Registration Probabilities and Pulse-Height Distributions of Coincidences in Optical Particle Counters

In conventional optical particle counters chains of particles' coincidences may occur as a result of continuous view volume illumination and sampling. Because of these coincidences, probability of the presence of several particles in a view volume given by the well-known Poisson distribution cannot be interpreted as a coincidence registration probability. Particle registration probability distribution accounting for chain coincidences is calculated for optical counters with the first maximum of the counter pulse chosen for counting. Registration probability of the doublets is obtained numerically also for counters with the choice of the global maximum of the counter pulse for counting. Pulse-height probability density functions (pdf) are calculated for square, triangular, and Gaussian pulses. It is shown that pdfs of apparent multiplets are resolved in counters with rectangular response pulses and sufficiently high resolution. However, in counters with any nonrectangular response pulses the coincidences have overlapping pulse height distributions. It is shown that counters with the Gaussian pulses allow measuring 30% higher aerosol concentrations compared with the square pulse counters. Aerosol size distribution distortions invoked by coincidences are estimated. A method of counter view volume determination using coincidences is suggested.