At a discontinuity or impedance mismatch, e.g., in a Transmission line, the ratio of the incident power to the reflected power. Note 1: Reflection Loss is usually expressed in dB. Note 2: The reflection loss, L r, is given by where Z 1 and Z 2 are the respective impedances, and the vertical bars designate absolute magnitude.

In an optical fiber, the loss that takes place at any discontinuity of refractive index, especially at an air-glass Interface such as a Fiber endface, at which a fraction of the optical signal is reflected back toward the source. Note: This reflection phenomenon is also called “Fresnel reflection loss,” or simply, “Fresnel loss.” At normal incidence, the fraction of reflected power is expressed by the formula, where N 1 and n 2 are the respective indices of refraction.