The ratio (in dB) between the signal-to-noise ratio applied to the input of a microwave component and the signal-to-noise ratio measured at its output. NF is an indication of the amount of noise added to a signal by the component during normal operation. Lower noise figures mean less degradation and better performance.
The noise figure is a measure of the degradation of the signal-to-noise ratio (SNR) between the input and output ports of a device. As a standard reference the source temperature is assumed to be 290K.

The noise figure is most often expressed in decibels:
NF (dB) = 10 logl0 FC (cascaded noise figure)
Throughout this note, noise figures are in decibels unless stated otherwise.

Noise figure may also be expressed as an Effective Noise Temperature in Kelvin:
Te = 290 [log-1 (NF dB/10) -1]