Launch numerical aperture

In telecommunication, launch numerical aperture (LNA) is the numerical aperture of an optical system used to couple (launch) power into an optical fiber.

Note 1: LNA may differ from the stated NA of a final focusing element if, for example, that element is underfilled or the focus is other than that for which the element is specified.

Note 2: LNA is one of the parameters that determine the initial distribution of power among the modes of an optical fiber.

Source: from Federal Standard 1037C and from MIL-STD-188