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. [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. [ref]
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
.| Quantity | What is resolved? | Units |
|---|---|---|
| Radiant flux | Total radiation power in the specified beam or collection | |
| Radiant intensity | Power per outgoing solid angle | |
| Radiance | Power per projected area and per solid angle | |
| Irradiance | Incident power per receiving area | |
| Radiant exitance | Outward emitted power per emitting area |
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. [ref]
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
.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. [ref]
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
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Radiometry is the science of measuring optical radiation, which includes electromagnetic radiation in the visible and invisible spectrum, such as ultraviolet and infrared light. It encompasses various concepts and units related to the detection and quantification of light energy, including its intensity, power, and distribution across different wavelengths. Key terms in radiometry include: 1. **Radiant Power (or Radiant Flux)**: The total amount of energy emitted by a light source per unit time, measured in watts (W).