The sensitivity of the electric field distribution to the spectrum of the incident radiation in highly biased, high-resistivity, semiconductor structures of the MSM (metal-semiconductor-metal) type illuminated by nonmonochromatic light is investigated theoretically. It is shown that in the presence of deep impurity levels the field distribution depends strongly on the spectral composition of the incident light. The frequency interval corresponding to optical thicknesses of the order of unity is found to significantly influence the space charge in the bulk of the structure and the electric field distribution E(x), even when the fraction of energy in this region of the spectrum relative to the total flux is extremely minimal. The trapping of holes by a deep impurity level in the bulk of the structure forms a positive space charge and produces qualitatively new field distributions, which increase near the dark electrode with a positive curvature of the function E(x). The impurity trapping of electrons near the illuminated anode imparts a negative space charge to the impurity levels. This phenomenon induces a substantial increase of the field in the electrode sheath and forms in the vicinity of the anode a region wherein the field varies only slightly. All the prominent features disclosed by the calculations in the electric field distributions are observed in experiment.