Methods, systems and computer readable media for computing small particle size distributions of particles in a process stream comprising a sample dilute colloid. A reference matrix of pre-computed reference vectors is provided. Each reference vector represents a discrete particle size or particle size range of a particle size distribution of particles contained in a dilute colloid. Each reference vector represents a reference extinction spectrum over a predetermined wavelength range. A measurement vector representing a measured extinction spectrum of the sample particles in the sample colloid is provided, wherein the measured extinction spectrum has been spectrophotometrically measured over the wavelength range. The particle size distribution and particle concentrations of the particles in the sample colloid are determined using the reference matrix, the measurement vector and linear equations.