The photoconductivity of semiconductors was considered under the following general conditions: the surface recombination velocity was not equal to zero, the space charge near the surface is taken into account, the value of absorption coefficient was not limited. Solving both the continuity and Poisson equations for an illuminated thick specimen, we obtained a solution which suggests a new combined measurement in order to determine various optical constants. The absorption coefficient and the diffusion length of electrons in CZ grownn-type Si (Na= 1012-1013cm−3) were measured at room temperature, using the new technique.