With this counting method, the oil flows past a light source and, depending on the size of the individual particles, reduces the intensity of the light ray detected by a photodiode. The change in intensity of the light ray triggers voltage changes on the photodiode, which are a direct measure of the particle size. However, a prerequisite for correct determination is that the particles pass the light ray one after the other. With this measuring method, air bubbles and water droplets can falsify the result.
Since laser sensors from different manufacturers emit light of different brightness, the "bright" sensors may have relatively high numbers, especially in fresh oils, while older sensors do not see such subtle differences in oil-soluble "particles". Therefore, differences in counting fresh oils in different laboratories using laser sensors and automatic particle counters (APC) will often occur even despite the best calibration of the equipment.