A proposed technique for color filtering in a liquid-crystal or other flat-panel display device would make it possible to brighten the display without increasing the amount of light supplied from behind the panel. The need for the proposed technique arises as follows: At present, each pixel in a typical color liquid-crystal display device contains three dye filters: red, green, and blue. Each filter transmits its single primary color and absorbs the other colors, so that less than one-third of the available light is used for viewing. In addition, the liquid-crystal display uses polarized light, so that half of the incident unpolarized illumination is necessarily wasted. The net result is that less than one-sixth of the incident unpolarized light is utilized. One does not have the option of increasing the illumination substantially to brighten the display because the increase in heat generated by absorption of light in the filters could harm the display device.

Figure 1. Light Not Used in the Display at a given location would be reflected by a surface-plasmon color filter for use elsewhere. In the example shown here, incident ray A would give rise to reflected ray B, which would be reflected twice by the collimating reflector to become ray C.

In the proposed technique, one would replace the 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 reflected from the filters would be reused as illumination. The overall effect should be an increase in brightness and efficiency.

Figure 1 illustrates this concept as applied to a liquid-crystal panel back-lit by a lamp with a collimating reflector. Light reflected from a color filter on the panel would return to the collimating reflector, where it would be reflected twice and sent to a different location on the panel. Of course, neither the original light from the lamp nor the light reflected from the panel would be collimated perfectly as shown in simplified form in the figure; all incident and reflected beams of light would have some angular spread. This spread would be beneficial in that it would make the illumination more nearly uniform across the panel.

Figure 2. Alternating Rows of Microprisms in a surface-plasmon color filter in a pixel would transmit and reflect light in mutually orthogonal polarizations. This arrangement would make it possible to utilize both polarization components of unpolarized light, whereas heretofore, only one of them has been utilized.

Figure 2 shows a proposed configuration of an in-pixel surface-plasmon color filter, which would contain long, narrow microprisms in odd-numbered rows and shorter prisms oriented perpendicularly to them in even-numbered rows. Light that was p- or s-polarized to the longer prisms would be s- or p-polarized, respectively, to the shorter prisms. Each prism would pass light of only one polarization and reflect light of the other polarization. Thus, the polarized light not utilized in each pixel would be sent back to the collimating reflector and redistributed elsewhere on the panel, where some of it would be utilized in other pixels.

This work was done by Yu Wang of Caltech for NASA's Jet Propulsion Laboratory. NPO-20435

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Low-absorption color filters for flat-panel display devices

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This article first appeared in the December, 1999 issue of NASA Tech Briefs Magazine.

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