Thermal Index changes

 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 temperatureWavelength, λ, in µmP is air pressure i PascalsP0 is Normal pressure (0.101325*1e-6 Pascals)T is temperature in C°The Dnair/DT is about -1∙10-6 /C°. Ohara has a reference.Relationship between relative and absolute index of refractionAbsolute index of refraction is used if your lens is in a vacuumRelative index of refraction is used if your lens is in airSchott, an optical glass manufacturer, reports both absolute and relative index of refraction.Absolute index of refractionThe full nonlinear model of change in index of refraction with change in temperature isThis equation is used by Zemax to model the changes in index of refraction. Embedded in this equation is temperature changes to air's index of refraction.Infrared index of refraction changes with temperatureThe linear component of Dn/DT for infrared materials is usually an order of magnitude larger than visible glasses. ThereforeDn/DT absolute ~ Dn/DT relativeIn 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:Schott Report #19Other visible glass manufactures:OharaCDGMHoya