Technique for measuring the refractive index and its change with temperature in the infrared*

A simple method for measuring refractive index in the infrared is described. It uses autocollimation from the front surface of a prism and requires only that the receiver be moved to measure a new deviated ray when either the wavelength of the source or the temperature of the sample is changed. An error analysis that includes consideration of the evacuated sample space and wedge in the chamber windows shows that errors as low as one part in the fourth decimal place can be obtained.