Radiometry
= Radiometry
{wiki}
<Radiometry> measures the <energy> and <power> carried by <electromagnetic radiation>, including <light>. It asks how much radiation a source supplies, where it travels, and how much reaches a receiving surface. The measurements are physical rather than weighted by the sensitivity of the human eye: equal <radiant fluxes> at different <wavelengths> need not look equally bright. https://www.nist.gov/publications/radiometry-and-photometry-review-vision-optics[Yoshihiro Ohno, Radiometry and Photometry: Review for Vision Optics, 2000]{ref}.
The five quantities below answer different questions about the same radiation. The <International System of Units> makes the distinctions visible: a watt is a joule per second, while a steradian measures <solid angle>. https://www.nist.gov/pml/special-publication-811/nist-guide-si-chapter-4-two-classes-si-units-and-si-prefixes[NIST Guide to the SI, Chapter 4: derived units]{ref}.
|| Quantity
|| What is resolved?
|| Units
| <Radiant flux> $\Phi$
| Total radiation <power> in the specified beam or collection
| $\mathrm W$
| <Radiant intensity> $I$
| <Power> per outgoing <solid angle>
| $\mathrm{W\,sr^{-1}}$
| <Radiance> $L$
| <Power> per <projected area> and per <solid angle>
| $\mathrm{W\,m^{-2}\,sr^{-1}}$
| <Irradiance> $E$
| Incident <power> per receiving <area>
| $\mathrm{W\,m^{-2}}$
| <Radiant exitance> $M$
| Outward emitted <power> per emitting <area>
| $\mathrm{W\,m^{-2}}$
Resolving <wavelength> or <frequency> adds a further “per unit interval”, as in <spectral radiance>. The notation is consistent with NIST's table of spectral quantities. In particular, <radiant flux> is <power>, whereas the existing astronomical <radiative flux> is <power> per <area>. https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=901354[NISTIR 6657, A Rational Methodology for Estimating the Luminous Flux Based upon Color Primaries from Digital Projection Displays, 2009, Table 1]{ref}.
This bookkeeping makes an <optical camera>'s collected signal calculable from a source's <radiance> and the geometry of the receiving aperture. It also lets radiation measurements from different instruments be compared. For example, NIST describes separate calibrations of <spectral radiance> and spectral <irradiance> for instruments used to observe Earth. https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=104550[Johnson, Brown and Rice, Metrology for remote sensing radiometry, §§3.4.1–3.4.2]{ref}.