MEASUREMENT OF THE EMISSIVITY OF QUARTZ GLASS.

The technique employed for measuring the total emissivity of a material partially transparent to thermal radiation consists in measuring the temperatures of the sample and of a blackbody at which the radiant fluxes are equal. The apparatus consists of a water-cooled blackened vacuum chamber accommodating a tubular graphite furnace with a partition in the center. The furnace is heated by an inductor fed from a high-frequency generator. A sample in the form of a disc is suspended inside the upper section of the graphite furnace used as the blackbody; when the temperature stabilizes, the graphite furnace is dropped and the sample screened by a shutter. The measurement time is reduced to 40 ms by using a fast recorder. The total normal emissivity of grade KSG quartz glass was measured from 700 to 1300 K on samples 2 to 10 mm thick. Data are also given for other grades of quartz glass. Measurement errors are estimated.