A portable dust particle counting and size analyzing instrument with immediate readout would enable number‐vs‐size distributions of airborne particulate matter to be made known quickly in locations difficult of access, such as underground mines. This device operates on the principle of light scattering by individual particles. Battery operation, solid state circuitry, and miniaturized optical design allow the entire instrument to be housed in a briefcase. Total weight is less than 7 kg and over‐all dimensions are 43×33×20 cm. The shaping of the hole edges and conduits and the location of critical flow channels within the photometer head permit stable flow of aerosol past the light field volume where light scattering takes place. A photomultiplier tube receives the pulses of scattered light. The resulting current pulses, height discriminated and continuously integrated, activate milliammeters representing particular (adjustable) size ranges and calibrated in numbers of particles per cubic centimeter.