Optical: systems and elements – Single channel simultaneously to or from plural channels – By partial reflection at beam splitting or combining surface
Reexamination Certificate
2000-01-18
2001-07-17
Epps, Georgia (Department: 2873)
Optical: systems and elements
Single channel simultaneously to or from plural channels
By partial reflection at beam splitting or combining surface
C359S636000, C359S640000, C353S020000, C353S031000, C353S033000, C353S034000, C349S005000, C349S008000, C349S009000
Reexamination Certificate
active
06262851
ABSTRACT:
BACKGROUND OF THE INVENTION
To date, a variety of optical projection systems have been proposed. Each of these display systems typically includes (1) an input polarizer, (2) one or more spatial light modulators, and (3) one or more output analyzers. An input polarizer linearly polarizes unpolarized light. One type of input polarizer is a polarizing beam splitter (“PBS”), which polarizes unpolarized light by splitting it into transmitted P-polarized light and reflected S-polarized light. P-polarized light is light that is parallel to the plane of incidence (which is defined by the incident and reflected rays), while S-polarized light is light that is perpendicular to the plane of incidence.
A spatial light modulator (SLM) receives the light that an input polarizer linearly polarizes. An SLM often includes an array of picture elements (also called pixels) that the SLM individually controls to modulate the light passing through the pixels. An SLM is typically formed by positioning a layer of liquid crystal material between two electrodes. One of the electrodes is segmented into an array of pixel electrodes to define the pixels of the SLM, while the other electrode is usually not segmented.
There are two varieties of SLM's: reflective and transmissive. In both varieties, the direction of an electric field applied between each pixel electrode and the other electrode determines whether the corresponding pixel changes the polarization of light falling on the pixel. Hence, in both varieties, the incident light is modulated by changing the polarization of light falling on certain pixels while leaving unchanged the polarization of the light falling on other pixels.
An output analyzer receives the light transmitted or reflected by an SLM. Output analyzers are polarization-selective devices similar to the input polarizers. Polarizing filters and PBS's are two types of output analyzers. An output analyzer allows a certain polarization state of the light to pass, while discarding the remaining polarization states. Hence, output analyzers are placed at the outputs of SLM's to obtain the pattern of modulation of the SLM's, and thereby generate images. An observer will not perceive an image unless an analyzer follows an SLM, because the SLM does not attenuate the incident beam of light, but rather simply modulates the lights polarization state.
Projection displays generate color images by modulating, analyzing, and combining component color bands. Display devices typically use a few component colors (such as the primary additive colors, red, green or blue) to generate a multitude of colors for display. A component color band is a portion of the light spectrum corresponding to a component color. When all the component color bands are added, they produce white light. Conversely, component color bands can be extracted from white light.
To generate color images, projection displays not only use input polarizers, SLM's, and output analyzers, but they also use other devices. For instance, color projection systems often either use (1) a light source for each component color band (e.g., three light sources for the three primary additive colors, red, green, and blue), or (2) a single source of white light with a prism or other color separation device that separates incident white light into component color bands (e.g., into red, green, and blue light).
The component color bands are then used to illuminate one or more SLM's, which modulate the incident light for each color band. The modulated color bands are then recombined to produce a full-color image. The recombination may take place sequentially or simultaneously.
I. Color-Field Sequential Display Systems.
Color-field sequential systems create an image by sequentially projecting red (“R”), green (“G”), and blue (“B”) images.
FIG. 1
presents one prior art color-field sequential system. This display system
100
uses a mechanical color filter wheel
105
positioned between a light source
110
and a light valve
115
(which includes an SLM and an analyzer).
As shown in
FIG. 2
, the filter wheel
105
is divided into three filter sections, each acting as a pass filter for one of the three primary additive colors. By rotating the filter wheel, successive red, green, and blue light are generated to illuminate the light valve. The light valve is then modulated to generate successive red, green, and blue images. The eye-brain system fuses the successively-projected color images into a single blended polychromatic image, if the eye is stationary and the successive color patterns are projected at a high rate.
The eye, however, is not always stationary and often moves, and this movement can cause the viewer to see artifacts, called color sequential artifacts (“CSA”). For instance, the viewer might see spurious images (such as flashes of red, green, or blue light). CSAs are not only annoying, but also present safety concerns (e.g., they may cause epileptic attacks).
Increasing the projection rate of the images can minimize color sequential artifacts. However, at high rates, the mechanical color filter
105
does not operate reliably and introduces noise and vibration into the system. Electronic color switches can be used in place of the mechanical filter
105
, but the electronic switches require complicated processing and driving circuitry, and are somewhat inefficient at their high switching rates. Finally, sequential system
100
does not generate good color contrast because its light valve
115
cannot be cost-effectively designed to operate perfectly for each of the three generated color bands.
II. Simultaneous Projection Display Systems.
Simultaneous projection display systems create a color image by optically superimposing multiple partial-color images to the same location. In addition to using light sources, input polarizers, color-separating devices, SLM's, and output analyzers, simultaneous projection systems also use color-recombining devices (such as dichroic prisms) to recombine each of the component color images in a coordinated way.
Simultaneous projection systems may be divided into single-pass and double-pass systems. Double-pass systems use the same device for both the color separation and recombination operations, while single-pass systems use different devices for these operations.
A. Single-Pass Systems.
FIG. 3
presents one prior art single pass system. The light from the light source is separated into three color bands using dichroic filters. A separate light valve (formed by an SLM and an output analyzer) modulates each color band. The modulated color bands are then recombined using dichroic filters.
There are several disadvantages to this architecture. For example, this system is somewhat bulky and relatively expensive since it uses many components. Also, its projection lens is complex and costly since it needs a projection lens with large back-focal length due to the relatively large distance from the panel to the lens. The dichroic filters used for the recombination operations also introduce aberrations and distortion in the generated images.
FIG. 4
presents another prior art single-pass system. This system receives R, G, and B light either from three sources of light (as shown in FIG.
4
), or from a color-separator (not shown) that separates these different color bands from white light. System
400
also utilizes three PBS's
420
,
425
, and
430
. These PBS's serve as input and output polarizers. Specifically, the PBS's initially receive unpolarized light from light sources
405
,
410
, and
415
. They transmit the P-polarized light out of the system, while reflecting the S-polarized light towards the SLM's
435
,
440
, and
445
.
The SLM's then modulate and reflect the received light back to the PBS. On the second pass through, the PBS's serve as output polarizers (i.e., output analyzers). The analyzers (1) reflect and thereby reject the S-polarized light (corresponding to the light having a polarization that the SLM's did not change), and (2) transmit
Epps Georgia
Hewlett-Packard Co.
Spector David N.
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