Method and apparatus for measuring particle size

Described is a method for measuring the particle size of ultra-small particles (202) which are suspended in a fluid. Laser light (121, 123) is guided via a first optical path (160) to a suspension (201), and light (125) scattered by the particles (202) in the suspension (201) is guided to an optical detector (140) via a second optical path (170). The optical detector (140) generates an electrical measuring signal (Sm) which is presented to a signal processing device (300). According to the concept of the invention, the signal-to-noise ratio of the measuring signal (Sm) is lowered by presenting to the optical detector (140) a noise signal in the form of a light signal (126) via a third optical path (180), so that the measuring signal (Sm) is always a heterodyne measuring signal. As a result, the uniformity, reliability and accuracy of the measuring signal is improved.Applied Science