Refractive index is a dimensionless parameter which describes the ratio of the speed of light in an optical material to the speed of light in either air or vacuum. It is usually written as:

n = c/v

where n is the refractive index, v is the speed of light in the material and c is the speed of light in either air of vacuum. There are two common definitions used of refractive index:

• The index of a glass relative to vacuum is called the absolute index
• The index of a glass relative to air at a specified temperature and pressure (usually 20° or 25° C and 1 Atmosphere) is called the relative index

Which definition is used is irrelevant as long as the same definition is used consistently, everywhere within the optical system. Refraction depends on the index ratio between two materials, and so it does not matter whether the absolute or relative index is used, as long as the same definition is used consistently.

ZEMAX, in common with the vast majority of the optics industry, works with the relative index definition. In ZEMAX, you can define a system temperature and pressure, such as 20 degrees C and 1.0 ATM. This tells ZEMAX that any "air" space (any surface or object where the GLASS or MATERIAL property is blank) has an index of exactly 1.0. All wavelength data is referenced to air at the system temperature and pressure.

The refractive index is generally a function of wavelength. ZEMAX uses dispersion formulas of the form n(l) = some polynomial. The formula should yield a value for n that is the index of refraction relative to air at a pressure of 1.0 at some temperature called the glass reference temperature. It doesn't matter if the actual glass as measured is cold or hot, or in vacuum, or whatever. All that matters is that the resulting index computed from the dispersion formula be relative to air at 1 atmosphere pressure at the reference temperature, and that the dispersion formula is a function of the wavelength in micrometers as measured in air with pressure = 1.0 atmospheres at the reference temperature. The formula for air that ZEMAX uses is given in the ZEMAX User's Guide.

The refractive index is also generally a function of temperature. The variation in index is modeled using a non-linear formula. For a full description, see the ZEMAX User's Guide.

ZEMAX optionally allows each surface to have a surface temperature and pressure that is different from the system temperature and pressure. Once ZEMAX knows the system temperature and pressure, wavelengths, surface temperature and pressure, glass reference temperature, dispersion,  and thermal variation data, ZEMAX can compute the correct relative index to use for ray tracing. The method involves the following steps:

-Scale the wavelength to air at the reference temperature of the glass and a pressure of 1.0 atmosphere.

-Compute the relative index of the glass at the reference temperature from the dispersion formula.

-Compute the index of air at the reference temperature of the glass.

-Compute the absolute index of the glass (relative to vacuum) at the reference temperature of the glass.

-Compute the change in absolute index of refraction of the glass at the surface temperature.

-Compute the index of air at the system temperature and pressure.

-Compute the index of the glass relative to the air at the system temperature and pressure.

The end result, which is the index of the glass at the surface temperature and pressure relative to air at the system temperature and pressure, is what is used by ZEMAX for ray tracing.