A calibrated piston source of light,which simulates a cylindrical-volume luminosity source,has been introduced to measure the absolute concentration of illuminant particles. It is proved that this Piston Source Method is one of the simplest and most convenient ways to measure the absolute concentration of illuminant particles,especially in real time measurements of O2(a1△) in a singlet oxygen generator. The relative error of O2:(a1△) concentration is less than 20%.