Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
2000-04-10
2003-10-28
Mengistu, Amare (Department: 2673)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S102000, C345S108000, C345S109000, C359S227000, C359S233000, C359S234000
Reexamination Certificate
active
06639572
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to direct view displays and more specifically to a paper white display having an array of hinged micromirrors adapted to switch between two states to alternately cover and uncover a contrasting background.
2. Description of the Related Art
Direct-view displays produce images that can be viewed directly without the aid of magnification or projection. The market for direct view displays spans a continuum of performance and price that includes the ultra high performance but very expensive flat-panel DTVs, moderately performing and priced laptop computers, and the lower performing but much cheaper personal digital assistants, electronic books and cellular telephones. The high-end displays offer high spatial and color resolution but are very expensive and consume a lot of power. The low-end displays offer less resolution but are relatively cheap and can be operated from battery power.
This low-end market is currently dominated by the multiplexed liquid crystal display (LCD) technology. Multiplexed LCDs sacrifice grey scale performance in favor of fabrication simplicity and power consumption by eliminating the thin film transistor (TFT) array used in Active Matrix LCDs (AMLCD), which dominate the laptop computer market. The liquid crystal panel is fabricated with orthogonal row and column addressing lines on opposite sides of the liquid crystals that are driven by row and column drive electronics. The row drivers enable the row addressing lines one row at a time while the column drivers apply selected voltages to all of the column addressing lines to apply a voltage across the cells in the enabled row. The voltage changes the transmissive characteristics of the liquid crystal, which in turn optically modulates the amount of light transmitted through the LCD.
Because liquid crystals respond relatively slowly to changes in the applied voltage, the cell modulation is proportional to the root-mean-square (rms) voltage applied across the cell throughout the frame time. Although the voltage applied during the row enable is very large, the background noise created by the applied voltages for the remaining n−1 rows greatly reduces the RMS value of the margin between the off-state and full on-state of the liquid crystal. For example, commercially available AMLCDs can resolve about 16 million different colors while similarly available multiplexed LCDs can resolve only 256 different colors. As the number of scanned rows increases, this disparity in grey scale color resolution grows.
These LCDs must be constantly refreshed, e.g. 30 times per second, which consumes a lot of power. Without a sustaining voltage they will decay from their modulated state to their relaxed state over time. Furthermore, the polarizers inherently required by LCDs absorb such a large fraction of the ambient light, typically 60%-70%, they are unable to produce the “paper white” quality desired by the industry. As such consumers must make do with cell phones and PDAs whose gray displays are difficult to read even under the best ambient lighting conditions. Power consuming backlights must be added to improve their readability to minimum acceptable levels.
Another class of displays that are prevalent and gaining market share in low-end applications are bistable displays. Bistable displays have two stable states, black and white. True bistable displays do not require a voltage to be applied to remain in either state and thus require no power when stable. Quasi bistable displays require an applied voltage to hold the stable state. Ideally, i.e. no leakage current, this would still require no power. However, in practice there is some amount of power consumed. In either case, since bistable displays do not require continuous refreshing they are very low power. This makes them ideally suited for hand-held applications such as cellular telephones, PDAs and electronic books. Adequate grey scale resolution can be achieved using standard half-toning techniques. However, known bistable displays suffer from the same problem as multiplexed LCDs, their white state tends to be gray rather than paper white. As a result, they require backlighting and their readability is limited even in the best ambient light conditions.
Kent Displays, Inc. is the leader in bistable Cholesteric LCDs. The bistability of cholesteric optical textures allows for high resolution on a low cost passive matrix with reduced power consumption since power is not needed to continuously refresh the image. The reflected colors of the cholesteric liquid crystal materials provide for a display that is readily viewed in sunlight or low ambient light without dedicated illumination. However, single layer cholesteric LCDs are colored and combining different color layers to get a neutral color dark state severely reduces the overall brightness of the display. These displays are very dim; black characters on a dark grey background instead of black on a white background.
Xerox PARC is developing a gyricon technology in which bichromal spheres are cast in a clear elastomer on a flexible substrate. The sphere dipole causes rotation in an electric field to show either the black or white surface of the sphere. The gyricon display is thin, flexible, exhibits a wide viewing angle and, like other bistable devices, requires no power to store the device. However, contrast ratios of only 6:1 have been achieved.
E Ink, Corporation is developing an alternative bistable display technology, electronic ink, in which the ink is made of microcapsules, each of which can change color with an applied electric field. More specifically the microcapsules are filled with a colored dye. Charged white particles are suspended in the dye. Orienting the electric field the right way causes white particles to be attracted to the surface so that the display appears white and vice-versa. E Ink claims to have achieved 75% brightness, 30:1 contrast ratio and a 180 degree viewing angle.
Iridigm Display Corporation uses a MEMS technology in which bridge-like elements move up and down in response to an applied voltage to achieve a bistable display. By changing an element's position from up to down, either constructive or destructive interference is created with an external light source. This allows each element to switch from reflective to absorbing, from green to black, for example. Each image pixel is composed of tens-to-hundred of bridge elements, which facilitates grey scale and reduces yield requirements. Iridigm's displays are fabricated on glass substrates using standard thin film transistor (TFT) materials and processing techniques, that allow them to construct aluminum bridge elements and a proprietary thin-film stack to control interference. However, because Iridigm's display is based on interference patterns it will be sensitive to viewing angle and will have difficulties achieving paper white quality.
A number of quasi-bistable electromechanical shutter technologies have been pursued and patented for direct view displays, but have not yet succeeded to large scale commercialization due to a variety of issues including fabrication, stiction, limited contrast ratio, poor optical efficiency, high cost and poor pixel uniformity U.S. Pat. No. 3,553,364 to Lee entitled “Electromechanical Light Valve” describes an electromechanical light valve in an array of many such valves for controlling the transmission of light in continuously changing patterns. Each light valve consists of a housing having grounded conducting walls for shielding the interior thereof from external electrostatic forces produced by surrounding valves and a leaf shutter mounted in the housing. The application of a voltage to the leaf shutters causes the shutter to be attracted to the grounded conducting walls. As the voltage differential increases, the angle the shutter deflects increases, which in turn allows less light to pass through the housing.
Lee's design always involves the leaf shutters touching one surface or another, e.g. the cond
Little Michael L.
Lyon John J.
Blakely , Sokoloff, Taylor & Zafman LLP
Mengistu Amare
Nguyen Jimmy H.
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