Liquid crystal cells – elements and systems – Liquid crystal system – Projector including liquid crystal cell
Reexamination Certificate
1999-09-09
2003-10-21
Kim, Robert H. (Department: 2871)
Liquid crystal cells, elements and systems
Liquid crystal system
Projector including liquid crystal cell
C349S005000, C349S009000, C353S033000, C353S034000
Reexamination Certificate
active
06636276
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to a high-performance projection display system using at least two reflective light valves and a light splitting and combining device, typically a polarizing beam splitter, and more particularly to a projection display system which uses a wavelength-selective retarder device to project high contrast images from reflective light valves to and from which lights go through two faces of the light splitting and combining device.
2. Discussion of Related Art
Reflective liquid crystal on silicon (LCOS) imagers are expected to become the lowest cost light valve (LV) technology for such applications as high definition television (HDTV), advanced television (ADTV), and monitors of medium-to-large diagonal about 28 inches or more. Though mature LCOS technology is expected to be cheaper than competing digital micro mirror device (DMD) and polysilicon technologies, its adoption is hampered at present by limitations which its reflective mode of operation imposes on projection optics.
These limitations may be understood as follows: A light which is unchanged in direction or polarization upon reflection from a reflective LV will necessarily retrace its path back to the light source due to the intrinsic reversibility of efficient passive optical elements. A light that remains in the illumination state is thus unable to contribute to the image. Each reflective light valve must therefore be illuminated in a dark-state light, and space must be provided above the light valve to separate the light which the LV converts to a bright state from the dark state. With LCOS light valves this separation is most commonly accomplished by a polarizing beam splitter (PBS), as shown in FIG.
1
. An illumination light supplied by a light source
10
enters one face (i.e. one port or channel)
12
a
of the PBS
12
and is redirected by a polarizer-coated PBS hypotenuse
14
(for example in reflection as shown in
FIG. 1
) to a second channel
12
b
of the PBS, where it illuminates one or more reflective light valves
16
. The bright-state light returned from these light valves is then passed by the hypotenuse coating
14
(for example in transmission) to exit the PBS through a third face
12
c
. A dark-state returned light is reflected by the hypotenuse coating
14
and returns to the source
16
.
One problem with the PBS arrangement shown in
FIG. 1
is that it forces separation of the light valve from a projection lens, so that a more complex lens is required for high-quality imaging. It is therefore desirable to minimize the space impact of the PBS. Moreover, when the projector employs multiple light valves, it is desirable to deploy the associated PBS's as efficiently as possible. A particular inefficiency of the arrangement shown in
FIG. 1
is that only three of the four available PBS faces or ports are used. This in turn increases the complexity of a system that employs the subsystem of
FIG. 1
to project color images from multiple light valves. For example,
FIG. 2
shows a prior art arrangement in which three PBS's
20
,
21
,
22
associated with three Lvs
23
,
24
,
25
(one for each of the red [R], green [G], and blue [B] colors) are integrated into a projection display system using a number of color separating and combining dichroics
26
,
27
,
28
. Each LV has a dedicated PBS, and the layout becomes fairly complex because the separating and combining dichroics
26
,
27
,
28
must essentially wrap the light around these PBS's
20
,
21
,
22
. In practice, additional lenses (not shown) must be added to accommodate the differing lengths of the various paths taken by the light.
A different example is shown in
FIG. 3
, comprising a prior art optical system in which R, G, and B lights are alternately delivered to a single light valve
30
by using a light wheel
32
positioned between a light source
34
and a PBS
36
. Though the system of
FIG. 3
is simple optically, the light valve
30
is not used in an efficient way because only one color is projected at a given time. Alternative versions of the
FIG. 3
system use, for example, a telecentric pair of micro-lens arrays, or scanning illumination optics, to simultaneously deliver the R, G, and B illumination bands to spatially separated sub-pixels in the light valve. Here the inefficiency manifests itself in the constraint that only one-third of light valve area can be made available for each color.
U.S. Pat. No. 5,517,340 (hereinafter the '340 patent), issued on May 14, 1996 to Doany et al. and hereby incorporated by reference, discloses a more efficient use of the PBS than the arrangement of FIG.
3
. As shown in
FIG. 4
, which is a reproduction of
FIG. 6
of the '340 patent, one can make use of two channels of a PBS to illuminate light valves. The embodiment shown in
FIG. 4
uses a “squirrel-cage” configuration for the color wheel, rather than the disk shown in FIG.
3
. In the arrangement of
FIG. 4
, a green light is always introduced in P polarization state. In other words, for green wavelengths the incident electric field is always polarized within the plane of the figure. The green light is thus directed through the PBS hypotenuse to illuminate the light valve at the right face of the PBS which is dedicated to green image information. Red and blue lights are always introduced in S polarization state. That is, red and blue lights are polarized perpendicular to the plane of incidence. More precisely the S polarized component of the illumination light is alternately switched in color between red and blue. Red and blue lights are therefore reflected by the PBS coating to the light valve at the top face of the PBS, which is alternately loaded with red and blue image information. Or, the red/blue illumination and the loading of red/blue image information can be scrolled along the light valve in a synchronous fashion.
The goal of the system shown in
FIG. 4
is to encode one set of colors or wavelengths of the illumination light in P polarization, in order that the PBS hypotenuse coating illuminates one LV in transmission with these colors, while encoding a second set of colors or wavelengths in S polarization, in order to illuminate a second LV in reflection. This assumes a PBS of the conventional kind where the hypotenuse coating reflects S polarization and transmits P.
However, a problem with the system of
FIG. 4
is that when illuminating light is returned from the green light valve in dark state, the hypotenuse coating of the PBS will inevitably reflect several percent of the dark state light out the bottom exit face of the PBS and onward to a projection screen. This is because a practical hypotenuse coating of the PBS is not capable of entirely transmitting back through to the source the green light that remains in the dark state upon reflection from the green LV. Available hypotenuse coatings will instead have a residual reflectivity in P polarization of at least a few percent averaged over practical angular ranges. Thus, the image will contain non-negligible residual green intensity even when the green light valve is in the black state, and contrast of the image will be degraded accordingly. The PBS also directs unwanted P-polarized green light onto the red/blue LV, further contributing to the residual dark-state intensity. One might contemplate putting a cleanup polarizer in the bottom output face to reduce the residual dark-state P polarization while passing bright-state S polarization, but unfortunately these polarization assignments are reversed for the red and blue lights. That is, for red and blue colors or wavelengths, P polarization is the bright state and thus cannot be reduced. A PBS operating in transmission can usually by itself adequately reduce the residual S-polarized light without a cleanup polarizer. Thus in the layout of
FIG. 4
high contrast can be provided in red and blue if the input red and blue lights are purely S polarized, but the problem of low green contra
Kim Robert H.
Schechter Andrew
Scully Scott Murphy & Presser
Trepp, Esq. Robert M.
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