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Show Light Sensitive Devices By H. A 1. 2. rough list of light sensitive devices might include the following: The skin of most animals is to The retina of eyes may have eyes have a T. PLUMBI sensitivity range some extent light sensitive. developed from super sensitive skin. of about one octave, with a Human maximum in yellow light. 3. Thermometers of various kinds can detect the heat energy transformed from light energy whenever light rays are stopped. The bolometer or resistance type thermometer, and the thermopile radiomicrometer are good examples. 4. The radiometer also has been used for light. exploring the solar spectrum of S. The selenium cell, where resistance varies with light intensity, was about the first truly light sensitive device of physics, if we neglect photography and photo-chemistry. 6. Then came the "photo-electric effect" and photo emissive tubes. These really light controlled valves which govern the emission of electrons from cold metals. They control all kinds of electric power by magneto electric relays, or by thyratron tube relays. are The application of photo-relays to industry is rapidly increasing. Today there are more than 300 common uses of such devices. They are used in photometers of ordinary light sources. and of invisible infra-red and ultra violet rays, and even for measuring the erythemal effects of ultra-violet. Photo-tubes are the keys to talking pictures, television, color analyzers, wired photos, and other modern marvels. They can be used for detecting, guarding, guiding, measuring, limiting and controlling every conceivable mechanical proc ess devised by man. They are tireless, constant, rugged and far more accurate and delicate than human eyes. Photo-tubes actually have a amount of photo-electric effect and transform a small light energy directly into electric energy; but mostly their electron emission is applied to the grid of an amplifying tube which controls an ex ternal source of power. 7. The light. "light sensitive cell" creates an electromotive force when exposed to This is a true photo-electric-effect in a film between two metal sur faces. It is a real transformer of light energy into electric current energy. It generates electric power for operating measuring or control equipment, and no auxiliary source of power is necessary, although amplification may be used. The G. E. light-sensitive cell consists of a layer of selenium between a steel plate and a transparent conducting cover plate. With 80 foot-candles it has an output of about 16 microwatts. 8. This light-sensitive cell combined with a 1 Engineer, General Electrical Co., properly calibrated micro-ammeter Salt Lake City, Utah. 43 |
