There are two common methods for referencing refractive index measurements: absolute and relative. Absolute measurements are made using vacuum as the reference material, and relative measurements use air as the reference, normally at Standard Temperature & Pressure (STP), which is 20 degrees C and 1 Atm. Further, wavelengths are also measured in a specified medium: the vacuum wavelength of the red He:Ne laser line is 0.632991 m in vacuum and .632816 in air at STP for example.

In ZEMAX, we define a "System" Temperature and Pressure for each optical design under the System...General...Environment tab:



There are two important points to note:

  • Wavelengths are always defined as being measured in 'air' at the System Temperature & Pressure. Therefore, if the System Pressure is zero, wavelengths entered in the Wavelength dialog box are vacuum wavelengths, and if the pressure is 1 (or any number other than zero) then wavelengths are measured in 'air' at that temperature and pressure
  • When the 'glass' column of a surface is left blank, it is assumed to be 'air', and the refractive index of 'air' is always exactly 1 at all wavelengths.

Note we write 'air' in quotation marks, because it is air at the System Temperature and Pressure. If the pressure is zero, then 'air' is a vacuum! In the rest of this article we will drop the emphasis for simplicity.

Note also that the surfaces in a design can be at different temperatures and pressure than the System values. There are no restrictions on the temperature or pressure a particular surface can be at. 

Most lens designs in ZEMAX utilize glasses which are found in a glass catalog file (e.g. N-BK7 from the SCHOTT.AGF catalog file). The index of refraction of such a glass at any given wavelength is determined using the appropriate dispersion formula and the corresponding dispersion coefficients for the glass. However, these coefficients provide values for the refractive index relative to air at the reference temperature of the glass (T0) and at a reference pressure of 1 atmosphere (P0 = 1 atm). How does ZEMAX then calculate the index when the operating temperature (T) and pressure (P) of the glass differ from the reference values?