Thermally cycling visible index of refraction (n) is not as easy as changing a thickness with temperature. The index of air is dependent on temperature as well. Since optical power is really dependent on the delta index of refraction, in some cases temperature changes in air need to be considered as well. For visible glasses where the index change with temperature (Dn/DT) is very small, the index change of air HAS to be includedl. Air index of refraction as a function of temperature Wavelength, λ, in µm The Dnair/DT is about -1∙10-6 /C°. Ohara has a reference. Relationship between relative and absolute index of refraction Absolute index of refraction
Infrared index of refraction changes with temperature
In addition Dn/DT in the infrared is very difficult to measure and they have huge error bars! So a full nonlinear Dn/DT model is usually not used, but rather a single numeric value is used => Infrared materials model index changes with a single Value of Dn/DT instead of a bunch of coefficients. Reference: Other visible glass manufactures: |