Photobleaching is the destruction of a photochemical fluor by high-intensity light. In microscopy, photobleaching may complicate the observation of fluorescent molecules, since they will eventually be destroyed by the light exposure necessary to stimulate them into fluorescing. This is especially problematic in time-lapse microscopy.

However, photobleaching may also be exploited to study the motion and/or diffusion of molecules, for example via the FRAP technique.

Commonly photobleaching must be controlled either by reducing the intensity or time-span of light exposure, increasing the concentration of fluorophores, or by employing more robust fluorophores that are less prone to bleaching. To a reasonable approximation, a given molecule will be destroyed after a constant exposure (intensity of emission X emission time X number of cycles).

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