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Fluorescence lifetime imaging
A fluorophore which is excited by a photon will drop to the ground state with a certain probability based on the decay rates through a number of different decay pathways. To observe fluorescence, one of these pathways must be by spontaneous emission of a photon. In the ensemble description, the fluorescence emitted will decay with time according to
F(t) = F0e - t / τ
In the above, t is time, τ is the fluorescence lifetime, F0 is the initial fluorescence at t = 0, and ki are the rates for each decay pathway, at least one of which must be the fluorescence decay rate kf.
Fluorescence lifetime imaging yields images with the intensity of each pixel determined by τ, which allows one to view contrast between materials with different fluorescence decay rates (even if those materials fluoresce at the exact same wavelength), and also produces images which show changes in other decay pathways, such as in FRET imaging.
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