SOLUTION

The sensitivity of a photoelectric cell is measured in terms of Current per unit of luminous flux and Current per unit area of the surface.

PHOTOELECTRIC DEVICE

• When light strikes certain light-sensitive materials, it may cause them to give off electrons, it may change their ability to conduct electricity, or it may cause them to develop an electrical potential, or voltage, across two surfaces.
• Devices that rely on these effects for their operation are called photoelectric devices.

Photoelectric devices can be adapted to a variety of applications.

• As an optical switch that senses the interruption of a light beam, a photoelectric device can open doors automatically or activate a counter on an assembly line.
• Because the electric current put out by photoelectric devices varies with the intensity of the incoming light, the devices can be used as sensors to turn lights on at night and off during the day.
• Photoelectric devices can be made sensitive to certain wavelengths of incoming light and used in photographic light meters. They are also used in the reproduction of movies and soundtracks.
• Photoelectric units can also be made to operate in the infrared and ultraviolet regions of the electromagnetic spectrum (see radiation). The light beams in burglar alarms, for example, are usually outside the visible spectrum.
• The sensitivity of a photoelectric cell can be expressed in terms of luminous sensitivity (output current per incident visible light flux; μa/lumen) or power falling on the cell per square centimeter of the sensitive area, a quantity known as irradiance (output current per incident radiation power, (μa/,μw).