Abstract: An optical particle detector is disclosed using a dark field optical system in which an air sample is illuminated and forward scattered light is collected in a zone centered about but excluding on-axis light. The collected light is then sensed to detect the presence of air borne light scattering particles. Particles of greatest interest are smoke particles. The system includes a self-contained light source and a photo detector suitable for operation by a dry cell. The optics are designed for high light gathering efficiency consistent with minimum space requirements and the internal design minimizes scattered light to achieve maximum smoke detection sensitivity.