1. A method of determining a changing spatial distribution of particles at multiple points in time following to each other at intervals, the method comprising the following steps for each of the points in time: recording real two-dimensional images of the particles with different mapping functions at the respective one of the points in time; providing an estimated spatial distribution of the particles; calculating virtual two-dimensional images of the estimated spatial distribution applying the different mapping functions; determining differences between the virtual two-dimensional images and the real two-dimensional images of the same mapping functions; and varying the estimated spatial distribution of the particles for reducing the differences to obtain an approximated spatial distribution which is approximated to the actual spatial distribution of the particles at the respective one of the points in time; wherein the estimated spatial distribution of the particles for at least one of the points in time is provided in that the locations of the individual particles in an approximated spatial distribution obtained for one other of the points in time are shifted dependently on how the locations of the individual particles have changed between at least two approximated spatial distributions obtained for at least two others of the points in time.