Note 1 to entry: The luminous turbidity factor can be expressed by

${\displaystyle T_{\text{v}}=\frac{-\ln (E_{\text{vs}}/E_{\text{v}})}{a_{\text{v}}m}}$

where E_vs is the direct-beam illuminance on a horizontal surface (at the ground), E_v is the extraterrestrial horizontal illuminance, a_v is the luminous extinction coefficient, and m is the relative optical air mass.

Note 2 to entry: The luminous turbidity factor has unit one.