photocathode. An electrode in an electron tube that releases electrons when exposed to light. Photocathodes are used in television cameras to convert light energy into electrical energy.
photochemistry. The branch of chemistry that deals with the effects of light on chemical reactions.
photoconductive cell. A photoelectric device that changes its resistance as the amount of light striking it changes. When the cell is in total darkness, its resistance is maximum, and as light falls on it, its resistance decreases.
Photoconductive cells using either cadmium sulfide or cadmium selenide as the active elements are called light-dependent resistors (LDRs), and one of their uses is in photographic light meters.
photodiode. A semiconductor diode whose conduction characteristics change when its PN junction is exposed to light. A photodiode in total darkness acts as an open circuit; no current can flow.
photoelectric characteristics. The characteristics of a material that change when the material is exposed to light.
Some materials (photovoltaic materials) produce a voltage when exposed to light, and others (photoconductive materials) change their resistance when exposed to light.
photoelectricity. Electricity produced by light. When light strikes certain types of semiconductor materials (for example, selenium and cadmium oxides), light energy is absorbed into the material, and electrons are forced to move through it. This movement of electrons is photoelectricity.
photoelectric material. A material that emits electrons when light shines on it. Some of the alkaline metals, such as cesium, lithium, and rubidium, are photoelectric.
photoemissivity. The characteristic of a material that causes it to emit electrons when it is exposed to light.
photofet. A junction field effect transistor in which the gate-to-channel junction acts as a photodiode. The light sensitivity of a photofet is about ten times as great as that of an ordinary phototransistor.
photon. A massless particle of an electromagnetic field. A photon, which is also called a quantum of light, has both energy and momentum.
photonegative characteristics. The characteristics of a material that cause its resistance to increase when light shines on it.
photophone. A device that changes variations of light intensity into sound. A beam of light can be modulated by the output of a microphone. If this modulated light beam is shone on a photoelectric cell, its output current will duplicate the current produced by the microphone.
photo reconnaissance (air traffic control). Military activity that requires locating individual photo targets and navigating to the targets at a preplanned angle and altitude. Photo reconnaissance normally requires a lateral route width of 16 nautical miles and an altitude range of 1,500 feet to 10,000 feet AGL.
photoresistor. A semiconductor device that acts as a variable resistor whose resistance is determined by the amount of light shining on it through a built-in lens. The resistance of a photoresistor increases as the amount of light shining on it increases.
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