Optics: image projectors – Unitary plural refracting surfaces
Reexamination Certificate
1999-03-30
2001-01-30
Dowling, William (Department: 2851)
Optics: image projectors
Unitary plural refracting surfaces
C353S098000
Reexamination Certificate
active
06179425
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to an image projection system comprising, in this order, an illumination system with a radiation source and an image display system with at least one reflecting image display panel for modulating, with image information, an illumination beam to be supplied by the illumination system, and a projection lens system, a polarization-sensitive beam splitter being arranged between the illumination system and the image display panel.
An image projection system of the type described in the opening paragraph is known, for example, from European patent specification EP 0 467 447, to which U.S. Ser. No. 08/976,517 corresponds. The image projection system described in this specification comprises an illumination system for supplying an illumination beam and an image display system with reflective image display panels for modulating this light beam in conformity with image information to be projected. The display panel may be, for example a liquid crystalline display panel comprising a layer of liquid crystalline material which is provided with a two-dimensional array of pixels which are driven by means of, for example electrodes.
In the known reflective LCD projection systems, the different directions of polarization are separated from each other by means of a polarization-sensitive beam splitter. The part of the light modulated by the display panel, which part must give rise to dark pixels in the image, in other words, the light which is modulated by pixels of the display panel corresponding to dark pixels in the image, is deflected from the light path by the beam splitter and is thus lost. This is at the expense of the peak brightness in the image.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an image projection system in which a relatively high peak brightness is realized.
To this end, the image projection system according to the invention is characterized in that the illumination system comprises an extra optical system for at least partly re-illuminating the image display system with the light reflected by the image display system to the illumination system, the extra optical system comprising means for redistributing light coming from a pixel of the image display system across a plurality of pixels of the image display system.
The present invention relates to a reflective LCD projection system in which a polarization-sensitive beam splitter (PBS) polarizes the incident light before it is incident on the display panel and analyzes the light modulated by the display panel before it is projected.
The display panel may be, for example a TN (twisted nematic)-based display panel. Such a panel may operate, for example, as follows. When a given voltage is present across the liquid crystalline layer for pixels in the bright state, the direction of polarization of the incident light is rotated. This light is reflected by the layer to the PBS which in its turn reflects the light towards the projection lens. When a different voltage is present across the liquid crystalline layer for pixels in the dark state, so that the layer is deprived of the possibility of rotating the plane of polarization, this light is reflected by the display panel and subsequently passed by the PBS towards the radiation source, because the direction of polarization has not changed.
The light which is modulated by a pixel representing a dark or grey pixel in the image is now not deflected from the light path by the beam splitter but is again sent towards the radiation source and thus recuperated. This light will as yet have an opportunity of being incident on a pixel representing a bright pixel.
In the image projection system described above, the light intended for dark or grey pixels is thus not lost but is re-used.
To prevent ghost images from being produced during this re-use, the illumination system is provided with an extra optical system.
A preferred embodiment of the image projection system according to the invention is characterized in that the integrator system comprises a first and a second integrator plate, and in that the extra optical system comprises a third integrator plate, a lens system and a reflecting element, the third integrator plate being situated in the plane of the second integrator plate, in that the lens system is arranged between the integrator system and the image display panel for at least partly imaging an image of the second integrator plate reflected by the image display panel on the third integrator plate, and in that the reflecting element is arranged between the third integrator plate and its focal plane.
In this way, the second integrator plate is imaged in the plane of the second integrator plate, after reflection on the display panel and via the optical system. The third integrator plate is situated at the location of this reflected image, and an image of the light distribution of the first integrator plate is formed in its focal plane. The mirror ensures that this light distribution is reflected to the third integrator plate where it serves as illumination for the third integrator plate. The light distribution across the display panel after illumination via the third integrator plate is a superposition of the light spots in the first integrator plate.
An embodiment of the image projection system according to the invention is characterized in that an additional polarizer is arranged between the polarization-sensitive beam splitter and the projection lens system.
A higher contrast ratio in the projected image can thereby be achieved.
A further embodiment of the image projection system according to the invention is characterized in that the distance between the third integrator plate and the reflecting element is variable.
Since the light spots in the second integrator plate are mainly concentrated in the center of the individual lenses, the re-used light, which is responsible for the enhanced peak brightness, is also more centralized in the projected image. The light redistribution can be influenced by the above-mentioned measure.
Another embodiment, with which the same object as in the previous paragraph is achieved, is characterized in that the individual lenses of the third integrator plate have mutually different focal lengths.
A further embodiment of the image projection system according to the invention is characterized in that the illumination system comprises a polarization conversion system.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.
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Dowling William
U.S. Philips Corporation
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