Note 1 to entry: The basic law of radiometry and photometry is expressed by

${\displaystyle \text{d}\Phi =L\cdot \frac{\text{d}A\cos \theta \cdot \text{d}{A}^{\prime }\cos {\theta }^{\prime }}{l^2}}$

where the proportionality factor for this geometrical condition is equal to the radiance or luminance, respectively, within the elementary beam.

Note 2 to entry: The basic law of radiometry and photometry is based on the geometric extent.