An apparatus and method are provided for determining particle sizes and/or a number of particles in a fluid. The method includes emitting laser beams from a laser source, through a lens optical system having a laser absorption device, which absorbs unscattered laser beams emitted by the laser source. The laser absorption device is encompassed by first, second, and third lenses, wherein the first lens directs scattered laser beams onto the second and/or the third lens, wherein the second lens directs laser beams that impinge on the second lens proceeding from the first lens onto a first photodetector that generates first measurement signals, wherein the third lens directs laser beams that impinge on the third lens proceeding from the first lens onto a second photodetector that generates second measurement signals. An evaluation device is used to evaluate the measurement signals of the two photodetectors to determine the sizes of the particles.