Direct measurements of the deep defect density in thin amorphous films were made with the aid of the so-called absolute constant photocurrent method. It was explained how the optical (photocurrent) absorption spectrum could be measured directly in absolute units (/cm); without additional calibration, and undisturbed by interference fringes. Computer simulations were used to demonstrate the absolute precision of the measurements and to clarify residual interferences which were sometimes observed. The latter were shown to be direct reflections of an inhomogeneous defect distribution.M.Vanecek, J.Kocka, A.Poruba, A.Fejfar: Journal of Applied Physics, 1995, 78[10], 6203-10