In calibrating instruments that measure reflectance and transmittance to a few tenths of a percent, it is desirable to have a sample that is smooth, uniform, nonabsorbing, and stable, so that the measured reflectance and transmittance will add to unity, and no light will be either absorbed or scattered outside the collecting optics. A properly prepared, thin, transparent, high index' film (such as CeO2 or TiO2) on a fused quartz substrate is suitable for a calibration sample. Equations are given to calculate the reflectance and transmittance both at normal incidence and nonnormal incidence, as well as to correct for multiple reflections in the substrate and reflection from the back surface of the substrate. Factors such as aging and optical thickness nonuniformity, which can introduce systematic errors into the calibration, must be minimized by proper choice of materials and preparation techniques.
Jean M. Bennett and E. J. Ashley, "Calibration of Instruments Measuring Reflectance and Transmittance," Appl. Opt. 11, 1749-1755 (1972)