| quantity defined by the formula where dΦe is the radiant flux transmitted by an elementary beam passing through the given point and propagating in the solid angle dΩ containing the given direction; dA is the area of a section of that beam containing the given point; θ is the angle between the normal to that section and the direction of the beam|
unit: W · m -2 • sr -1
Notes 1 to 5. In the five following notes the symbols for the quantities are without subscripts because the formulas are also valid for the terms 845-01-35 and 36.
Note 1 – For an area dA of the surface of a source, since the intensity dI of dA in the given direction is dI = dΦ/ dΩ, then an equivalent formula is , a form mostly used in illuminating engineering.
Note 2 – For an area dA of a surface receiving the beam, since the irradiance or illuminance dE produced by the beam on dA is dE = dΦ/ dA, then an equivalent formula is , a form useful when the source has no surface (e.g. the sky, the plasma of a discharge).
Note 3 – Making use of the geometric extent dG of the elementary beam, since dG = dA • cos θ• dΩ, then an equivalent formula is L = dΦ/ dG.
Note 4 – Since the optical extent G • n2 is invariant, then the quantity L • n -2 is also invariant along the path of the beam if the losses by absorption, reflection and diffusion are taken as zero. That quantity is called the basic radiance or basic luminance or basic photon radiance.
Note 5 – The relation between dΦ and L given in the formulae above is sometimes called basic law of radiometry and photometry:
with the notation given here and at 845-01-33.