Optics: image projectors – Polarizer or interference filter
Reexamination Certificate
1998-09-14
2001-08-14
Dowling, William (Department: 2851)
Optics: image projectors
Polarizer or interference filter
C353S031000, C353S033000, C349S009000
Reexamination Certificate
active
06273567
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to color LCD projectors and, in particular, to such a projector that provides high brightness in a compact form factor.
BACKGROUND AND SUMMARY OF THE INVENTION
Color liquid crystal display projectors generate display images and project them onto display screens, typically for viewing by multiple persons or viewers. The display images may be formed by transmitting light from a high-intensity source of polychromatic or white light through or reflected from an image-forming medium such as a liquid crystal display (LCD).
FIG. 1
is a schematic diagram of a prior art multi-path reflective color liquid crystal display projection system
10
that utilizes color separating mirrors
12
R,
12
BG, and
12
G in combination with polarization selective polarizing beam splitters
14
R,
14
G, and
14
B and reflective liquid crystal displays
16
R,
16
G, and
16
B to provide a high resolution, high brightness display.
Projection system
10
includes a light source
18
that directs white light through a polarizer (or polarization converter)
20
that provides polarized light to a pair of crossed dichroic mirrors
12
R and
12
BG. Dichroic mirror
12
R reflects red light components along a red optical path
22
R that is folded by an achromatic fold mirror
24
R. Green and blue light passes through mirror
12
R. Mirror
12
BG reflects blue and green light components along a blue-green optical path
22
BG that is folded by an achromatic fold mirror
24
BG. Red light passes through mirror
12
BG. Mirror
12
G reflects green light components along a green optical path
22
G and allows the blue light components to propagate along a blue optical path
22
B. As a result, mirrors
12
R,
12
BG, and
12
G cooperate to separate polarized red, green and blue light components and deliver them to polarizing beam splitters
14
R,
14
G, and
14
B.
Each polarizing beam splitter
14
includes a pair of right-angle prisms having their inclined faces positioned against each other with a polarization selective dielectric film (not shown) positioned therebetween. As is conventional for polarizing beam splitters, P-polarized light passes through the dielectric film and S-polarized light is reflected. S- and P-polarizations are conventional nomenclature referring to a pair of orthogonal linear polarization states in which, with regard to a polarization selective dielectric film, S-polarized light can be said to “glance” off the film and P-polarized light can be said to “pierce” the film. Polarizer
20
transmits the red, green and blue light components as predominantly S-polarized light, so nearly all the light received by polarizing beam splitters
14
R,
14
G, and
14
B is reflected by the dielectric films to reflective liquid crystal displays
16
R,
16
G, and
16
B.
In one implementation, reflective liquid crystal displays
16
are quarter wave-tuned (i.e., with 45°-60° twists) twisted nematic cells and reflect light from each pixel with a polarization that varies according to the control voltage applied to the pixel. For example, when no control voltage is applied (i.e., the pixel is in its relaxed state), the pixel imparts maximum (i.e., a quarter wave) phase retardation that results in a polarization rotation for suitably aligned polarized light. Each pixel imparts decreasing polarization rotation with increasing control voltage magnitudes until the pixel imparts no rotation (i.e., the pixel is isotropic).
In the relaxed state of a pixel, the polarization state is reversed when the light is reflected, so that the S-polarized light becomes P-polarized light. The P-polarized light then passes through the dielectric film of the polarizing beam splitter toward a crossed-combining prism
26
(also known as an X-cube) to be incorporated into the display image. With non-zero control voltages, the pixel reflects the light with corresponding proportions of P- and S-polarizations. Control voltages of greater magnitudes in this example cause greater portions of the light to be reflected with S-polarization, with all the reflected light having S-polarization at the greatest control voltage. The portion of the light with S-polarization is reflected by the dielectric films in polarizing beam splitters
14
back toward the illumination source and are not incorporated into the display image.
Such a multi-path reflective color liquid crystal display projection system
10
can provide improved imaging characteristics over more conventional projections systems that use transmissive liquid crystal displays. Reflective liquid crystal displays do not suffer from the low transmissivity characteristic of transmissive displays, and hence the relatively low brightness of their projection systems. Moreover, the reflective liquid crystal displays are relatively easier to fabricate and miniaturize than conventional transmissive liquid crystal displays, which can allow lower production costs and smaller, more portable projection systems.
While it may have advantages over conventional transmissive projection systems, such a multi-path reflective color liquid crystal display projection system
10
suffers from disadvantages that impair its imaging characteristics. One of crossed mirrors
12
R and
12
BG is actually formed with two mirror halves that are positioned behind and in front of the other of mirrors
12
R and
12
BG. Proper alignment of the mirror halves is very difficult and rarely achieved. As a consequence, the images reflected by the mirror halves are mis-aligned, which can result in readily discernible mis-alignments in the image halves. The relatively common misalignment between the mirror halves introduces, therefore, generally unacceptable image errors that may appear as ce-coupled image halves that are improperly joined along an apparent seam.
One implementation of a multi-path reflective color liquid crystal display projection system according to the present invention utilizes two color separating mirrors in combination with two polarizing beam splitters and reflective liquid crystal displays to provide a high resolution, high brightness display. The projection system includes a light source that directs white light through a polarization converter that provides S-polarized light to a first angled dichroic mirror. In one implementation, the dichroic mirror reflects two color components (e.g., green and one of the red and blue components) and passes one color component (e.g., the other of the red and blue components). The dichroic mirror provides a one-to-two color separation in which the green light component is reflected with one other light component.
A quarter wave plate and an achromatic mirror are positioned behind and parallel to the dichroic mirror and cooperate to convert the light that passes through the dichroic mirror (e.g., red light) from S-polarization to P-polarization. The P-polarized red light then passes through the dichroic mirror along the same optical path as the S-polarized green and blue color components.
A second angled dichroic mirror directs a selected one of the red blue components (e.g., blue) to a polarizing beam splitter that includes a pair of right-angle prisms having their respective inclined faces positioned against each other with a dielectric film therebetween. The dielectric film in the polarizing beam splitter is polarization-selective and may be achromatic or color-tuned. With a color-tuned dielectric film, the polarizing beam splitter transmits all color components of light other than the selected component (e.g., blue), regardless of polarization, while functioning as a conventional polarizing beam splitter for the selected color (e.g., blue light). Accordingly, the polarizing beam splitter reflects S-polarized blue light toward a reflective liquid crystal display, and any P-polarized blue light passes out of the polarizing beam splitter with the non-blue light (i.e. red or green light).
The remaining color components (e.g., red and green) pass through the second angled dichroic mirror to a second polarizing beam splitter havi
Booth David K.
Conner Arlie R.
Dowling William
Ipsolon LLP
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