Optics: image projectors – Polarizer or interference filter
Reexamination Certificate
2000-02-24
2001-06-26
Dowling, William (Department: 2851)
Optics: image projectors
Polarizer or interference filter
C353S081000
Reexamination Certificate
active
06250762
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an image projection system comprising an illumination system, a modulation system having at least two reflecting image display panels of the non-diffusing type for modulating light generated by the illumination system in conformity with image information to be projected, and a projection lens system. The image projection system includes an element having a color-separating as well as a color-recombining effect, and a polarizing beam splitter which is situated between the illumination system and the element and between the element and the projection lens system.
A reflective image display panel of the non-diffusing type is understood to mean a reflective liquid crystalline image display panel of the non-diffusing type or a DMD or the like.
An image projection system comprising two or three reflective image display panels may be given a very compact construction if the color separation and the color recombination are effected by one and the same optical system. The optical system may comprise, for example, a polarizing beam splitter. Since the other optical elements of such an optical system are situated between the reflective display panels and the polarizing beam splitter, it is undesirable that a change of polarization would be effected by the color-separating and color-recombining element. However, this is the case in practice, so that the ultimate image has a too low contrast and strong color deviations for all luminance levels between white and black. This is caused by the polarization-dependent transmission of the color-separating faces of the color-separating element and the geometrical decomposition of the polarization vector on all oblique faces due to non-perpendicular incidence. Each color channel causes a specific change of polarization as a function of the wavelength and the direction of propagation through the color-separating element. In addition to polarization changes of the light, light having a given direction of polarization may be reflected in an unwanted direction, In addition to strong color shifts in a color channel, this also causes unwanted optical crosstalk when light having a certain wavelength reaches a reflective display panel which is meant to modulate light of another wavelength.
The cause of the above-mentioned polarization effects is found in the relatively large angle of incidence on the color-separating faces of the color-separating element. Dependent on the element used, this angle of incidence may be between 10-20° and 45°. For a large angle of incidence, it is difficult to have an equal transmission characteristic of a color-separating coating for both s-polarized and p-polarized light. The smaller the angle of incidence, the smaller the above-mentioned problem will be. For small angles of incidence, the difference of transmission between p and s-polarized light can be minimized more easily. But also in this case, for example for a plumbicon prism, there is still a change of polarization.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an image projection system in which the above-mentioned drawbacks are obviated.
To that end, the image projection system includes at least one polarization-compensating element is situated between the element and the image display panels.
After minimizing the differences between the transmission for s and p-polarized light in the desired wavelength range and for the desired viewing angles, by making use of a color-separating element suitable for this purpose, the remaining rotation of polarization induced by the color-separating element is reduced by means of polarization-compensating elements.
A preferred embodiment of the image projection system according to the present invention is characterized in that at least one polarization-compensating element is situated between the polarizing beam splitter and the element.
The solution to the above-mentioned problem is thus found in the addition of polarization-compensating elements. Said elements may be provided to the entrance face of the color-separating element and/or to one or more of the three exit faces of the element. “To the entrance face” is understood to be between the polarizing beam splitter and the color-separating element and “to the exit faces” is understood to be between the color-separating element and the display panels. Direct optical contact is not required.
A further embodiment of the image projection system according to the present invention is characterized in that the polarization-compensating element is a birefringent element.
A very suitable compensating element may comprise a birefringent element or a combination of birefringent elements. A birefringent element or a combination of birefringent elements ensures that a change of polarization by the color-separating element is substantially eliminated within the wavelength range for a plurality of propagation directions.
A further embodiment of the image projection system according to the present invention is characterized in that the birefringent element has a biaxial symmetry.
When the color-separating element has a viewing angle-dependent behavior which is different for the horizontal viewing directions with respect to the vertical viewing directions, the polarization-compensating element is preferably a birefringent element having a biaxial symmetry.
A further embodiment of the image projection system according to the present invention is characterized in that the birefringent element has a tilted optical axis.
If there is a difference between the positive and negative viewing directions, an element having a tilted optical axis is advantageous.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.
REFERENCES:
patent: 5374968 (1994-12-01), Haven et al.
patent: 5986815 (1999-11-01), Bryars
patent: 6082861 (2000-07-01), Dove et al.
patent: 6089718 (2000-07-01), Hashizume
Dowling William
U.S. Philips Corporation
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