used. Earlier inventors had proposed red light for the purpose and the use of selenium cells in determining printing exposures, but without confining the radiation to the infrared. Siemens and Halske100 proposed that infrared be used in the examination of materials, such as photographic films, which are spoiled by visible light. The infrared image of the object being examined is converted into a visible image by a television device, using screens which change color or develop color on exposure to heat rays, phosphorescence quenching, and so on. Clearly, the visible image could be photographed if a permanent record were required.
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