In optics, photobleaching (sometimes termed fading) is the photochemical alteration of a dye or a fluorophore molecule such that it is permanently unable to fluoresce.
However, photobleaching may also be used prior to applying the (primarily antibody-linked) fluorescent molecules, in an attempt to quench autofluorescence.
Loss of activity caused by photobleaching can be controlled by reducing the intensity or time-span of light exposure, by increasing the concentration of fluorophores, by reducing the frequency and thus the photon energy of the input light, or by employing more robust fluorophores that are less prone to bleaching (e.g. Cyanine Dyes, Alexa Fluors or DyLight Fluors, AttoDyes, Janelia Dyes and others).
At light intensities used in single-molecule fluorescence imaging (0.1-1 kW/cm2 in typical experimental setups), even most robust fluorophores continue to emit for up to 10 seconds before photobleaching in a single step.
For some dyes, lifetimes can be prolonged 10-100 fold using oxygen scavenging systems (up to 1000 seconds with optimisation of imaging parameters and signal-to-noise).