Experimental determination of the effect of temperature on refractive index and optical path length of glass

With the Fizeau interferometer method, the change in optical path length with temperature is measured at six wavelengths for twenty new Corning optical glasses. The sample, a polished block with a few seconds of wedge, is held in a double-walled chamber with temperature control provided over the range -20 degrees C to +80 degrees C by a circulating water-glycol mixture. A Gaertner interferometer viewing apparatus is modified by addition of a monitor head that permits simultaneous photoelectric fringe detecting and viewing. Measurement of thermal expansion with the Plummer-Hagy high precision dilatometer permits the extraction of the temperature coefficient of absolute refractive index from the optical path measurement. The coefficient of refractive index relative to standard air at the temperature of the glass is found by further data reduction using Edlén's equations. The experimental values of the index change as a function of temperature are curve-fitted by a FORTRAN program, and the slopes are computed to give instantaneous values of the coefficients. The average residual for the fit is about 1.0 x 10(-6). The precision of the absolute coefficient values is estimated to be +/-0.05 x 10(-6). Representative data for one glass are presented to illustrate the results available from this work.