This paper describes a two chopper technique for making fluorescence lifetime measurements when the pump source and the fluorescence are at the same wavelength. By pumping directly into the emitting state, delays due to relaxation from higher lying pump bands are avoided, while at the same time, the technique eliminates the problem of detector saturation when pumping directly into the fluorescing level.
K. A. Ingersoll, "Two-chopper technique for measuring fluorescence lifetimes," Appl. Opt. 15, 61-61 (1976)