The photoelectric measurement of coal dust stains on filter paper

Abstract

Deposits of coal dust collected on filter paper from airborne clouds have been studied in connexion with their light screening power. This depends on the nature of the paper as well as the size and concentration of the particles. Nine kinds of paper were investigated, two of them in detail. Some idea has been obtained of the qualities necessary in a filter paper in order that the dust deposits will have maximum optical density. Theoretical expressions are derived for light screening according to how the particles are dispersed in the paper. Maximum screening would result if each particle shadowed individually, but this is only possible in very sparse deposits; otherwise, overlapping of the particles has to be allowed for. Diffraction of light by the particles and the effects of diffusion of light in the paper are discussed.