Electric eye is a device that either produces an electrical current or allows a current to flow when light shines on it. The strength of the current depends upon the amount of light that falls on the electric eye. When the light stops, so does the current.
Electric eyes can be made more sensitive to light than is the human eye. They can see objects when a person would see only total darkness. Electric eyes respond not only to visible light, but also to infrared and ultraviolet light. Some electric eyes react so rapidly to changes in light that they can “see” bullets in flight.
What electric eyes do.
An electric eye can act as a switch to turn another device on or off. For example, an electric eye placed opposite a light across a conveyor belt can count objects on the belt. Each time an object comes between the light and the electric eye, the current flowing from the eye stops. A counting device connected to the eye then adds to the total. Electric eyes can turn on street lights or house lights by sensing when the sun goes down. They can also set off burglar alarms and perform similar jobs.
Electric eyes can also measure amounts of light. The electric eyes on some cameras measure the light and then properly adjust the shutter. Motion-picture projectors use electric eyes to produce sound from special patterns on the film. Digital cameras use an array of many electric eyes to sense light. A computer later processes and combines the electrical information gathered from the individual eyes to render a digital photograph.
How electric eyes work.
Scientists call electric eyes photoelectric cells. There are three basic kinds of photoelectric cells—phototubes, solar cells, and photoconductive cells.
Phototubes
are special vacuum or gas-filled tubes that contain photosensitive materials. These materials give off electrons when light strikes them. Every phototube has a cathode made of photosensitive material, and an anode. When light strikes the cathode, it causes electrons to flow to the anode and creates a current. See Photomultiplier tube.
Solar cells,
also called solar batteries or photovoltaic cells, are made of semiconductor materials (see Semiconductor). When light shines on a solar cell, electrons are given off. They produce a current in a circuit connected to the cell.
Photoconductive cells
are also made of semiconductor materials, but they do not create an electric current as solar cells do. Instead, when light shines on them, their resistance (opposition to the flow of current) decreases.
