1. The ratio of the amplitude of a wave transmitted through an interface to that of the wave incident upon it. This ratio can be greater than one.
2. A measure of the amplitude of a wave passing through an interface restricted to the case of normal incidence.
3. Because (with the reflection method) we are usually interested in energy which has passed through an interface twice (once going down and once returning upward), a two-way transmission coefficient T* for normal incidence is often used:
where R is the reflection coefficient at normal incidence, , , V1, and V2 are the densities and velocities in the upper and lower media.
4. A ratio of the energy densities (which involves the square of transmission coefficients as defined above).