PHOTOELECTRIC DEVICES
The photoelectric device is an application of the photoelectric effect. The basic principle is to liberate electrons from a metal surface by exposing it to photons in a light beam and then to measure the number of electrons with electronic circuitry. The photoelectric device, like the photographic emulsion, can·be made to respond to different wavelength regions by varying the metals used in making the surface of a device. The biggest advantage of the photoelectric device is that it can be manufactured to have a very large dynamic range of response; in addition its response is linear to the number of incident photons for a fictitious device. With modern electronics it is possible to adapt the photoelectric device to count individual photons or to use a mosaic of devices to form a picture much as a photographic plate does.
As an illustration of the photoelectric device's importance as a radiation detector, only about 15 percent of the nights of observing on the 5.1-meter Hale telescope are devoted to photographic work. On 85 percent of the nights some kind of photoelectric detecting device is being used.
