Solid state based illumination source for a projection display

Details Solid state based illumination source for a projection display Solid state based illumination source for a projection display

1999-01-29

2002-04-02

Metjahic, Safet (Department: 2858)

Optics: image projectors

Composite projected image

Reexamination Certificate

active

06364487

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to light sources, and more particularly, to a solid state light source for use in projectors and the like.
BACKGROUND OF THE INVENTION
Systems for projecting images onto screens are known in the art. Systems based on incandescent or arc lamps are often used in projection displays. However, such sources have limited lifetimes and poor output stability over the life of the light source. In addition, spectral components, such as UV and IR, that are not needed for display purposes must be filtered out of the emissions to avoid damage to other components or the viewer's eyes.
Sources based on combinations of Light Emitting Diodes (LEDs) or semiconductor lasers that emit red, green and blue light can be used as a solid state light source. Since LED and laser emissions are typically spectrally narrow band light sources, the use of color filters for the display can be omitted. Also the optical output of semiconductor lasers and LEDs can typically be electrically modulated at a rate that is sufficient for a time sequential color display.
Unfortunately, individual solid state light sources do not provide sufficient brightness for many projection display applications; hence, arrays of LEDs must be utilized to obtain sufficient output. A significant fraction of the light generated in an LED array is lost. One common type of projection display utilizes a liquid crystal-based spatial light modulator. Such modulators require the use of linearly polarized light. Since the emission of conventional light sources is unpolarized, the light source includes a polarizing filter that transmits light in the desired linear polarization state and absorbs the perpendicular state. As a result, 50% of the radiance of the light from the source is lost.
Broadly, it is the object of the present invention to provide an improved solid state light source and projection display.
It is a further object of the present invention to provide a light source that provides a white light output utilizing LEDs of different radiances and colors.
It is a still further object of the present invention to provide a polarized light source with an optical output power larger than 50% of the output power of the original unpolarized light source.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.
SUMMARY OF THE INVENTION
The present invention is an illumination source for use in projectors and the like. The illumination source includes a light source that generates a two-dimensional emission pattern having a light intensity that varies as a function of position in the emission pattern. A collector collects the light from the light source and illuminates an exit aperture therewith. The illuminated exit aperture has a two-dimensional emission pattern with a light intensity that varies as a function of position in a manner that is more uniform as a function of position than the emission pattern of the light source. An imaging optical element images the exit aperture onto a surface. The collector is preferably a compound parabolic concentrator or a compound elliptical concentrator. In one embodiment of the invention, a partially reflecting film is placed between the exit aperture and the imaging optical element. The partially reflecting film reflects light of a first polarization state back into the collector and transmits light of the orthogonal polarization state. In another embodiment of the invention, a quarter wave plate is introduced between the exit aperture and the partially reflecting film to further increase the radiance of the source.


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“Monolithic GaN Light Emitting Diode Arrays for Display Illumination”, IBM Technical Disclosure Bulletin, vol. 39, No. 9, Sep. 1996, pp. 227-229.

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