Liquid crystal cells – elements and systems – Particular structure – Interconnection of plural cells in series
Reexamination Certificate
1998-10-02
2004-03-16
Parker, Kenneth (Department: 2712)
Liquid crystal cells, elements and systems
Particular structure
Interconnection of plural cells in series
C349S005000
Reexamination Certificate
active
06707516
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to color display systems. More specifically, the present invention relates to a single-panel field-sequential color display system for generating high resolution, full color images.
2. Background of the Related Art
There is a need for low cost, high resolution color display systems for use in large screen, high-definition television sets, computer monitors, data projectors and other commercial, industrial, training and entertainment display products.
Full color display is generally implemented using one of five techniques: (1) spatially using color filter arrays; (2) temporarily using sequential color techniques; (3) additively using multiple optical paths; (4) subtractively using stacked display panels; or (5) additively using stacked display panels.
In spatial color systems, each full-color pixel in a full-color display is subdivided into at least three pixels, with one pixel dedicated to each additive primary color. In a cathode ray tube display system, the sub-pixels are implemented with phosphors that are excited by an electron gun, causing them to become luminous. A color filter array consisting of red, green and blue spectral filters is registered to the phosphor pixel array. Similarly, in spatial color display systems that utilize a spatial light modulator, a color filter array is registered to the active pixel elements of the spatial light modulator, such that the transmission or reflection level of each primary color can be locally controlled.
One problem with spatial color systems is that the sub-pixels must be sufficiently small so that they are not individually resolvable by the viewer. The resulting spatial integration of the sub-pixels by the eye yields a perceived full-color image. In addition, because each full-color pixel must be subdivided into three sub-pixels, the spatial light modulators used in spatial color systems require three times the number of pixels than those used in monochrome displays.
In additive split-path color display systems, the three additive primary colors (red, green and blue) are displayed using three separate panels (channels) e.g., three spatial light modulators. The three panels project three different color representations of the same image simultaneously such that the three separate images overlap at an image plane. The three color images “add” up to give an accurate full color representation of the image. The main problem with this approach is cost, size and weight. Three separate image sources are required, each with its own set of associated optics. The higher the number of panels, the larger the system. In addition, a complicated combination optics is usually required with this approach.
In a stacked display system, three optical paths are effectively created without wavefront shearing. There are two types of stacked display systems: (1) additive, where each display panel contributes red, green and blue light; and (2) subtractive, where pixels in each display panel subtract red, green or blue light. The term subtractive is appropriate because such systems are analogous to color film. Although all of the light travels along the same physical path, only specific layers of the structure manipulate light in each wavelength band. In practice, a full-color display consists of a stack of three co-registered transmissive display panels, e.g., spatial light modulators, each responsible for independently determining the local transmission of one additive primary color.
Because there is only one physical path, each stage must be made independent of the other using wavelength selective effects. Luminance modulation with a liquid crystal display requires both a polarized input and an effective voltage-controlled analyzing polarizer. Thus, color independent luminance modulation is typically achieved by wavelength selectively controlling the degree of input polarization, and/or the wavelength selectivity of the analyzer.
Compared to split-path display systems, stacked display systems have unique design challenges. In order to obtain high optical throughput, the optical transmission losses of the display panels must be low, the transmission losses of any passive color control elements must be low, and images must be efficiently relayed between display panels. In stacked direct view display systems, there are additional complications associated with color quality and parallax when the display is viewed off-normal.
In field-sequential color display systems, sub-frames are displayed, with each sub-frame comprising the distribution of an additive primary color in a full-color image. In single-panel field-sequential color display systems, a single image source or panel is used. The three additive primary color images are displayed in three separate sub-frames sequentially during one display frame. Display frame rates are typically 60 Hz ({fraction (1/60)} of a second per frame). The three additive primary color images are displayed in sequence at a rate that is three times the frame rate (typically ≧180 Hz) or higher so that all three additive primary color images are displayed over the course of one display frame. The eye integrates the sub-frames temporally, yielding a perceived full-color image. This technique is preferable over additive or subtractive three-panel systems in terms of cost and complexity because only one display panel is used.
The main disadvantage of field-sequential color display systems is reduced light output (luminance). This is due to the fact that each separate color image is displayed for only one-third of a frame as compared to a full frame in an additive or subtractive three-panel system. In addition, since the intensity distribution of the image will change according to which color is being displayed, the image source, e.g., spatial light modulator, must be able to respond or switch in {fraction (1/180)} of a second or less as opposed to {fraction (1/60)} of a second in an additive system (all three color image sources remain static for one full frame in a additive split-path system).
In a single-panel field-sequential color display system, the spatial light modulator must be sequentially illuminated with red, green and blue light in synchronism with the driving of the spatial light modulator with red, green and blue image information. This is typically accomplished by sequentially filtering a broad band (white) light source with a color filter, for high brightness applications, or a set of three lasers or three LEDs that can be individually modulated at ≧180 Hz.
A color wheel is commonly used as the color filter in single-panel field-sequential color display systems employing a lamp. The color wheel may be divided into thirds, with one-third passing red light, one-third passing green light, and one-third passing blue light. The color wheel is positioned between the light source and the spatially light modulator, and is rotated so that each primary color illuminates the spatial light modulator while the spatial light modulator is driven with the image data for that color.
One disadvantage of using a color wheel is that the color display sequence is fixed and cannot be changed without changing the color wheel. In addition, as the color wheel rotates from one color filter to the next, the spatial light modulator must be blanked for an amount of time that depends on the size of the illumination spot on the color wheel and its rotation rate to avoid color mixing. This blanking time can be longer than the amount of time it takes to load image data into the spatial light modulator, which reduces the display system brightness and hence the optical efficiency. In addition, color wheels require motors, controllers and fans to operate, and contribute to the size, weight, cost, and power consumption of the display system.
Spatial light modulators (SLMs) that can be used with the above-described display systems include transmissive and reflective liquid crystal devices (LCDs) using amorphous silicon thin film transistors and single crystal
Johnson Kristina M.
Sharp Gary D.
Colorlink, Inc.
Fleshner & Kim LLP
Parker Kenneth
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