Sometimes the ratio of the amplitude of the transmitted electric field to the amplitude of the field incident at the optically smooth planar interface between two optically homogeneous media. 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.