A particle sizing method which allows for counting and sizing of particles within a colloidal suspension flowing through a single-particle optical sizing sensor SPOS apparatus using pulse height detection and utilizing non-parallel and non-uniform illumination within the sensing region of the flow cell. The method involves utilizing a deconvolution process which requires the SPOS apparatus to be characterized during a calibration phase. Once the SPOS apparatus has been characterized, the process of deconvolution after a data collection run, recursively eliminates the expected statistical contribution to the pulse height distribution PHD histogram in all the lower channels from the highest channel height detected, and repeating this for all remaining channels in the PHD, removing the contributions from largest to smallest sizes.