The photoconductance of polycrystalline silicon films at photon energies smaller than the band gap has been measured as a function of intensity applying a 1.3 μm wavelength semiconductor laser. The observed photosignal increases superlinearly at low intensities and saturates above about 1.5 W/cm−2. This distinct nonlinearity is caused by a significant energy dependence of optical to thermal cross sections of trap states in the band gap. Assuming a three‐level‐rate equation model, grain boundary trap densities were evaluated.