The spectral power (infinitesimal power per unit frequency bandwidth) received from a solid angle of the sky on a surface of area is
where| [
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spectral power, | |
| [rad] | angle between |
|
| [rad] | angle between |
|
| [Hz] | frequency of observation, | |
| [
|
sky brightness at position (
|
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| [sr] | infinitesimal solid angle of sky
|
|
| [
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infinitesimal surface area. |