# Noise-equivalent flux density

The NEFD can be related to a light detector's Noise-Equivalent Power for a collection area A and a photon bandwidth $\nu$ by:
$NEFD = \eta \frac{NEP}{A \nu}$, where a factor $\eta$ (often 2, in the case of switching between measuring a source and measuring off-source) accounts for the photon statistics for the mode of operation.