Incident earth radiation recorded in mercury-cadmium-telluride photoconductive detectors is usually a nonlinear function of the incident signal. This nonlinear characteristic must be taken into account for interferometric data transformation processes to avoid measurement errors of the incident light intensity. In this work, we investigate the effects of nonlinearity contributed by various factors, especially the nonlinearity contributed by the illuminated and nonilluminated detector areas, which to our knowledge was not investigated earlier for a constant current biased detector circuit. The effects of wire resistance and detector preamplifier loading as well as the impact of off-centered detector illumination compared to the centered detector illumination on the nonlinearity are also investigated. Computer simulation results are presented for constant current biased detector circuits to estimate the effects of nonlinearity. Finally, we show that the nonlinearity can be estimated by using a third-order parametric equation.