Color-filtering and beam-scanning devices based on electro-optical switching of internal-reflection states have been proposed for use in display and measurement applications. Associated digital circuits would apply electronic control signals to spatial segments of these devices to obtain discontinuous spatial and/or spectral displacements of reflected and/or transmitted light beams. If this description seems a little too general, it is because the basic device concept is rather general; it could be implemented in numerous different optical and electronic configurations.

The proposed devices would take the places of the galvanometer-driven mirrors, rotating prisms, color-filter wheels, and other optomechanical devices that have been used in some beam-scanning and -filtering apparatuses until now. Unlike the optomechanical devices, the proposed devices would contain no moving parts. Relative to the optomechanical devices, the proposed devices would offer advantages of high speed and light weight.

Figure 1. A Unique Output Color could be selected by applying voltage to an electrode coterminous with one of the color filters. The electric field would increase the index of refraction of the electro-optical material by an amount sufficient to stop total internal reflection at the selected filter, thereby making light pass from the left prism, through the filter, to the right prism.

Figure 1 illustrates a color-filter device of the type proposed, with a simple design chosen for explaining the basic principle of operation. A ferroelectric liquid crystal or other suitable electro-optical material would be sandwiched between two long Dove prisms. A continuous thin film of indium tin oxide on the sandwich contact surface of the left prism would serve as an electrode for applying an electric field to the electro-optical material. A thin film of indium tin oxide would also be applied to the sandwich contact surface of the right prism, but this film would be divided into segments to form electrodes corresponding to discrete color/beam-scanning pixels. Each electrode segment would be coated with an interference or other suitable optical filter to define the color of the pixel.

The device would be positioned and oriented so that white light entering through the lower end of the left prism would strike the sandwich contact surface at an angle slightly greater than the minimum angle for total internal reflection in the absence of applied voltage. Therefore, in the absence of applied voltage, the white light would bounce along inside the left prism through a number of total internal reflections; none of the light would be coupled to the right prism and the light would be reflected out, still white, at the top of the left prism.

Figure 2. A Unique Output Path could be selected by applying voltage to an electrode coterminous with one of the small prisms on the right side. Except for the prism configuration and the lack of filters, this device would function similarly to that of Figure 1.

If a sufficient voltage were applied between the single large electrode on the sandwich contact surface of the left prism and one of the pixel electrodes on the sandwich contact surface of the right prism, then the index of refraction of the electro-optical material in that pixel would increase sufficiently to raise the minimum angle of incidence for total internal reflection above the actual angle of incidence. In that case, the incident light would strike the color filter in the affected pixel, so that light of one color would pass through the filter into the right prism, while the complementary color would be reflected back into the left prism. The end result would be that light of the color passed by the filter would travel along the right prism by total internal reflection and would leave the right prism at its top end, while light of the complementary color would behave similarly in the left prism and would leave that prism at its top end. If the pixels contained filters of different colors, then one could select a unique output color by applying voltage to the pixel of that color.

Figure 2 illustrates a simple non-color-discriminating beam-scanning device. This device would be similar to the device of Figure 1, except that there would be no color filters and instead of one long prism on the right side, there would be multiple small prisms - one prism for each pixel. As in the example of Figure 1, in the absence of applied voltage, light would travel along the left prism by total internal reflection and would emerge from the top of the left prism. In this case, however, the left prism would be configured so that the light emerging from the top would be traveling rightward instead of leftward. If a sufficient voltage were applied to one of the pixel electrodes, then the light would pass through to the right side and would emerge through the upper right face of the pixel for that prism. Thus, one could discontinuously scan (digitally switch) a beam of light among discrete parallel paths by applying voltage sequentially to different pixel electrodes.

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

Technology Reporting Office
JPL
Mail Stop 122-116
4800 Oak Grove Drive
Pasadena, CA 91109
(818) 354-2240

Refer to NPO-20240


NASA Tech Briefs Magazine

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

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