A device is provided that can be inserted into a spectrophotometer, in order to measure the thickness and refractive index of a thin film that is on a sample plate. A pair of identical parallelogram prisms diverts the spectrophotometer beam to measure the attenuated total reflection, and returns an output beam that is in the original beam path, independent of wavelength and rotation angle. The attenuated total reflection of the thin film sample plate is measured in a prism coupling geometry, as a function of wavelength and angle. From this data, combined with normal incidence transmission data, the thickness and refractive index can be extracted.