Page 43 - Curriculum Visions Dynamic Book

Photoelectric devices
A photoelectric device consists of a small piece of photosensitive material, such as cadmium sulphide, connected to a battery.
When light shines on cadmium sulphide, its electrical conductivity increases sharply. This allows it to be used as an automatic switch in such situations as dusk-to-dawn street and home lighting and as the device in automatic cameras.
Cadmium sulphide becomes a conductor when light reaches it because some of the light particles (photons) can make the electrons energetic enough to be free to flow, almost as though they were in a metal.
Cadmium sulphide is especially sensitive to changes in visible light.
How an automatic exposure system works
This is a diagram of a circuit used in an automatic camera. The iris in the camera determines the amount of light that reaches the film. A number of small fragments of cadmium sulphide are placed in the path of the light to obtain an average value of the light entering the camera. The light causes a change in the conductivity (resistance) of the cadmium sulphide.
The cadmium sulphide devices are used as though they were variable resistors. A small electronic circuit reads the values of the cadmium sulphide “resistors” and causes the iris to be adjusted until the amount of light reaching the film is appropriate for the correct exposure.
phosphor: any material that glows when energized by ultraviolet or electron beams such as in fluorescent tubes and cathode ray tubes. Phosphors, such as phosphorus, emit light after the source of excitation is cut off. This is why they glow in the dark. By contrast, fluorescors, such as fluorite, emit light only while they are being excited by ultraviolet light or an electron beam.
photon: a parcel of light energy.
semiconductor: a material of intermediate conductivity. Semiconductor devices often use silicon when they are made as part of diodes, transistors or integrated circuits.
 Light rays enter through the camera lens.
 Motor adjusts the aperture of the iris.
 Light falls on the cadmium sulphide chip, changing its conductivity.
 An electronic circuit reads the conductivity value and engages the motor that adjusts the iris.
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