This paper presents the calculation of the optical properties of thin films of CdS from spectral transmittance measurements using the method of Swanepoel.CdS thin film was grown by evaporation technique which has better advantages in optical and crystalline properties compared to other manufacturing techniques as chemical bath deposition (CBD) and sputtering.The experimental procedure is presented for the transmittance spectrum of CdS thin film through a VIS-IR Oriel spectrophotometer that was automated using virtual instrumentation for data acquisition.After applying the mathematical Swanepoel model, the optical properties of the CdS thin film were obtained: 1499,08nm thick, an absorption coefficient of 2x104 cm -1 for a wavelength of 500nm and an optical gap of 2,38eV.These results indicate that the CdS thin film is suitable for use as optical window in the manufacture of solar cells.