Abstract: A tubular test chamber of square cross-section minimizes reflection of stray light from the interior surface of the test chamber onto the detector during measurement by an optical system which is based upon right angle light scattering by the dust particles of a test sample. A rotary solenoid is utilized to open a trap door through which a sample is dropped into the test chamber during energization so that the internal spring of the solenoid can be used to bias the door closed upon deenergization thereof. The electronic circuitry which supports the measurement process provides an output in the form of a single "characteristic number" for each sample measured and has a novel feedback circuit used in auto-zeroing of the device.