 Radiometric energy balance

A radiometric energy balance accounts for radiant energy crossing the specified measurement boundary during a specified interval. For a passive optic, suppose the relevant transient has finished, no electromagnetic energy remains stored, and no additional energy source is present. Incident radiation is partitioned into all outgoing radiation and energy transferred to matter. Resolving outgoing paths givesThe channels must be disjoint: a scattering term cannot include energy already counted by a reflection or transmission collector. The definition of radiant energy concerns radiation emitted, transferred or received, so the location and collection geometry belong in each term.^[ref]

For an original example, inject $10\mu \mathrm{J}$ and measure $6\mu \mathrm{J}$ in the transmitted path plus $2\mu \mathrm{J}$ in the specular reflection. Calling the missing $2\mu \mathrm{J}$ absorbed is premature. If a separate collector finds $1.5\mu \mathrm{J}$ of diffuse outgoing radiation outside those channels, the inferred absorbed energy is only $0.5\mu \mathrm{J}$. The uncollected scattering in a radiometric energy balance would otherwise be misidentified as optical absorption.

A physical check compares calibrated incident and outgoing energy readouts while controlling apertures, backgrounds and the observation window. The scattered component may need a different collection geometry from the specular beam. Energy absorbed by matter can remain stored or later leave as heat or radiation; a more general control-volume balance must explicitly include storage, other heat transfer and work. It must not count absorbed energy twice by also treating the same material-energy increase as an independent loss.