Apparatus for the Measurement of the Normal Spectral Emissivity in the Infrared

The measuremen t of the spectral emlSSlVl ty of heated substances may be accomplished by comparing the energy emitted by the substance under study 'with that emitted by a standard substance for radiation at various wavelengths. For such a standard one usually uses a blackbody, that is, a hollow body provided with a hole for viewing the radia tion inside. The inside surface of the blackbody must be maintained at a uniform temperature. The wavelength separation may be accomplished using a double-beam monochromator or a singlebeam monochromator. Th e specimen under investigation may be formed in to a rod, bar, ribbon , or other shape. Some workers have even formed the specimen into a blackbody. For such a specimen both the in slde surface (the blackbody) and the outside surface (the specimen) are observed . The double-beam monochromator and associated method have certain advantages especially with relation to the elimination of the effects of absorption by carbon dioxide and water vapor, but the optical adjustment is critical as compared with a single-beam instrument. Furthermore through the use of a single se t of optics and equal path lengths in the single-beam monochromator the effects of both atmospheric absorption and differential reflectivity in the auxiliary optics are practically eliminated. The apparatus described herein is of the latter type. The specimen may vary in temperature from poin t to point depending upon its shape, size, and method of heating. If made into the form of a blackbody the inside temperature may dIffer from that on the outside. A convenient shape is that of a long thin ribbon heated electrically by contacts at the two ends. The data presented in this paper apply to samples of this type for which the temperature is measured by means of thermocouples in metallic contact with the opposite face from the pomt of radifint energy measurement. 2. Apparatus and Method