Photocathodes convert light into electrons using the photoelectric effect. Apart from solid photocathodes which are applied to HPD's, also gaseous and liquid photocathodes are known. The plot on the right shows the quantum efficiency vs wavelength for some commonly used photocathodes. All shown cathodes are produced by vacuum evaporation in an ultra high vacuum. The highes qantum efficiency (about 25-30% at 400 nm) is achieved by the bialkali cathode (K2CsSb). The multialkali cathodes (sometimes called S20) have a response which extends more to the red part of the visible light spectrum. The solar blind CsTe cathodes are normally reserved for special UV applications.
Quantum efficiency of some common photocathodes