Measurement of temperature and emissivity of specularly reflecting glowing bodies
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INTRODUCTION
THE measurement of thermodynamic temperature by noncontact methods necessitates knowledge of spectral emissivities. It has been pointed out by several authors II'z'31 that emissivity is a function of both temperature and wavelength. Traditionally, emissivities are measured by comparing the emittance of a radiating body to that of a black-body. However, the techniques that utilize this method do not lend themselves well to measurements of emissivity on liquid metals, alloys, and ceramics, which is the goal of our work at Rice University and the Houston Area Research Center. The emitted light from any glowing body is necessarily polarized, t*] By measuring the ratio of the polarization component intensities, it is possible to extract the emissivity of the body. On this basis Nordine ~ has suggested a method, originally proposed by Tingwaldt, ]4Jfor the measurement of emissivity and thermodynamic temperature. This involves the measurement of polarization states of emitted light at 45 deg with respect to the plane normal. The ratio of the orthogonal components of the polarizations of the emitted light then yield an emissivity for the material at 45 deg. Simultaneous measurement of radiance brightness at this angle and at normal incidence then produce both the normal incidence spectral emissivity and thermodynamic temperature. Our work involves studies of liquid sample droplets generated in an RF levitation coil. The requirement that the observation be limited to light emitted at 45 deg will not be satisfied if the droplet undergoes shape oscillations or rotation. Since the droplet changes surface geometry, it is possible that light reaching the sensor is not emitted at 45 deg alone and has to be excluded by some other methods. The additional requirement of extremely small aperturing to obtain approximately flat regions on the curved sample surface reduces the signalto-noise ratio of these measurements.
(3. E HANSEN, Research Associate, S. KRISHNAN, Graduate Student, R.H. HAUGE, Senior Scientist, and J. L. MARGRAVE, Professor, are with the Department of Chemistry, Rice University, Houston, TX 77251. This paper is based on a presentation made in the symposium "Experimental Methods for Microgravity Materials Science Research" presented at the 1988 TMS-AIME Annual Meeting in Phoenix, Arizona, January 25-29, 1988, under the auspices of the ASM/MSD Thermodynamic Data Committee and the Material Processing Committee. METALLURGICALTRANSACTIONS A
Measurement of the elliptical state of polarization of reflected light from the surface of a radiating sample yields a measure of the reflectivity of the surface. This can be accomplished in several ways. One method involves measuring the ratio of the intensities of the orthogonally polarized components of the reflected electric vector. By measuring this ratio at two different angles of incidence, the reflectivity, emissivity, and related optical properties (such as refractive index and dielectric constants) can be extracted from the Fresnel conditions, t5'61An alter
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