80% of the unpolarized entrance light is finally passed through the cholesteric polarization filter as, for example laevorotatory polarzed light. Such a polarization filter having a high share of polarized light which can be utilized is characterized in that the cholesteric filter comprises a dextrorotatory or a laevorotatory cholesteric layer in which the wavelength range, in which the transmitted light is polarized, is larger than the wavelength range of the luminescent light, the wavelength range of the cholesteric filter, in which perpendicularly incident light is polarized, and the wavelength. range of the luminescent light starting at approximately equal, low wavelengths.
The reflecting property of a luminescent layer provides the possibility of replacing the interference filter of the arrangement known from DE-OS 38 36 955 by a combination of two cholesteric layers polarized in opposite directions, which combination can be manufactured in a very simple manner. A corresponding solution according to the invention is characterized in that the cholesteric filter is a light collection filter comprising successively arranged cholesteric layers, one of which polarizes in a dextrorotatory sense and the other polarizes in a laevorotatory sense in the same wavelength range, the wavelength range for this combination of layers, in which perpendicularly incident light is completely reflected, being slightly above the wavelength range of the luminescent light.
It is advantageously possible to arrange a polarizing cholesteric filter according to the invention and a cholesteric light collection filter according to the invention one after the other and to superimpose their physical effects. A simple solution using two layers with the same effect is characterized in that the cholesteric filter comprises a first polarizing layer and a second polarizing layer having an opposite sense of rotation, in which the wavelength range, at which light passing through the first layer is polarized, is larger than the wavelength range of the luminescent light and, in the case of perpendicularly incident light, starts approximately at the same low wavelength as the wavelength range of the luminescent light, and in that the wavelength range, in which the second layer polarizing in the opposite sense of rotation polarizes perpendicularly incident light, is slightly above the wavelength range of the luminescent light.
An advantageous embodiment of the invention is characterized in that the light source forms part of a cathodoluminescent lamp for LCD-projection television (
FIG. 3
) in which a &lgr;/4 element is arranged between the light source and the LCD element.
In accordance with a further advantageous embodiment, light sources according to the invention may form part of a flat colour display screen having a pattern of light sources of different colours. In a further embodiment the light sources may be arranged behind a substrate and a &lgr;/4-element as well as an absorbing polarizer may be arranged in front of the substrate.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment described hereinafter.
REFERENCES:
patent: 3669525 (1972-06-01), Adams et al.
patent: 3697152 (1972-10-01), Adams et al.
patent: 4073571 (1978-02-01), Grinberg et al.
patent: 4882617 (1989-11-01), Vriens
patent: 4900133 (1990-02-01), Berman
patent: 5016985 (1991-05-01), Kalmanash et al.
patent: 5089883 (1992-02-01), Welker et al.
patent: 5193015 (1993-03-01), Shanks
patent: 5235443 (1993-08-01), Barnik et al.
patent: 0154953 (1985-09-01), None
patent: 0302619 (1989-02-01), None
Melamed et al “Selected Optical Properties of Mixtures of Cholesteric Liquid Crystals” Applied Optics-vol. 10, No. 5—May 1971—pp. 1103-1107.*
Adams et al. “Cholesteric Films as Optical Filters” Journal of Applied Physics—vol. 42—No. 10—Sep. 1971, pp. 4096-4098.*
Adams et al., “Lossless Polarizer” Xerox Disclosure Journal—vol. 1—No. 3—Mar. 1976—pp. 85-86.*
F. J. Kahn “Cholesteric Liquid Crystals For Optical Applications” Applied Physics Letters—vol. 18, No. 6—Mar. 1971—pp. 231-233.*
Philips Research Bulletin on Materials 1991, “Scanning tunnelling microscopy” pp. 5-16. No date.
Hikmet Rifat
Raue Ralf
Welker Thomas
Bartlett Ernestine C.
Duong Tai V.
Parker Kenneth
Spain Norman N.
U.S. Philips Corporation
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