The angle within which the core of an optical fiber will accept light injected by a light source, the boundary of the angle of acceptance is equal to the critical angle. Illustrated in three dimensions, the angle of acceptance becomes the cone of acceptance.Within that angle, a light source can inject an optical signal into the fiber core and the signal will remain in the core, reflecting off of the interface between the core and cladding, as illustrated in Figure A-6. At a more severe angle, i.e., outside the cone, the signal will penetrate the interface and enter, and perhaps be lost in, the cladding. The angle of acceptance and, therefore, the cone of acceptance are determined by the difference in index of refraction (IOR) between the core and cladding.The mathematical sine of the angle of acceptance is known as the numerical aperture (NA). See also cone of acceptance, critical angle, IOR, and numerical aperture.