The physical processes taking place in PIN photodiodes exposed to spatial radiation have been investigated in the infrared band. The modelling and simulation of the diodes was done by means of numerical algorithms using the coupled Poisson and continuity equation to obtain the behaviour of electronic devices. The effects of the radiation studied were atomic displacement damages. The present work aims at a comparison of the behaviour of differently constructed PIN photodiodes after exposure to 10 MeV protons and 1 MeV neutrons having fluences in both cases in the range of 1 × 10 13 to 2.5 × 10 14 particle cm −2 . We have also included in our simulation the effects produced by the light illumination on the photodiodes; in this case the intensities used ranged from 0 to 2 mW cm −2 . We have obtained as a result of our research one expression that relates the total reverse current and the incident radiation.