AbstractThe response of optical particle counters to nonspherical particles is analyzed theoretically and on the basis of experimental results. Theoretical approximations valid for particle diametersd≪ λ andd≫ λ (where λ is the wavelength of light) are used to derive some general predictions about the influence of the particle shape on light scattering. These predictions are compared with experiments on six optical particle counters using nonspherical particles. The instruments differ in the kind of illumination (laser or incandescent light), the mean scattering angle and the receiver aperture. When the particle size is smaller than the wavelength the light‐scattering diameter of a nonspherical particle comes very close to its volume‐equivalent diameter. For irregularly shaped particles larger than the wavelength better conditions than giving a “projected area” response cannot be achieved with an optical arrangement for single‐pa