1. A method for computing small particle size distributions, comprising: providing a reference matrix of pre-computed or pre-measured reference vectors, each reference vector representing a discrete particle size of a particle size distribution of particles contained in a dilute colloid, each reference vector representing a reference extinction spectrum over a predetermined wavelength range; providing a measurement vector representing a measured extinction spectrum of the sample particles in the sample colloid, wherein the measured extinction spectrum has been spectrophotometrically measured; determining at least one of a particle size, the particle size distribution and at least one particle concentration of the particles in the sample dilute colloid, using the reference matrix, the measurement vector and linear equations; and smoothing results calculated for the determining at least one of a particle size, the particle size distribution and at least one particle concentration of the particles in the sample dilute colloid.