Efficiency of light-scattering aerosol particle counters.

The counting efficiency as a function of particle size has been calculated for a particular forward scattering aerosol particle counter that uses white light illumination for spherical particles with indexes of refraction 1.33-0i, 1.40-0i, 1.50-0i, 1.592-0i, 1.50-0.05i, 1.50-0.1i, 1.50-0.5i, and 1.95-0.66i. The counting efficiency differs from 100% because of statistical effects: photoelectron statistical broadening, variation in aerosol speed through the illuminated volume, nonuniform illumination of the sampled aerosol, and nonuniformity of the photomultiplier tube photo cathode surface. These spectra broadening effects have been determined for the particle counter by measurement of monodisperse aerosols, and the efficiency calculation is based on these measurements. For the particle counter studied, the efficiencies vary from 70%; to slightly over 100% in the size ranges where the counter response is single valued. The efficiency for materials for which the response is not single-valued fluctuates wildly in those size ranges from about 20% to more than 10,000%. The efficiency as a function of particle size discriminator level setting is sensitive to particle refractive index and to the aerosol size distribution locally about that particle size. The method for calculation of the efficiency can be applied to light-scattering aerosol counters available commercially.