However, there are differences in the measurement of the fluorescence signal between a CELIF and a typical LIF experiment. Only a fraction of the laser light is transmitted through the cavity entrance mirror and is subsequently interacting with the sample. As a consequence, the initial fluorescence is very small in comparison to single-pass LIF. The light confined in the cavity undergoes up to several thousand round trips, interacting with the sample twice on each of them. Each of these interactions induces further fluorescence so that the integrated fluorescence leads to an appreciable LIF signal. Effectively, the cavity stretches the short laser pulse into the exponential decay characteristic for the cavity.

Without loss of generality, we base the following derivation on a fixed excitation wavelength and on a sample distribution that is symmetric with respect to a LIF probe volume that is placed at the center of the cavity. The photon loss per single pass is ${\mathcal{L}}= \sigma \rho s $, with the sample length, $s$, given that ${\mathcal{L}}\ll 1$. The light intensity at the center after entering the cavity is $$\label{eq:ILIF0}
I_0{^\text{LIF}}= I_\text{L} T {\mathcal{F}}\left( 1 - \frac{{\mathcal{L}}}{2}\right),$$ where $I_\text{L}$ is the laser intensity incident on the cavity entrance mirror 