Method for measuring mean particle size of the bulk powder using speckle patterns.

We report a method for measuring the mean size of fine particles in a bulk state using the correlation properties of two speckle patterns. With this method, the correlation between the two speckle patterns produced from the same region of the upper surface of the powder bed under two coherent illuminations is measured using double-exposure speckle photography and a cross-correlated method. It is found to be directly related to the surface-area mean diameter of particles over the bed surface and the incidence angle difference between two illuminations. This technique has the advantage of simplicity in the measuring procedure, including the pretreatment of the sample powder.