(b) was calibrated to the simultaneously recorded CRD spectrum (a). The CELIF spectra (d) and (f) were calibrated with respect to spectrum (b). For details of the BPEB spectroscopy see ref. .[]{data-label="fig:BPEBspectra"}](Fig04){width="8.6cm"}

Discussion {#sec:discussion}
==========

Our work has shown that recorded CELIF signals are linearly related to absorbances derived from CRD measurements. In this single laser beam setup, the cavity ring-down part provides the absolute scale for the LIF measurement without the requirement of any external calibration. With this combination of the two techniques, the high sensitivity of LIF greatly extends the accessible absorbance measurement range of CRDS on an absolute scale due to a greatly improved signal-to-noise ratio. The presence of the cavity introduces changes to the LIF technique that we will discuss in the following sections.

Sources of noise and error
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Fig. \[fig:BPEBspectra\] clearly shows the improvement in quality of the CELIF spectra compared to the CRD spectra. We will first discuss the sources of noise (or error) in both techniques. Shot-to-shot fluctuations in the sample density affect both techniques similarly as the same molecules are probed.

For the CRD technique, mode fluctuations inside the cavity lead to increased noise on the ring-down transient, which, together with 