Launch angle

1. The angle, with respect to the normal, at which a light ray emerges from a surface.

2. The beam divergence at an emitting surface, such as that of a light-emitting diode (LED), laser, lens, prism, or optical fiber end face.

3. At an end face of an optical fiber, the angle between an input ray and the fiber axis[?].

Note: If the end face of the fiber is perpendicular to the fiber axis, the launch angle is equal to the incidence angle when the ray is external to the fiber and the refraction angle when initially inside the fiber.

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