Logarithm of the ratio of two values of a field quantity, usually amplitude. Levels are expressed in different ways: $L_F=\ln (\frac{F}{F_0})N_{\text{p}}$, where $N_{\text{p}}$ is the symbol for the unit neper coherent with the SI, or $L_F=20{\log }_{10}(\frac{F}{F_0})\mathrm{dB}$, where $\mathrm{dB}$ is the symbol for the unit decibel.