Based on common measurements of refraction, it is found that the precision of minimum deviation condition is better than that of vertical incidence condition according to our analysis of the sensitivity and uncertainty of them.Analysis shows that for the same deviation and apex angle, the uncertainty of the minimum deviation method is only 1/2 of the uncertainty of the vertical incidence method. Another factor which would also effects uncertainty is the bandwidth of optical source. From analyzing dispersion of ZnSe by Herzberger dispersion equation, it is found that the dispersion of ZnSe at the wavelength of 5~10 μm is approximately 5×10-4 μm-1. For a given accuracy, the optical source’s bandwidth is less than 20 nm.