Refractive indices of infrared-transmitting substrate materials calculated using standard spectrophotometer transmittance curves.

A method is described for quickly determining refractive indices of ir-transmitting substrates and window materials. Using two or more known sample thicknesses, spectrophotometer transmittance curves are run on standard laboratory equipment. Equations are developed that permit direct computation of the index from these data. The technique is applicable to other wavelength regions as well and can be extended to be used in the determination of indices of liquids. Calculations can be continued through regions of anomaly, i.e., through strong spectral absorption bands if desired. The method is mathematically rigorous, and accuracy of computed results will usually depend only upon spectrophotometer transmittance reading and sample thickness measurement accuracies. The third decimal place is attainable when suitable samples are available. A dispersion curve for polyethylene is presented, as well as approximate indices for several plastics at randomly selected wavelengths. We have found the method to be a very useful laboratory tool.