Liquid crystal projectors

Details Liquid crystal projectors Liquid crystal projectors

1999-08-06

2001-07-17

Dowling, William (Department: 2851)

Optics: image projectors

Unitary plural refracting surfaces

C353S020000

Reexamination Certificate

active

06260972

ABSTRACT:

The invention relates to liquid crystal projectors, and particularly although not exclusively to liquid crystal projectors suitable for projecting a colour television or video image onto a wall or screen.
Some prior art documents relating to this field will first be mentioned.
The paper entitled
Ultra
-
High
-
Efficiency LC Projector Using Light Illuminating System
, by Y. Itoh, J. -I. Nakamura, K. Yoneo, H. Kamakura, and N. Okamoto of Seiko Epson Corp., published in SID 97 Digest pages 993-996 describes the most efficient illumination system (see
FIGS. 1 and 2
) currently known for commercially available liquid crystal (LC) projectors. This system comprises two arrays of lenses, which will be referred to as lenslet arrays hereinafter. The paper discloses a homogeniser element (two lenslet arrays) that incorporates a method for substantially converting the unpolarised light from an arc lamp into one linear polarisation state. A disadvantage of this element is that it increases the extent of the light beam which translates into a loss in optical brightness of the final projected image. The present invention utilises a similar homogeniser element that minimises the increase in beam extent, which enables a higher thoughput system to be created.
U.S. Pat. No. 3,296,923 and U.S. Pat. No. 4,497,015 relate to conventional optical integrators and are based upon a first and second two-dimensional lens array, both lens arrays being arranged in a similar rectangular grid. In these two patents homogeniser elements increase the optical beam extent, and do not provide a method for converting the polarisation into mainly linearly polarised light. Since the homogeniser does not polarise the light and also increases the beam extent, a liquid crystal projector incorporating such a component is relatively inefficient.
U.S. Pat. No. 5,662,401 relates to source extent modification in cylindrically symmetrical systems to improve light throughput. U.S. Pat. No. 5,418,583 describes a non-identical set of two lenslet arrays used in conjunction with an arc light source and a parabolic reflector to utilise the optical extent of the output beam effectively. The light from the first lenslet array is focussed into (i) a rectangular distribution, or (ii) a circle with the smallest possible diameter; the aim being to minimise the ‘dead space’ in the optical distribution and hence extent. However, this arrangement does not incorporate a polarisation conversion scheme.
British Patent Application No. 9718741.3 and British Patent Application No. 9800018.5 relate to the concept of extent modification and homogenising lenslet arrays.
U.S. Pat. Nos. 5,662,401 and 5,418,583 and British Patent Application Nos. 9718741.3 and 98999 all describe optical homogeniser elements that minimise the increase in optical extent. However they do not incorporate polarisation conversion technology as outlined in the paper above (Y. Itoh et al).
According to the invention there is provided a liquid crystal projector comprising a light source, a first lenslet array comprising a plurality of first lenslets, a second lenslet array comprising a plurality of second lenslets and being con-focal with said first lenslet array, and at least one liquid crystal panel, wherein at least some of said first lenslets are non-identical with each other, and further comprising a polarising array comprising a plurality of polarising elements each arranged to polarise light passing through a respective second lenslet.
In one embodiment of the invention said first lenslet array produces an array of source images in a plane, and the area over which said array of source images extends is smaller than it would be if the first lenslets were identical with each other.
This is advantageous because by filling the gaps in the intensity distribution at the image plane of the first lenslet array, the extent of illumination is reduced allowing greater throughput of light in systems limited by the extent of source size. Such an arrangement can also be used with current technology and state of the art system designs, and need not add any further cost. The optical extent of the light source is used more efficiently offering the following advantages:
(i) a larger source may be used, with a concomitant increase in optical power;
(ii) a brighter projected image is obtained for systems that are “etendue limited” using a convention homogenisers and polarisation conversion elements;
(iii) slower projection lenses can be used, providing a cost saving; and
(iv) smaller light valve panels can be used which translates into a smaller overall optical system.
The word “extent” defines the size of the optical beam in the system and is mathematically represented as the product of the beam size multiplied by the solid angle (divergence) of the beam. The extent of a projection system is limited by the size of the light valve and the f-number of the projection lens; if the extent of the projection system is smaller than that of the illuminating beam then the system is “etendue” limited and hence some light will be lost.
In a further embodiment of the invention said first lenslets are arranged in a number of lines, said first lenslet array produces a plurality of images of said source arranged in stripes, and at least some of said stripes contain source images from more than one of said lines so that the number of said stripes is less than the number of said lines.
In a further embodiment of the invention each first lenslet is paired with a corresponding second lenslet to form a pair of lenslets, and the spatial arrangement of first lenslets within said first lenslet array is different from the spatial arrangement of second lenslets within said second array.
In a further embodiment of the invention, for each pair of lenslets the physical centre of the first lenslet is the optic axis of the second lenslet, and the physical centre of the second lenslet is the optic axis of the first lenslet.
Each first lenslet may be rectangular in shape, as may each second lenslet.
Each second lenslet may be slightly larger than an image of said source produced at the second lenslet by a corresponding first lenslet.
Some light may be lost as a result of overfilling of at least some of said second lenslets.
Said plurality of polarising elements may be an array of polarising beam splitter elements located behind said second lenslet array.
An aperture array may be located in front of said array of polarising beam splitter elements.
Said aperture array may comprise a plurality of elongate apertures arranged parallel to each other.
The liquid crystal projector may further comprise three separate liquid crystal panels for modulating red, green and blue light respectively.
The liquid crystal projector may further comprise a projection lens arranged to project a television image onto a wall or screen.
The overall shapes of the first and second lenslet arrays may be different, for example the shapes may be generally square and generally rectangular respectively.


REFERENCES:
patent: 5418583 (1995-05-01), Masumoto
patent: 5662401 (1997-09-01), Shimizu et al.
patent: 6000802 (1999-12-01), Hashizume et al.
patent: 6024451 (2000-02-01), DeVaan et al.
patent: 6062695 (2000-05-01), Kakuda et al.
patent: 6084714 (2000-07-01), Ushiyama et al.
patent: 0812115 (1997-12-01), None

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