= Photobleaching
{wiki}
<Photobleaching> is an irreversible loss of fluorescent capacity caused by illumination. It changes the number of active <fluorophores> over experimental <time>; <fluorescence lifetime> concerns the delay after an individual excitation. Reversible dark states and ordinary return to the <ground state> are different processes.
In a stipulated population-loss model, let $T$ denote elapsed illumination <time> and $t$ denote delay after a pulse. If active population declines as $e^{-k_{\rm b}T}$ while surviving <fluorophores> retain the same decay rates, then $I(t;T)=A_0e^{-k_{\rm b}T}e^{-t/\tau}$. Brightness falls while $\tau$ stays fixed. This example does not assert that all bleaching follows <exponential decay>: chemical changes and selective loss from a mixture can also change the observed decay shape. The existing <FRET photobleaching controls> illustrate why deliberate bleaching requires careful interpretation.
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