It is known experimentally that illumination conditions can significantly affect the spectral response behavior of hydrogenated amorphous silicon (a‐Si:H) Schottky barrier detector and solar cell structures. This behavior is examined for the first time using a first‐principles numerical analysis to explore the repercussions of the relative intensities and absolute intensities of the bias light and the monochromatic light. It is demonstrated that to avoid the experimental difficulty of an ill‐defined spectral response, which can arise when lock‐in detection is used, the bias light intensity should be an order of magnitude larger than the monochromatic light intensity. More importantly, the numerical modeling demonstrated that spectral response can fundamentally depend on illumination conditions due to field redistribution and recombination redistribution. It is further shown that these redistributions depend on the homogeneity of the intrinsic absorber layer; that is, the behavior with bias light is found to be different if a thin surface layer is postulated to exist at the front of the intrinsic layer. It is also demonstrated that the spectral response can be greater in the shorter wavelengths for materials with a higher midgap density of states. This result can explain the experimentally observed effects of prolonged light soaking.