1. A device for determining particle sizes of particle streams through a measuring volume, the device comprising: an optical measuring arrangement detecting the measuring volume; and a computing means connected to the optical measuring arrangement, wherein the optical measuring arrangement comprises a first optical measuring system with a first dot matrix sensor and with a lighting means, wherein the lighting means transilluminates the measuring volume and the first dot matrix sensor and the lighting means form a transmitted-light arrangement, and wherein the computing means determines projection areas of particles within the transilluminated measuring volume from image data of the first dot matrix sensor, and wherein the optical measuring arrangement comprises a second optical measuring system with a second dot matrix sensor for detecting a diffraction pattern of the particles, and wherein the computing means determines a size distribution of the particles in the measuring volume on the basis of the projection areas and the diffraction pattern, wherein the computing means determines the size distribution from particle sizes determined on the basis of the projection areas, and the computing means determines the particle size on the basis of the detected diffraction pattern, wherein aberrations of an optical system of the second optical measuring system, including a radial center shift of a spectrum as a consequence of one or more of a lateral and a radial shift of an object in an object space, is represented on the second dot matrix sensor as a function of a radial distance from a location of an optical axis and illumination in the object space is scanned with constant radius, wherein a shifted and weighted diffraction pattern is accumulated over a round angle, and wherein a mean dot spread function is calculated by integration over the radius via said computing means.