Aphototube or photoelectric cell is a type ofgas-filled orvacuum tube that is sensitive to light. Such a tube is more correctly called a 'photoemissive cell' to distinguish it fromphotovoltaic orphotoconductive cells. Phototubes were previously more widely used but are now replaced in many applications by solid state photodetectors. Thephotomultiplier tube is one of the most sensitive light detectors, and is still widely used in physics research.
Phototubes operate according to thephotoelectric effect: Incomingphotons strike aphotocathode, knockingelectrons out of its surface, which are attracted to ananode. Thus current is dependent on the frequency and intensity of incoming photons. Unlikephotomultiplier tubes, no amplification takes place, so the current through the device is typically of the order of a fewmicroamperes.[1]
The light wavelength range over which the device is sensitive depends on the material used for the photoemissive cathode. Acaesium-antimony cathode gives a device that is very sensitive in the violet to ultra-violet region with sensitivity falling off to blindness to red light. Caesium on oxidisedsilver gives a cathode that is most sensitive to infra-red to red light, falling off towards blue, where the sensitivity is low but not zero.[2]
Vacuum devices have a near constant anode current for a given level of illumination relative to anode voltage. Gas-filled devices are more sensitive, but the frequency response to modulated illumination falls off at lower frequencies compared to the vacuum devices. The frequency response of vacuum devices is generally limited by the transit time of the electrons from cathode to anode.
One major application of the phototube was the reading of opticalsound tracks for projected films. Phototubes were used in a variety of light-sensing applications[3] until some were superseded byphotoresistors andphotodiodes.
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