Photon lifetime
= Photon lifetime
{title2=$\tau_c$}
The <photon lifetime> is the exponential decay time of stored <energy> in an unpumped <optical cavity>. If a round trip of duration $T$ retains an energy fraction $\rho$, then $\tau_c=-T/\log\rho$ and the <quality factor> is $Q=\omega\tau_c$. Field-amplitude decay takes twice this time. Output coupling contributes to the loss as well as absorption and scattering.