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In radiometry, radiant flux or radiant power is the measure of the total power of electromagnetic radiation (including infrared, ultraviolet, and visible light). The power may be the total emitted from a source, or the total landing on a particular surface.
The SI unit of radiant flux is the watt (W), which has dimensions of energy/time or, in SI units joules/second.
| Quantity | Symbol | SI unit | Abbr. | Notes |
|---|---|---|---|---|
| Radiant energy | Q | joule | J | energy |
| Radiant flux | Φ | watt | W | radiant energy per unit time, also called radiant power |
| Radiant intensity | I | watt per steradian | W·sr−1 | power per unit solid angle |
| Radiance | L | watt per steradian per square metre | W·sr−1·m−2 | power per unit solid angle per unit projected source area. Sometimes confusingly called "intensity". |
| Irradiance | E | watt per square metre | W·m−2 | power incident on a surface. Sometimes confusingly called "intensity". |
| Radiant exitance / Radiant emittance | M | watt per square metre | W·m−2 | power emitted from a surface. |
| Radiosity | J or Jλ | watt per square metre | W·m−2 | emitted plus reflected power leaving a surface |
| Spectral radiance | Lλ or Lν | watt per steradian per metre3 or watt per steradian per square metre per hertz | W·sr−1·m−3 or W·sr−1·m−2·Hz−1 | commonly measured in W·sr−1·m−2·nm−1 |
| Spectral irradiance | Eλ or Eν | watt per metre3 or watt per square metre per hertz | W·m−3 or W·m−2·Hz−1 | commonly measured in W·m−2·nm−1 |
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