= Fluorescent tracer
A <fluorescent tracer> is a marker detected through <fluorescence> whose distribution is used to infer transport, mixing or location. In a fluid experiment, a small amount of fluorescent dye can reveal where an injected portion of <water> travels. The marker's own motion and chemical interactions determine what it faithfully tracks.
A possible optical arrangement illuminates a sample at an excitation wavelength, rejects most excitation light with an emission filter, and records the remaining <fluorescence> with a detector. A <fluorometer> can turn this into a concentration measurement. For fixed optical conditions in a calibrated dilute regime, an illustrative model is
$$
S=S_{\mathrm{bg}}+kc,
$$
where $c$ is dye <mass concentration>. A <fluorescence calibration curve> establishes the usable range and background. Assuming linearity at every concentration would hide effects of absorption, quenching or detector response.
The same marker can measure a <volumetric flow rate>. For steady injection at rate $q$ and stock concentration $c_s$ into an upstream flow $Q$ with background concentration $c_0$, complete downstream mixing gives
$$
Qc_0+qc_s=(Q+q)c_d,\qquad
Q=q\frac{c_s-c_d}{c_d-c_0}.
$$
For $q=0.010\,\mathrm{L\,s^{-1}}$, $c_s=1000\,\mathrm{mg\,L^{-1}}$, $c_d=2\,\mathrm{mg\,L^{-1}}$ and $c_0=0$, the inferred upstream flow is $4.99\,\mathrm{L\,s^{-1}}$. The extra $0.01$ litre per second belongs to the injected stream.
The Bureau of Reclamation's manual describes this constant-injection method, requiring known injection rate, adequate downstream mixing and concentration measurement; fluorometers detect dye at levels too faint for visual comparison.<Water Measurement Manual by U.S. Bureau of Reclamation>[Chapter 12, §7, Tracer-Dilution Methods, injection, mixing and fluorometer measurement]{ref} <Adsorption>, <photobleaching> or incomplete mixing can invalidate an ideal conserved-tracer inference. Observing a bright pattern alone does not establish that the dye is a quantitatively faithful measure of every constituent's motion.
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