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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.^[ref]

An operational model explains why this need not reproduce absorption. For one dilute, unsaturated emitting species, write $a(\lambda )$ for absorbed fraction, $R_{\mathrm{in}}(\lambda )$ for incident photon rate, $\Phi (\lambda )$ for fluorescence quantum yield, and ${\eta }_{\mathrm{band}}(\lambda )$ for the fraction of emitted photons counted in the selected band. The background-subtracted count rate isOnly 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.

For the constant-rate model in fluorescence quantum yield, fluorescence lifetime is the mean residence time in the emitting excited state and the $1/e$ timescale of its exponential decay. The detected fluorescent photon rate after an ideal instantaneous pulse is $I(t)=\eta k_{\mathrm{r}}{N}_0{e}^{-t/\tau }$ for $t\ge 0$. Its amplitude and its decay timescale are separate quantities.

For a stipulated example, rates $(k_{\mathrm{r}},k_{\mathrm{nr}})=(0.05,0.20){\mathrm{ns}}^{-1}$ give $\tau =4\mathrm{ns}$ and $\Phi =0.20$. Rates $(0.20,0.05){\mathrm{ns}}^{-1}$ give the same $\tau$ but $\Phi =0.80$. Conversely, doubling $N_0$ doubles the ideal amplitude without changing $\tau$. Equal fluorescence lifetimes need not imply equal brightness or equal fluorescence quantum yields.

Different environments can supply several populations, giving $I(t)={\sum }_j{A}_j{e}^{-t/{\tau }_j}$ rather than one exponential decay. A fitted single timescale then depends on the selected model and detection band. FRET donor lifetime uses the change in a donor's decay when transfer introduces an additional loss route; the FRET branch develops that interpretation.

Fluorescence quantum yield counts emitted fluorescent photons per absorbed excitation photon. It differs from quantum efficiency, which describes the photodetector.^[ref]

For an original constant-rate model, let $k_{\mathrm{r}}$ be the radiative decay rate and $k_{\mathrm{nr}}$ the sum of competing rates without fluorescent photon emission. An initial population $N_0$ in the emitting excited state obeys the rate equationIntegrating $k_{\mathrm{r}}N(t)$ over all time givesHere every absorbed excitation is assumed to reach this emitting population; additional losses before that stage require another factor. Under steady weak excitation, absorbed photon rate $R_{\mathrm{abs}}$ and total detection fraction $\eta$ give $C=\eta \Phi R_{\mathrm{abs}}$. The count rate therefore mixes excitation, yield, and detection, whereas $\tau$ specifies a decay timescale.

A fluorophore is a fluorescent molecule or fluorescent part of a molecule. Its usefulness as a label depends on how efficiently it absorbs the chosen excitation light, its fluorescence quantum yield, and its resistance to photobleaching. A large fluorescence quantum yield alone does not guarantee a bright image if few excitation photons are absorbed or collected.