## Basic relation between received spectral power and sky brightness

The spectral power (infinitesimal power per unit frequency bandwidth) received from a solid angle of the sky on a surface of area is

| (72)

where

| [ ] | spectral power,
| [rad] | angle between and Zenith,
| [rad] | angle between and East,
| [Hz] | frequency of observation,
| [ ] | sky brightness at position ( ) and frequency ,
| [sr] | infinitesimal solid angle of sky ,
| [ ] | infinitesimal surface area.