Many interpolation equations used in the calibration of radiation ther
mometers are semi-empirical in nature, providing little information on
the characteristics of the thermometer. In this paper a systematic ap
proach is taken in the derivation of two general interpolation equatio
ns, the first based on Wien's law and the second on Planck's law. Beca
use these equations are based on physical principles they can be used
with increased accuracy and confidence over semi-empirical equations,
and their limitations are easily understood. The two general equations
provide a framework for the development of specific equations for rad
iation thermometers and for the understanding of the relationships of
interpolation equations to the effective wavelength formulation curren
tly employed in radiation thermometry.