In fiber optics, half the vertex angle of that cone within which optical power may be coupled into bound modes of an optical fiber. Note 1: The axis of the cone is collinear with the fiber axis, the vertex of the cone is on the fiber end-face, and the base of the cone faces the optical power source. Note 2: The acceptance angle is measured with respect to the fiber axis. Note 3: Rays entering an optical fiber at angles greater than the acceptance angle are coupled into unbound modes.