Any of a class of instruments for sizing and counting large numbers of individual aerosols or hydrometeors to characterize populations, or also for imaging and classifying the shapes of individual hydrometeors by measuring the optical illumination, scattering, or attenuation of a laser beam by each particle and recording the result electronically. Included are optical scattering probes and optical imaging probes. Primarily airborne instruments, they can be adapted for stationary use at the surface for precipitation or fog monitoring, in some cases with forced ventilation to create a particle flux.