Measurement of total hemispherical emittance of transparent materials at low temperature

A transient calorimetric method of measuring the total hemispherical emittance of transparent materials in the temperature range 75 degrees K to 312 degrees K is described. Measured emittance values with an estimated accuracy of +/-5% are presented forvarious thicknesses of NaCl and fused quartz. The emittance of NaCl decreased from 0.67 at 75 degrees K to 0.34 at 312 degrees K. The emittance of quartz was approximately constant at 0.73 over the same temperature range. Increasing the thickness of NaCl resulted in slight increases in emittance; no thickness effect was observed in the quartz. Total normal emissivity values calculated for NaCl from optical constants were not appreciably different from the measured total hemispherical emittances.