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Important Radiometric Terms

1. Radiant energy, . Energy traveling in electromagnetic waves, measured in joules.
   1. Spectral radiant energy, , measured in joules per nanometer, .
2. Radiant Flux (radiant power), . The time rate of flow of radiant energy, measured in joules per second or watts .
   1. Spectral Radiant Flux, , measured in .
3. Radiant flux density, . The quotient of the radiant flux incident on or emitted by an element of surface at a point, by the area of the element. The preferred term for radiant flux density leaving a surface is exitance, . The preferred term for radiant flux density incident on a surface is irradiance, . Measured in watts per square meter, .
   1. Spectral radiant flux density, . In terms of exitance it is . In terms of irradiance it is . Measured in .
4. Radiant intensity, . The radiant flux proceeding from a source per unit solid angle in a given direction. Measured in watts per steradian, .
   1. Spectral radiant intensity, . Measured in .
5. Radiance, . Power per unit projected area perpendicular to the ray per unit solid angle in the direction of the ray. Measured in .
   1. Spectral radiance, .

      .

      Measured in .