Sensor apparatus for single-particle optical sizing of particles in a fluid suspension, comprising: means establishing flow of said suspension through a physically well-defined measurement flow channel (35, 166); illumination means (40, 160, 194, 250, 262) comprising a laser light source and a lens for effectively directing a relatively narrow beam (41, 162, 252, 261) of light, the incident beam alone in conjunction with the front and back windows (36, 37) of the measurement flow channel defining an optical sensing zone, the side walls (38, 39) of said measurement flow channel (35, 166) being of no consequence with respect to definition of the optical sensing zone, said beam (41, 162, 252, 261) of light and said optical sensing zone being of such size relative to the size of said measurement flow channel (35, 166) that said sensor apparatus responds to only a fraction of the total number of particles flowing through said measurement flow channel (35, 166), whereby said sensor apparatus responds effectively to a relatively concentrated fluid suspension, and said beam (41, 162, 252, 261) having a maximum intensity portion and a continuum of lesser intensities for positions spaced transverse to said maximum intensity portion, whereby some of said particles have trajectories through said maximum intensity portion, others of said particles have trajectories through said lesser intensity positions, and still others of said particles may have trajectories outside said zone; detector means (D LS, D LE) for photo-detecting light from said zone to provide pulse height signals, each responsive to a particle flowing through said zone, said pulse height signals being functions of the sizes and trajectories of detected particles, particles of a given size providing a maximum pulse height signal when flowing through said maximum intensity portion and lesser pulse height signals when flowing through said lesser intensity positions of said zone, said pulse height signals collectively, forming a pulse height distribution PHD; and means (191, 259, 270) for mathematically decomposing said pulse height distribution into a linear combination of individual PHDs corresponding to uniform particles, to extract therefrom a particle size distribution PSD of said particles in said fluid suspension. means establishing flow of said suspension through a physically well-defined measurement flow channel (35, 166); illumination means (40, 160, 194, 250, 262) comprising a laser light source and a lens for effectively directing a relatively narrow beam (41, 162, 252, 261) of light, the incident beam alone in conjunction with the front and back windows (36, 37) of the measurement flow channel defining an optical sensing zone, the side walls (38, 39) of said measurement flow channel (35, 166) being of no consequence with respect to definition of the optical sensing zone, said beam (41, 162, 252, 261) of light and said optical sensing zone being of such size relative to the size of said measurement flow channel (35, 166) that said sensor apparatus responds to only a fraction of the total number of particles flowing through said measurement flow channel (35, 166), whereby said sensor apparatus responds effectively to a relatively concentrated fluid suspension, and said beam (41, 162, 252, 261) having a maximum intensity portion and a continuum of lesser intensities for positions spaced transverse to said maximum intensity portion, whereby some of said particles have trajectories through said maximum intensity portion, others of said particles have trajectories through said lesser intensity positions, and still others of said particles may have trajectories outside said zone; the incident beam alone in conjunction with the front and back windows (36, 37) of the measurement flow channel defining an optical sensing zone, the side walls (38, 39) of said measurement flow channel (35, 166) being of no consequence with respect to definition of the optical sensing zone, said beam (41, 162, 252, 261) of light and said optical sensing zone being of such size relative to the size of said measurement flow channel (35, 166) that said sensor apparatus responds to only a fraction of the total number of particles flowing through said measurement flow channel (35, 166), whereby said sensor apparatus responds effectively to a relatively concentrated fluid suspension, and said beam (41, 162, 252, 261) having a maximum intensity portion and a continuum of lesser intensities for positions spaced transverse to said maximum intensity portion, whereby some of said particles have trajectories through said maximum intensity portion, others of said particles have trajectories through said lesser intensity positions, and still others of said particles may have trajectories outside said zone; detector means (D LS, D LE) for photo-detecting light from said zone to provide pulse height signals, each responsive to a particle flowing through said zone, said pulse height signals being functions of the sizes and trajectories of detected particles, particles of a given size providing a maximum pulse height signal when flowing through said maximum intensity portion and lesser pulse height signals when flowing through said lesser intensity positions of said zone, said pulse height signals collectively, forming a pulse height distribution PHD; and means (191, 259, 270) for mathematically decomposing said pulse height distribution into a linear combination of individual PHDs corresponding to uniform particles, to extract therefrom a particle size distribution PSD of said particles in said fluid suspension.