Optics: image projectors – Miscellaneous
Reexamination Certificate
1999-10-01
2001-08-14
Dowling, William (Department: 2851)
Optics: image projectors
Miscellaneous
C353S084000, C349S005000, C348S742000
Reexamination Certificate
active
06273571
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to display system architectures using a device for selectively transforming partially polarized light, more particularly to a retarder stack for transforming at least partially polarized light for input into various optical devices such as electro-optic modulators, magneto-optic modulators or other optical components.
2. Background of the Related Art
Display systems create color in a number of different ways. In a typical cathode ray tube system, the colors are a result of phosphors being excited by an electron gun, causing them to become luminous. The choice of material from which the phosphor is made helps determine which color is viewed when the phosphor is activated. Additionally, they can have more than one gun, where the nature of the electron beam is changed to form a certain color.
Systems that do not use cathode ray tubes, such as spatial light modulator based systems, use other methods. Spatial light modulator based systems include liquid crystal device (LCD) systems, digital micromirror device (DMD) systems, and grating light valve systems, among others. These systems typically project an image from an array of individual elements, each corresponding to a picture element (pixel) dot on the displayed image.
One method in which they create color images is by having one array for each color, typically red, green and blue. The red image is created by activating the appropriate elements on the array to project an image of red (ON elements) and dark or black spots (OFF elements), at the display surface, while doing the same for green and blue. The three images are projected and converged into one image on the display surface, creating the colored image. The mix of red, green and blue to achieve the right colors is achieved by controlling the amount of time each color is ON for each pixel.
Another method is to use one array and have some type of filtering of the source light from which the image is created. This has the advantage over the three device system in that it requires fewer components and is therefore cheaper and can be made more compact. The typical method illuminates the array in a color sequence, relying upon the human eye to integrate the colors into the proper mix, as in the three device system. Each image for each color is projected to the display surface in the same place as the other colors' images, allowing for the same integration effects as in the three device systems.
However, the color filter can be problematic. A typical solution is to use a color wheel divided into thirds, one for each color, and to place it between the light source and the array. As the color wheel spins, each color illuminates the array while the data for that color is being displayed.
This sequence is fixed and cannot be changed easily if events such as channel changes occur, requiring extra processing and adjustments to allow the system to continue to function. The transitions between the colors in sequence require the light to be turned off while the color changes. The amount of time the light is off depends upon the size of the wheel and the rate at which it rotates. This time can even be longer than the amount of time it takes to load data into the array, reducing the system efficiency.
Additionally, color wheels require motors and a controller to operate and with their own bulk contribute to the size and weight of the display system, as well as the cost. Therefore, a solution is needed that provides color sequential systems with a means for filtering light that is smaller, lighter and faster than the present solutions.
The manipulation or transformation of polarized or partially polarized light is essential in a wide variety of optical systems. Especially with the onset of integrated optics and the processing of optical signals, it is necessary to predictably manipulate the polarization of light before that light proceeds to the next stage in an optical system.
Many optical systems require input light be completely or nearly completely polarized and have a known bandwidth and polarization such as input light from lasers or light emitting diodes (LEDs). It is important, however, to be able to selectively transform portions of relatively wide bandwidth light for eventual use in optical systems. For example, it is desirable to be able to selectively shift a certain band of frequencies within a wide band of frequencies comprising white light.
Color display and color filters are examples of optical systems which utilize wide bandwidth light such as white light to function.
Color Displays
Color display is generally provided by spatial or temporal multiplexing of the additive primary colors, red, green and blue. In a spatial multiplexed display, each color pixel is divided into three subpixels, one for each primary color. Ideally the pixels are small enough compared to the viewing distance from the eye that the colors are spatially integrated into a single full-color image. As a result of subdividing each pixel, the spatial resolution of the display is reduced by a factor of at least three. In temporal multiplexing, colors are sequentially switched between the three primary colors, and if the switching rate is fast enough the eye temporally integrates the three images to form a single full-color image. In both cases, the color filter is typically combined in series with a binary or display capable of generating a gray scale which is spatially aligned and temporally synchronized with the color filter to modulate the intensity of each color. To display white with spatial multiplexing, all three subpixels simultaneously transmit a primary; with temporal multiplexing the three primaries are sequentially transmitted. In either case, at best only one third of the input intensity can be displayed.
In subtractive display, color is produced by stacking three monochrome displays (for example Plummer, U.S. Pat. No. 4,416,514 and Conner et al., U.S. Pat. No. 5,124,818). Polarization components are placed between each display panel, such that each panel ideally independently controls the transmission of an additive primary color. Subtractive displays have the advantage that every pixel is a three-color pixel and that the display does not, in principle, suffer the throughput loss associated with spatial or temporal multiplexing. However, previous implementations generally could not completely independently modulate each color. Additionally, they utilized pleochroic dye polarizers as the only color selective polarization components between each display panel. Due to the poor performance of pleochroic dye polarizers, including poor color contrast, high insertion loss and shallow transition slopes, the benefits of subtractive displays have not before been realized.
Color Filters
There are two basic classes of liquid crystal color switching filters: polarization interferences filters (PIFs) and switched-polarizer-filters (SPFs). The basic unit of an SPF is a stage, consisting of a color polarizer and a two-state neutral polarization switch. This class is intrinsically binary tunable, such that each filter stage permits switching between two colors. Stages are cascaded in order to provide additional output colors. Color polarizers used in SPFs include single retardation films on neutral linear polarizers and pleochroic color polarizing filters. The polarization switch can be a liquid crystal (LC) polarization switch preceding a static polarization analyzer. The switch optimally provides neutral polarization switching. The chromatic nature of the active element degrades performance and is ideally suppressed in an SPF.
Shutters based on color polarizer consisting of a neutral-polarizer followed by a single retarder are well reported in the art (for example in U.S. Pat. No. 4,002,081 to Hilsum, U.S. Pat. No. 4,091,808 to Scheffer and U.S. Pat. No. 4,232,948 to Shanks). While the polarizer/retarder structure can be described as a complementary color polarizer in the sense that it is possible to produce two distinct hues b
Johnson Kristina M.
Sharp Gary D.
ColorLink, Inc.
Dowling William
Fleshner & Kim LLP
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