Care must be taken to ensure high emissivity to minimize error.

A laboratory blackbody cavity has been designed and built for calibrating infrared radiometers used to measure radiant temperatures in the range from about 200 to about 273 K. In this below-room- temperature range, scattering of background infrared radiation from room-temperature surfaces could, potentially, contribute significantly to the spectral radiance of the blackbody cavity, thereby contributing a significant error to the radiant temperature used as the calibration value. The spectral radiance error at wavelength λ is given by

[l – ε(λ)][B(T_c,λ) + B(T_a,λ)],

where ε(λ) is the effective spectral emissivity of the cavity, B(T,λ) is the ideal spectral radiance of a body at absolute temperature T according to Planck’s radiation law, T_c is the temperature in the cavity, and T_a is the ambient temperature. Examining the above expression shows that by making ε(λ) as close as possible to unity, one can minimize the spectral-radiance error and the associated radiant-temperature error. For example, it has been calculated that to obtain a radiant-temperature error of 1 K or less at a cavity temperature of 200 K, ambient temperature of 300 K, and wavelength of 6 μm, one has ε(λ)>0.999 (see Figure 1). A 1 K radiant-temperature error is more than sufficient for atmospheric and cloud studies, which is a common application of infrared radiometers.