= Radiant energy
{title2=$Q_{\rm e}$}
\b[Radiant energy] is the amount of <energy> emitted, transferred or received as <electromagnetic radiation>. It is measured in <joules>. For the specified beam or collection of rays crossing a boundary,
$$
Q_{\rm e}=\int_{t_0}^{t_1}\Phi_{\rm e}(t)\,dt,
$$
where <radiant flux> $\Phi_{\rm e}$ is radiation <power>. The receiving geometry and time interval are part of the statement; dividing by a receiving area produces <radiant exposure>, rather than another total energy.<International Lighting Vocabulary by CIE>[Entry17-21-041, definition and notes1 and4]{ref}
For an original passive-optic example, stipulate $5.0\,\mathrm{mJ}$ incident during an interval, with no remaining change of stored energy afterward. Suppose $3.0\,\mathrm{mJ}$ is transmitted, $1.25\,\mathrm{mJ}$ reflected and $0.75\,\mathrm{mJ}$ absorbed. Their sum is the incoming $5.0\,\mathrm{mJ}$. A detector collecting only the transmitted path receives $3.0\,\mathrm{mJ}$, even though the source supplied more. This <radiometric energy balance> specifies the collection boundary instead of equating every energy readout in the apparatus.
A thermal detector converts absorbed radiation into an observable temperature response; a calibrated model relates that response to received energy. A documented NIST arrangement uses a slow thermal reference and calibrated beam splitting to establish pulse-energy measurements.<Calibration Service for Low-level Pulsed-Laser Radiometers at 1.06 µm: Pulse Energy and Peak Power by Rodney W. Leonhardt>[§3.4, printedp.6; §4, printedpp.9–10]{ref} Absorbed energy, incident radiant energy and energy in an uncollected branch must remain distinct in that calibration.
Two sources can deliver the same radiant energy with different <radiant flux> histories or spectra. Their total energies agree only for the specified integration and collection; this alone does not determine peak power, <radiant exposure> at a point or detector response.
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