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**Source: **STP38703S

The basic measurement problems of radiation thermometry are discussed, with emphasis on the physical processes giving rise to the emissivity effects observed in real materials. Emissivity is shown to derive from bulk absorptivity properties of the material. Blackbody radiation is produced within an opaque isothermal material, with partial internal reflection occurring at the surface. The reflective properties of surfaces are discussed in terms of Fresnel's equations, and roughness effects are explained in terms of multiple reflections and Kirchhoff's law. Attention is drawn to the polarization effects implicit in Fresnel reflection and their possible usefulness in radiation thermometry. The effects of atmospheric absorption and emission, dust, and vapors are considered; it is shown that such problems are minimized if spectral bands are restricted to atmospheric windows, with particulates removed by purge gases. The role played by windows in the sight path is also considered, and the effects of, and remedies for, reflected extraneous radiation are discussed. The radiometer measurement equation is developed and discussed as the basis for radiometer instrumentation. The key remaining problems are identified as the determination of spectral emissivity in situ and correction for the effects of reflected extraneous (background) radiation.

**Keywords:**

absorptivity, atmospheric absorption and emission, atmospheric windows, blackbody radiation, complementarity of emitted and reflected radiation, emissivity, Fresnel reflection, high power of , T, Lagrange invariant, optical constants, optical roughness, polarized emission/reflection, photodetection, Planck radiation, radiation thermometry, radiometer measurement equation, radiometric temperature measurement, radiometry, ratio thermometers, reflected radiation, reflectivity, reflection by diffraction, roughness effects, subsurface emission, temperature measurement, window effects, Wien's law

**Author Information:**

Nutter, GD *Assistant Director/Program Supervisor, Instrumentation Systems Center, University of Wisconsin—Madison, Madison, WI*

**Committee/Subcommittee:** E20.02

**DOI:** 10.1520/STP38703S