Direct measurement of the deep defect density in thin amorphous silicon films with the help of the ‘‘absolute’’ constant photocurrent method is demonstrated here. We describe in detail how the optical (photocurrent) absorption spectrum can be measured directly in absolute units (cm−1) without additional calibration and undisturbed by interference fringes. Computer simulation was performed to demonstrate absolute precision of the measurement and to explain residual interferences which are sometimes observed. The residual interferences are shown to be direct fingerprints of an inhomogeneous defect distribution. ©1995 American Institute of Physics.