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.

The September 19, 2005 release of ZEMAX deleted an obsolete control: the  "Use Temperature, Pressure" checkbox on the Environment Tab of the General Data dialog. When this box was checked, ZEMAX adjusted the index data to the current environment. Since this adjustment is always correct based upon the documented way of defining dispersion data, the checkbox is considered obsolete and was removed in the September release.

Unfortunately, we became aware that at least one user had incorrectly defined the reference temperature of some custom glass data, and this change therefore altered the incorrect index data in an unexpected way. This problem will only occur if

a) The "Use Temperature, Pressure" checkbox was off,

b) User defined dispersion data had been added that was referenced to air at 20 degrees C and 1.0 ATM (which ZEMAX uses as the reference if the checkbox is off), and

c) The user defined dispersion data incorrectly defined the glass reference temperature as the actual working temperature of the glass rather than the temperature to which the index data was referenced, which must be 20°  if the checkbox is off.

In summary, if ZEMAX's own thermal adjustments were disabled, and if the reference temperature of any custom glass data was not 20 degrees, then the September 19 release will return index data different than earlier versions of ZEMAX. To maintain back compatibility with these incorrectly set up files, the checkbox has been reinstated and renamed “Adjust Index Data To Enviornment”.

It is recommended that this switch always be checked ON.

Warning messages will now be issued if this feature is checked OFF and the glass reference temperature does not match the surface temperature.It is strongly recommended that any such warning message be addressed and resolved.

Summary

• The refractive index of air (a blank entry in the material column) is always exactly 1.0 at all wavelengths at the system temperature and pressure.
• If the glass reference temperature and pressure of 1.0 atmosphere does not match the surface temperature and pressure, ZEMAX must adjust the index using the method described.
• There are 3 important temperatures and pressures:
• The system temperature and pressure is the air temperature and pressure that has an index of exactly 1.0, and wavelengths are measured in this air.
• The surface temperature and pressure is the environment in which a particular glass is used.
• The reference temperature defines the air to which the index given by the dispersion formula is relative. It also defines the media in which the wavelength argument of the dispersion formula is measured.

References

ZEMAX User's Manual, Chapter 19, "Thermal Analysis"