The photomultiplier is an extremely sensitive light detector providing a current output proportional to light intensity. Photomultipliers are used to measure any process which directly or indirectly emits light. Large area light detection, high gain and the ability to detect single photons give the photomultiplier distinct advantages over other light detectors.
The diagram below will help you understand how a photomultipler works.
The photomultiplier detects light at the photocathode (k) which emits electrons by the photoelectric effect. These photoelectrons are electrostatically accelerated and focused onto the first dynode (d1) of an electron multiplier. On impact each electron liberates a number of secondary electrons which are in turn, electrostatically accelerated and focused onto the next dynode (d2). The process is repeated at each subsequent dynode and the secondary electrons from the last dynode are collected at the anode (a). The ratio of secondary to primary electrons emitted at each dynode depends on the energy of the incident electrons and is controlled by the inter-electrode potentials. By using a variable high voltage supply and a voltage divider network, to provide the inter-electrode voltages, the amplitude of photomultiplier output can be varied over a wide dynamic range.