| 摘要 |
A phototropic cell comprises spaced transparent plate-like members, a spacing member extending around the periphery of the plate-like members to define therewith a chamber and a phototropic film extending across the chamber and secured to the spacing member. The rate of opaquing and recovery may be increased by incorporating in the cell a humectant film, e.g. of polyvinyl alcohol and glycerol, or by heating means. In one modification electrically conductive coatings are incorporated and attached to a high frequency potential source suitably triggered, e.g. by means of a photo-electric cell. Ultra-violet and infra-red filters may be incorporated. Two phototropic films of different sensitivity may be incorporated, the more sensitive inner film being protected against burning out by the less sensitive outer film. Suitable phototropic materials are disclosed in Specifications 1,008,536 and 1,008,537, e.g. comprising solid solutions of polyvinyl alcohol, polyvinyl acetal, polyvinyl butyral, cellulose acetate or cellulose acetate hydrogen phthalate and a transition metal halide or an inorganic heteropolyacid. Films of these materials when oriented by stretching become strongly dichroic when opaqued, so that the efficiency of the cell may be increased by using two oriented films with the stretch axes perpendicular.
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