A type of optical particle probe that measures the forward scattering caused by each particle or ensemble of particles intercepting a laser beam. The scattering is converted to an equivalent size via calibration and scattering theory for spheres. Examples are the passive cavity aerosol spectrometer probe, normally for sizing 0. 1–3. 0-μm diameter aerosols, the forward scattering spectrometer probe, normally for sizing either 0. 3– 20-μm or 0. 5–47-μm aerosols and cloud droplets, and the optical cloud drop spectrometer, normally for sizing 2–200-μm cloud droplets.