Method for detecting soiling

A method for determining a soiling of a side window of a chamber containing a fluidic sample with dispersed particles, wherein the particles are irradiated with light, in particular with a laser, through an inlet window of the chamber, wherein a force is exerted on the particles using the light, which force influences a movement of the particles that is dependent in particular on a size of the particles, wherein the movement of the particles is detected with a camera arranged outside of the chamber based on a scattered light of the particles which passes through the side window, wherein a size of the particles is ascertained via a speed of the particles, after which a target scattered light intensity is calculated based on an intensity of the light acting on the particles and the ascertained size of the particles, after which the target scattered light intensity is compared with a measured actual scattered light intensity and, based on a difference of the target scattered light intensity from the actual scattered light intensity, the soiling of the side window is determined.