Abstract: 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.

Abstract: For the determination of fine particle size distributions by scattered light measurements, an assembly of suspended particles is illuminated with coherent light, the scattered light pattern is focused on a light sensitive multi-element detector of a registration device and is transformed there to electrical signals. From the digitized electrical signals, the particle size distribution is computed with a computer. For the simplification of the setup and for the improvement of the accuracy of the computable particle size distribution, it is provided that the registration device is a video camera and that the particle size distribution is computed via the solution of a Fredholm integral equation from the signals produced by the light intensities at the pixels of the video camera. The video camera is a CCD camera.