The reliable detection of optical radiation is an essential element of laser technology. Most standard optoelectronics textbooks cover the subject of photodetection to some degree. The sensitivity of a photodetector can be defined in terms of the incident optical power required to achieve a given signal-to-noise ratio (SNR). A figure of merit commonly used is the noise-equivalent power (NEP), defined as the mean optical power required to be incident on the detector to generate an SNR of unity. The responsivity of a detector indicates the change in the output of the detector per unit change in the incident optical power and is, therefore, unrelated to noise considerations. When photons of sufficient energy are incident on a solid, electrons are emitted from the surface. Electrons emitted by the photocathode are accelerated towards the first dynode by a large potential difference. Dynode materials are chosen for their low work functions to enable efficient secondary emission.