We report a simple interferometric method for simultaneous, insitu measurement of both the thickness and the depth resolved optical losses of thin transparent films. The experimental arrangement is simple requiring only a laser and a detector regardless of the substrate. The originality of our method is based on the evaluation of the optical losses from the envelopes of the laser signal, while the thickness is measured conventionally by counting the number of signal periods. The technique has been tested while growing ZnO films in a planar rf magnetron sputtering system. Using the above method, the evolution of optical losses is easily observed during deposition, leading to the possibility of a real time control of the film quality. Both theoretical calculations and experimental results are presented in this letter.