A detector is a device that is capable of transforming the energy

distribution into a ﬂow of interpretable information. It senses an

optical signal within its ﬁeld-of-view, regardless of its angle of

incidence, and converting it into an electrical signal (current or

voltage). The process involved here is the quantum process, which

is statistical. Given a certain electric ﬁeld, the number of charges

N released is statistically related to the exposure W (wave power multiplied by time) by Einstein’s relation, W = N¯ω, with N as an integer having mean value N¯ , and obeying Poissonian statistical distribution. The probability of certain number of photons, Nph, being detected in a certain time interval, P(Nph), in which the average number of photons detected is N¯ph, is given by

P(Nph) = (N¯ph) Nph

Nph!