Measuring the Light Infinitesmal

In this world of digital displays, almost all transducers generate analog signals. Whatever the device, whether it is making electrical, magnetic, mechanical, or optical measurements, i-ts output signal is likely to be analog rather than digital. One of the few exceptions is the photomultiplier tube (PMT). Its digital nature stems from the 1905 discovery by Einstein that the ejection of electrons from illuminated suriaces may be explained if light is considered to comprise individual particles or photons. The rain of photons on the cathode of a PMT produces characteristic charge pulses at the anode, the "shot effect" {Fig 1 ). Although the shot effect entails nothing but burdensome noise in ordinary de measurements, it is the basis of a technique, called photon counting (PC). Discrete pulses from the PMT are first sent to amplifying circuitry which discriminates between electrons that originate at the cathode of the PMT and those generated elsewhere down the dynode string. The amplified pulses can then be converted to a count-rate in analog form or, alternatively, can be fed directly to digital computational instrumentation for processing and storage.