Real Time Measurement of the Size Distribution Of Particulate Matter by a Light Scattering Method

The body of information presented in this paper is directed to those individuals concerned with the measurement of the size distribution of particulate matter in air. The light scattering instrument described herein is characterized by the fact that it can accurately size particles almost independently of their index of refraction. The basic concept involves the simultaneous measurement of the intensity of light scattered by a single particle at two small scattering angles. The ratio of the two intensities is directly related io ine size of ihe pariicle, and for scattering angles of 5° and 10° the effective range of the instrument is 0.2 to 4 μm. The air flows through the optical system at such a rate that approximately 25 μs are required to determine the size of each particle, and concentrations as high as 10⁴ particles/cc can be measured without dilution and without serious coincidence effects. By employing a multichannel analyzer as the data storage and readout device it is possible to detect changes in particulate size distribution within a few seconds. Calibration of the instrument has been performed using polystyrene latex spheres and materials having a wide range of index of refraction and shape including carbon black, iron oxide and spores.