transmission coefficient
The incident and transmitted fields are plane harmonic waves and the interface is large in lateral extent compared with the wavelength of the illumination. May also be the ratio of transmitted to incident irradiances, that is, the ratio of the normal (to the interface) component of the transmitted Poynting vector to that of the incident Poynting vector. A better term for this quantity is transmissivity. The transmission coefficient (and hence transmissivity) depends on the angle of incidence of the illumination, its wavelength (by way of the wavelength-dependence of the relative refractive index of the two media), and its state of polarization (
see also reflection coefficient). These coefficients taken together are sometimes called the "Fresnel formulae" or "Fresnel relations." Transmission coefficient may mean the ratio of any transmitted to incident irradiance (transmissivity). Transmission coefficient, transmissivity, transmittance, and transmission function are used more or less synonymously but not always consistently. Within the same work two or more of these terms may mean the same physical quantity. Context is a guide to the exact meaning of different authors in different fields and on different occasions, but is not always sufficient to decipher them. Moreover, these terms are not restricted to electromagnetic waves but may be applied to transmission of acoustic and other waves.
This vector specifies the instantaneous rate of propagation of electromagnetic energy. The amount of energy transmitted per unit time across a unit area, with normal n, is n · S. In optics, irradiance is the Poynting vector averaged over a time sufficiently long compared with the period but short when compared to the characteristic response times of radiometric instruments.
