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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,where radiant flux ${\Phi }_{\mathrm{e}}$ 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.^[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.^[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.

Uncollected scattering in a radiometric energy balance is outgoing scattered radiation that falls outside the measured channels. Assigning every missing joule to optical absorption then overestimates the absorbed energy. The radiometric energy balance must count disjoint transmitted, reflected, scattered and absorbed terms for the same interval.

For an original collection model, stipulate $100\mu \mathrm{J}$ incident, $30\mu \mathrm{J}$ transmitted and $20\mu \mathrm{J}$ specularly reflected. A separate scattered-light receiver collects $q=10\mu \mathrm{J}$. If its calibrated capture fraction is $c=0.25$, the scattered energy is $q/c=40\mu \mathrm{J}$, leaving $10\mu \mathrm{J}$ absorbed. If all that is known is $0.20\le c\le 0.50$, scattering can range from $20$ to $50\mu \mathrm{J}$ and absorption from $0$ to $30\mu \mathrm{J}$. One receiver reading cannot determine both the total scattering and its capture fraction.

The fraction $c$ is an energy fraction, not automatically the receiver's fraction of solid angle. Converting geometrical coverage into an energy fraction also needs the scattering distribution.

An integrating sphere provides broad angular collection through repeated diffuse reflection. Actual reflectance/transmittance arrangements depend on port geometry, baffles and whether the specular component is included. The primary guide describes excluding that component through a port and using baffles to prevent direct viewing of the illuminated sample.^[ref] The apparatus therefore needs its own response calibration; calling a receiver integrating does not establish perfect collection.