Spectral emittance and reflectance of powders

The emittance of well-sized corundum powders in the range between 1-30 micro has been measured by ir interferometry in the 7.5-25 micro region of the spectrum (400-1300 cm(-1)). The spectral features persist for all particle sizes with some interesting relative intensity variations. The temperature of the radiating surface is determined by setting the emittance equal to unity at 1035 cm(-1). At this point it is known that both orientations of corundum have a refractive index close to unity and the absorption coefficient is quite small. Thus both the volume and the surface reflectance terms are negligible and hence the emittance is approx. 1. The same powders have been measured in reflectance using a Cary-White spectrometer. A quantitative comparison is made between the two techniques and Kirchhoff's law is shown to apply experimentally.