The pixel position-to-wavelength calibration function of film measurement devices (10) such as spectroscopic ellipsometers and spectroreflectometers may shift due to temperature and humidity changes and mechanical factors. One or more wavelength markers provided by the light source (20) or reference sample (90) may be used to correct the calibration function. The pixel positions (58) of one or more persistent wavelength markers are noted during the calibration process and the current positions of such markers are again noted to account for shifts due to various factors to correct the calibration function (60).
