Figure 1. The Jablonski diagram of fluorophore excitation, radiative decay and nonradiative decay pathways. E denotes the energy scale; S0 is the ground singlet electronic state; S1 and S2 are the successively higher energy excited singlet electronic states. T1 is the lowest energy triplet state.

Figure 2. A typical fluorometer design. LS is the light source, EXO is the excitation optical train, SC is the specimen chamber, EMO is the emission optical train, and DET is the optical detector. Both the excitation and emission optical trains contain beam-shaping and collimation optics. For wavelength-resolved measurements, spectral selection optical components such as monochrometers and filters are included in the EXO and EMO. For polarization measurement, polarizers are added to EXO and EMO. For lifetime measurement, a laser light source is often used and high-speed electronics are integrated into the detector subsystem.