= Molecular brightness
{title2=$b$}
<Molecular brightness> is the detected <fluorescence> signal contributed by one labelled <molecule> or fluorescent particle under specified illumination and collection conditions. It is often expressed as detected <photons> per second per particle, or counts per particle per acquisition interval. It depends on the label, its environment and the apparatus, so it is not an intrinsic universal constant of the <protein> carrying the label.
For one uniformly observed species in a simple additive model, background-subtracted count rate $F$ and mean particle number $N$ satisfy $F=bN$, where $b$ is the <molecular brightness>. Ten particles with $b=1000$ counts per second and five particles with $b=2000$ counts per second both yield $F=10{,}000$ counts per second. Mean intensity alone cannot distinguish the two samples. A particle carrying two independent identical labels can approach twice the <molecular brightness> of one label, but quenching or incomplete labelling can invalidate that comparison.
<Yan Chen> and colleagues used the distribution of <photon> counts as particles diffuse through a tiny illuminated observation volume to estimate both <molecular brightness> and mean occupancy. This uses fluctuations beyond the average, with the illumination profile included in the measurement model.<The Photon Counting Histogram in Fluorescence Fluctuation Spectroscopy by Yan Chen and others>[1999, abstract and pp. 553–567]{ref}
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