= Fluorescence excitation spectrum
{title2=$F_{\rm ex}(\lambda_{\rm ex})$}
A <fluorescence excitation spectrum> scans excitation <wavelength> while detecting <fluorescence> in a fixed emission band. A <fluorescence emission spectrum> does the converse. The inspected spectrofluorometer example uses an excitation <monochromator> scan with the emission <monochromator> fixed, and reverses their roles for emission.<What are Absorption, Excitation and Emission Spectra? by Edinburgh Instruments>[What are Absorption, Excitation and Emission Spectra?, Excitation Spectra, PDF pp. 2–3, Figures 3–4; Emission Spectra, p. 4, Figures 5–6]{ref}
An operational model explains why this need not reproduce absorption. For one dilute, unsaturated emitting species, write $a(\lambda)$ for absorbed fraction, $R_{\rm in}(\lambda)$ for incident <photon> rate, $\Phi(\lambda)$ for <fluorescence quantum yield>, and $\eta_{\rm band}(\lambda)$ for the fraction of emitted <photons> counted in the selected band. The background-subtracted count rate is
$$
C_{\rm ex}(\lambda)=R_{\rm in}(\lambda)a(\lambda)\Phi(\lambda)\eta_{\rm band}(\lambda).
$$
Only after correcting incident <photon> rate, and with constant yield and emission-band collection, is this proportional to absorbed fraction. A nonfluorescent absorber contributes to absorption but not this readout. Thick samples, multiple emitting species, and reabsorption of <fluorescence> complicate the model.
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