Does a photodetector always measure the rate of arrival of photons?

Abstract

It is pointed out that in many cases the response of a photodetector depends on the angle of the incident light beam in a way that is different from the transmissivity of the photosensitive surface. The detector therefore does not directly measure the rate of arrival of photons under all circumstances. As an illustration, some experimental measurements of the angular response of a silicon photodiode are presented.