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Controlled extraction from light guides by means of ELE ctrophoretic modulation of total internal reflection Cheng, Ju-Chieh (Kevin)

Abstract

A prototype of a prism light guide using a new variable extraction technique is discussed. Prism light guides are hollow structures that pipe light by means of total internal reflection (TIR). High efficiency and uniform illumination can be achieved using a uniform extraction technique along the prism light guides. However, the shapes of extractors used for uniform extraction have to be designed specifically for different geometries of conventional light guides. Another limitation of the conventional systems is that the level of extraction along the guides cannot be controlled once the extractors have been built into the light guides. Motivated to generalize the shape of the extractor and to enable the control of the extraction/illumination level along the light guide, a new variable extraction technique has been developed. This extraction technique is based on frustration of TIR, electrophoresis, and photon scattering. A rectangular variable extractor based on the new technique has been made with highly scattering silica particles in a fluorinert suspension contained between two transparent conducting films. By changing the polarity of the electrical potential applied across the conducting films, the charged silica particles can be moved to prevent, or "frustrate," TIR (through interaction with light in the very thin evanescent wave region near the TIR interface inside the light guide) and cause scattering in which light gets extracted, whereas the opposite polarity will not affect TIR. As a result, by changing the strength of the applied potential the level of scattering or extraction can be varied for each extractor independently along the light guide. A Monte Carlo ray tracing analysis of the light guide using the new extraction technique has been performed, and the results are promising. Based on the ray tracing model, a scale prototype of the variable extractor light guide has been constructed. The prototype consists of a prism light guide with a rectangular cross section and eight variable extractors. To test it the light guide was then mounted on a model o f a series of eight offices with two light sources at each end of the guide, such that each extractor controls the illumination level in each office. A simulation algorithm was developed to predict the extraction or illumination level for different settings of the applied potential in each office, and the performance of the actual prototype was compared and agrees with the model prediction. A control algorithm was then programmed to adjust the level of extraction in each office according to the needs of the users, and the algorithm was successfully applied to the actual prototype.

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