A calorimetric method is described which has been used to measure the total hemispherical emissivity of crystal-oriented sapphire. The results of two series of measurements are presented, one with the crystal C axis parallel to the surface normal (0° orientation) and the other with the C axis perpendicular to the normal (90° orientation). In the temperature range of -65° to 100°C the emissivities vary from 0.496 to 0.567 for the 0°-oriented sapphire and from 0.471 to 0.552 for the 90°-oriented material. Error analysis indicates that these results should be accurate to better than ±3%.
A. M. WITTENBERG, "Total Hemispherical Emissivity of Sapphire," J. Opt. Soc. Am. 55, 432-435 (1965)