Level-crossing detection through measurement of the second factorial moment of photon-counting distribution

A level-crossing detection method is presented and developed based on measurement of normalized second factorial moment of the photon-counting distribution of the resultant fluorescence. An expression for the normalized second factorial moment is obtained, and the errors expected in the determination of signal width and crossing field are theoretically studied. The validity of the approximations performed to establish the theoretical model is proved by an experiment with a square-wave periodically modulated light beam.