text, a CRD measurement or spectrum is based on the ring-down times according to eq. \[eq:alpha\_CRD\].

We also integrate the CRD transient to extract $I{^\text{CRD}}$ (*cf.* eq. \[eq:ICRD\]) which is proportional to the light intensity inside the ring-down cavity (eq. \[eq:ILIF\_approx\]). The LIF transient which follows the ring-down decay is integrated to yield the LIF signal, $S{^\text{LIF}}$. According to eq. \[eq:CELIF\_norm\] the CELIF signal, $S{^\text{CELIF}}$, is obtained through a shot-to-shot normalization of $S{^\text{LIF}}$ with respect to $I{^\text{CRD}}$.

As alluded to earlier, CELIF is a LIF measurement where, without further calibration, only relative quantities are obtained. In CELIF this absolute calibration is provided by the absorption coefficient, $\alpha$, from the simultaneous CRD measurement, see eq. \[eq:alpha\_LIF\]. This calibration is particularly robust as both the LIF and CRD measurements use the same laser photons and sample molecules.

Results {#sec:results}
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![Simultaneously recorded (a) LIF and (b) CRD transients of the transition at 319.44 nm (*cf.* fig. \[fig:BPEBspectra\]) of BPEB. The LIF transient follows the same exponential decay as the CRD transient due to the negligible sub-ns fluorescence lifetime of the excited state[@Fujiwara2008] compared to the 1.80 $\mu$s ring-down time. The same integration limits were used to derive $S{^\text{CELIF}}$, see eq. \[eq:CELIF\_norm\]. Simultaneously 