Liquid crystal cells – elements and systems – Liquid crystal system – Projector including liquid crystal cell
Reexamination Certificate
2000-07-21
2003-03-11
Ton, Toan (Department: 2871)
Liquid crystal cells, elements and systems
Liquid crystal system
Projector including liquid crystal cell
C349S009000, C353S031000
Reexamination Certificate
active
06532044
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to color electronic (e.g., LCD) projectors and, in particular, to such a projector that includes equal-length color component paths and a separate projection lens assembly for each.
BACKGROUND AND SUMMARY OF THE INVENTION
Color electronic (e.g., 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.
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. The color component images are combined by an X-cube
26
and directed to a projection lens assembly
28
.
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
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 misaligned, which can result in readily discernible misalignments in the image halves. The relatively common misalignment between the mirror halves introduces, therefore, generally unacceptable image errors that may appear as de-coupled image halves that are improperly joined along an apparent seam.
Similarly, X-cube combiner
26
suffers from manufacturing limitations, such as an inability to perfectly form and join its components. In particular, such imperfections can arise at a central intersection region
29
where the X-cube components meet. Such imperfections are significant because they affect the central, most discernible region of an image.
FIG. 2
shows another prior light valve image projection system
30
as described in U.S. Pat. No. 5,327,270 of Miyatake. Projection system
30
includes three reflective liquid crystal panels
32
A,
32
B, and
32
C that have corresponding polarizing beam splitters
34
A,
34
B,
34
C, quarter wave plates
36
A,
36
B,
36
C, and projection lenses
40
A,
40
B,
40
C, respectively. Dichroic mirrors
42
A,
42
B,
42
C color separate the light from a light source
44
.
Color separation by successive dichroic mirrors
42
A,
42
B,
42
C eliminates image errors and artifacts that can be introduced by crossed mirrors
12
R,
12
BG in projection system
10
. Also, separate projection lenses
40
A,
40
B,
40
C eliminate the image errors and artifacts that can be introduced by X-cube
26
. To achieve such results, however, projection system
30
employs an in-line arrangement that is bulky and creates optical paths of different lengths for the different color components. The in-line arrangement of projection lenses
40
A,
40
B,
40
C creates relatively large separations between them, thereby imposing relatively large convergence angles that can introduce color component misalignments at the display screen.
Moreover, different path lengths are disadvantageous because the differences causes different magnifications of the ‘illumination pattern’ onto each of the three color channels. When different color channels receive illumination patterns of different magnifications, (e.g., if R illumination is bigger than G and B illumination) the intensity uniformity profiles will be different, and it will be difficult to achieve a uniform white field by superposition.
In accordance with the present invention, an electronic (e.g., LCD) projector combines multiple projection lens assemblies with equal color component optical path lengths to provide improved display images and a compact arrangement. In one implementation, the projector includes a successive pair of angled dichroic mirrors that fold the red and blue color components of light in opposed directions. The green color component of light passes through the dichroic mirrors toward a pixelated electronic light modulator, such as a liquid crystal display, and an associated projection lens assembly. The red and blue color components of light are each folded again to propagate parallel with the green color component toward a pixelated electronic light modulator, such as a liquid crystal display, and an associated projection
Cannon Bruce L.
Conner Arlie R.
Corning Precision Lens Incorporated
Ipsolon LLP
Ton Toan
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