Flux Density

Flux density can be expressed in either frequency or wavelength space.

In wavelength space:

$$F _\lambda = \frac {\Delta E}{\Delta t \Delta A \Delta \lambda}$$

In frequency space:

$$F _\nu = \frac {\Delta E}{\Delta t \Delta A \Delta \nu}$$

Converting from wavelength to frequency space
$$F _\nu = F _\lambda \frac {\Delta \lambda}{\Delta \nu}$$