Apparatus for determination of particle sizes and/or distributions of particle sizes

摘要

Apparatus for determination of particle sizes and/or distributions of particle sizes comprises a light source which radiates parallel light of high coherence through a measuring zone (14) in which the particles to be measured are disposed. The light beam diffracted at the particles is imaged by an imaging device (18) onto a photo-detector (20) which is coupled to an evaluating unit (22). The imaging device (18) is provided with several different focal lengths which can be selectably brought into the beam path of the overall device. At the start of a measuring process a control device causes the distribution of particle sizes initially to be determined with the use of the longest focal length and employment of evaluation mathematics valid for this focal length. After the evaluating unit (22) has ascertained that particle size fraction into which the largest measured particles fall, another focal length is, if necessary, brought into the beam path of which the measuring range still just reaches over the largest measured particles. Thereupon the apparatus determines the distribution of particle sizes again with the use of the previously selected focal length and evaluation mathematics matched to this, the distance between the photo-detector (20) and the imaging device (18) being matched to the respective focal length disposed in the beam path. Distributions of particle sizes of unknown samples are thus measured with higher resolution and accuracy.