ZEMAX Reference Temperature

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ZEMAX Users' Knowledge Base - http://www.zemax.com/kb

What is the "Reference Temperature"?
http://www.zemax.com/kb/articles/73/1/What-is-the-"Reference-Temperature"?
By Mark Nicholson
Published on 10/28/2005

When a glass is defined in a glass catalog, a "reference temperature" is also entered. The nature of this
temperature is sometimes misunderstood. This article explains the correct usage of the reference
temperature.

What is the Reference Temperature?
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
(λ) = 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.

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-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 "Adjust Index Data to Environment" Switch

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

Summary and References

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"

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