The power-spectrum density of light at the output of a photodetector is determined using a particle model. The model is the same as that introduced by Rice in determining the spectrum of shot noise. A time-varying Poisson statistic is used to represent the intensity variations. The conditions are shown under which the intensity variations of the detected light are representative of the intensity variations of the light in both time and frequency. An intrinsic signal-to-noise ratio of photon to photoelectron conversion is defined. Spectrum alternations due to delay are also determined. Finally, the mixing of light from two or more light sources is considered and the power-spectrum density determined.
SHERMAN KARP, "Spectrum of Detected Light: A Particle Model," J. Opt. Soc. Am. 55, 686-687 (1965)