Phototubes operate according to the
photoelectric effect: Incoming
photons strike a
photocathode, knocking
electrons out of its surface, which are attracted to an
anode. Thus current is dependent on the frequency and intensity of incoming photons. Unlike
photomultiplier tubes, no amplification takes place, so the current through the device is typically of the order of a few
microamperes. The light wavelength range over which the device is sensitive depends on the material used for the photoemissive cathode. A
caesium-
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 oxidised
silver 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. 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. ==Applications==