Amplitude transmittance ($t=U_{\mathrm{out}}/U_{\mathrm{in}}$)

Amplitude transmittance is the local ratio of transmitted to incident complex amplitude in a thin optical-element model. It includes wave phase as well as attenuation. For identical incident and transmitted media at normal incidence, the corresponding intensity transmittance is $|t{|}^2$; $t$ itself is not the transmitted power fraction. A pure phase screen has $|t|=1$.