The efficiencies and thus the brightnesses of flat-panel projectors based on liquid-crystal devices (LCDs) and digital mirror devices (DMDs) would be increased by the combination of a color-scrolling technique and a light-recycling technique, according to a proposal. These techniques were originally proposed separately for the purpose of increasing the efficiencies of LCD-based display devices.

The scrolling-color technique was reported previously in "Using Surface-Plasmon Filters To Generate Scrolling Colors" (NPO-20110),NASA Tech Briefs, Vol. 23, No. 2 (February 1999), page 14a. To recapitulate: by use of prisms and surface-plasmon tunable filters, white light from a lamp or other illumination source would be converted into a pattern of scrolling primary colors. The advantage of this scheme is that both polarization components and all colors would be utilized, whereas in prior schemes, most of the light power is wasted through color and/or polarization filtering.

Figure 1. Scrolling Colors Would Be Generated, and light not used to generate scrolling colors on the first pass would be recycled and so used on subsequent passes.

The light-recycling technique was reported in "Low-Absorption Color Filters for Flat-Panel Display Devices" (NPO-20435),NASA Tech Briefs, Vol. 23, No. 12 (December 1999), page 34. In this technique, one would replace traditional dye filters with surface-plasmon or interference filters, which are more reflective than absorptive. In addition, the filter and illumination optics would be arranged so that much of the light in all colors and both polarizations reflected from the filters would be sent back through the light-source optics to be reused as illumination.

The present proposal calls for, among other things, a white light source equipped with a reflector and a color-recycling and -scrolling panel (CRSP), as shown schematically in the upper part of Figure 1. The CRSP would contain an array of voltage-tunable or voltage-switchable filters, each of which would transmit one primary color [red (R), green (G), or blue (B), depending on the applied voltage] and reflect the other two primary colors. The reflected light would be bounced back and forth between the CRSP and the light-source reflector until light of each color impinged on its proper color filter. Thus, light in the three colors would be redistributed as needed and relatively little would be lost.

At the first phase of a three-phase operating cycle, the color-filter array could be energized to a color pattern as RGB . . . , for example. In the next phase, the color pattern could be changed to BRG . . . . In the third phase, the color pattern could be changed again to GBR . . . . The net result is that recycling and scrolling of colors would occur in combination. The number of color filters need not match the number of pixels in the display panel to be illuminated panel pixels; the only requirement on the number of filters is that it be an integer multiple 3.

Figure 2. A Flat-Panel Projector based on an LCD would utilize the scrolling-color and light-recycling techniques for maximum light-utilization efficiency.

The voltage-tunable or voltage-switchable color filters could be surface-plasmon tunable filters [see "Voltage-Tunable Surface-Plasmon Band-Pass Optical Filters" (NPO-19988),NASA Tech Briefs, Vol. 22, No. 8 (August 1998), page 18a]. Alternatively, they could be assemblies of interference or thin-metal film filters, high-index-of-refraction prisms, and total-internal-reflection switches [see "Digitally Tunable Color Filters and Beam Scanners" (NPO-20240),NASA Tech Briefs, Vol. 23, No. 9 (September 1999), page 65] that would include layers of an electro-optical material between prisms, as shown in the lower part of Figure 1. In the absence of applied voltage, the electro-optical layer in each switch would have a low index of refraction, so that total internal reflection would occur at the prism/switch interfaces. The application of a sufficient voltage to the switch would increase the index of refraction of the electro-optical layer sufficiently to allow light to pass through. By appropriate switching in this manner, one can cause light of the various colors to travel along various paths in the prisms to obtain various output color patterns.

Figure 2 presents an example of an LCD-based display system that would include a CRSP. The image of the CRSP would be projected into a monocolor LCD panel by a relay lens. A zoom lens would project an image of the LCD onto a screen. It would be necessary to use an electronic driver that would synchronize the scrolling of colors and the modulation of light by the LCD. Similar geometry can be applied to DMDs.

This work was done by Yu Wang of Caltech for NASA's Jet Propulsion Laboratory.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to

Intellectual Property group
Mail Stop 202-233
4800 Oak Grove Drive
Pasadena, CA 91109
(818) 354-2240

Refer to NPO-21052.

This Brief includes a Technical Support Package (TSP).

Unfortunately the TSP Light Recycling and Color Scrolling for Brighter Displays (reference NPO-21052-) appears to be missing from our system.

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