A loss of optical power that occurs at a splice or a pair of mated connectors when the numerical aperture of the "transmitting" fibre exceeds that of the "receiving" fiber, even if the cores are precisely the same diameter and are perfectly aligned. Note 1: The higher numerical aperture of the transmitting fibre means that it emits a larger cone of light than the receiving fibre is capable of accepting, resulting in a coupling loss. Note 2: In the opposite case of numerical aperture mismatch, where the transmitting fibre has the lower numerical aperture, no numerical aperture loss occurs, because the receiving fibre is capable of accepting light from any bound mode of the transmitting fiber.