| half the vertex angle of that cone within which optical power may be coupled into bound modes of an optical fibre
NOTE 1 – Acceptance angle is a function of position on the entrance face of the core when the refractive index is a function of radius in the core. In that case, it is:
where n (r) is the refractive index and n2 is the minimum refractive index of the cladding.
NOTE 2 – Power may be coupled into leaky modes at angles exceeding the acceptance angle.