Reflection Characteristics and Fluorescence of Powdered Materials

Abstract

Some experimental results on the reflection characteristics of powdered materials such as powdered glass and phosphors are described.
As for glass, absorption coefficients of several glass plates differing in chemical composition are first measured, then the plates are powdered, screened with silk cloths of 110 and 250 mesh. The reflectance of the powder is determined by comparing with that of smoked magnesium oxide measured beforehand.
For those samples of powder, the reflectances of which are less than 0.75, the grain size affects the reflectance very little. In the spectral region where the absorption coefficient is greater than 1×10³cm^-1, the reflectance ranges from 0.05 to 0.1.
If the layer of the powder is thick enough for the transmission to be ignored, and at the same time, if the reflectance is from 0.05-0.1 to 0.75, Kubelka's formula is found valid for the determination of absorption coefficient of the powder concerned. It is also proved experimentally that the absorption coefficient can be estimated by assuming the scattering constant to be 210cm^-1 regardless of the refractive index of the material and the wavelength of the incident light.
The ratio of the intensity of fluorescence emitted by the powder toward the source of incident light to the intensity of excitation can be much greater than the ratio taken on a transparent plate if observed under certain geometrical conditions.
By applying the above results to the data obtained on powdered phosphors, the energy efficiency of fluorescence is found the greatest in the spectral region in which the absorption coefficient is from 10²cm^-1 to 10³cm^-1.