Flux

Flux

The power of the emitting source on a unit surface area. Also known as bolometric flux or radiant flux in astronomy and irradiance in other fields. The SI units are watt per square meter.

[ F = \frac{\partial L}{\partial A} ]

Where \(F\) is flux, \(L\) is luminosity or power, and \(A\) is the area.

Spectral Flux

The power of the emitting source on a unit surface area for a unit wavelength. Also known as spectral irradiance in other fields. The SI units are watt per square meter per meter.

[ F_\lambda = \frac{\partial F}{\partial \lambda} ]

Where \(F_\lambda\) is the spectral flux and \(\lambda\) is the wavelength

You may also define the spectral flux in frequency instead of wavelength.