Power transmission coefficient ($\mathcal{T}=P_{\mathrm{transmitted}}/P_{\mathrm{incident}}$)

The ratio of transmitted to incident normal time-averaged power for a specified wave and interface. Converting a field-amplitude transmission coefficient into power requires the appropriate impedances or flux normalization. For a displacement wave on a string with impedance $Z$, $\mathcal{T}=(Z_2/Z_1)|t{|}^2$; using an electric-field amplitude in electromagnetic waves instead gives the corresponding electromagnetic flux ratio. Amplitude squared alone is sufficient only when the relevant incident and transmitted normalizations agree.