The total internal Reflection is what causes Light to be guided along the length of an optical fiber. Light must first fall inside an acceptable angle SO that it can enter into the fiber's core. Acceptance Angle or Numerical Aperture (NA) measures the Range of acceptance of light into a fiber. The angle over which a Fiber accepts light depends on the refractive indices of the Core and Cladding glass. Refraction bends a Ray of light entering a fiber so that it is at a smaller angle to the axis of the fiber than it was in air.

Alternate Definition of Numerical Aperature:

The sine of the Vertex Angle of the largest cone of meridional rays that can enter or leave an optical System or element, multiplied by the Refractive Index of the medium in which the vertex of the cone is located. Note: The NA is generally measured with respect to an object or image point and will vary as that point is moved.

2. For an Optical Fiber in which the Refractive Index decreases monotonically from N 1 on the axis to n 2 in the cladding, an expression of the extent of the fiber’s ability to accept, in its bound modes, non-normal incident rays, given by NA = (n 12-n 22)½. Note: In Multimode fibers, the term equilibrium numerical aperture is sometimes used. This refers to the numerical aperture with respect to the extreme exit angle of a ray emerging from a fiber in which equilibrium Mode distribution has been established.

3. Colloquially, the sine of the Radiation or acceptance angle of an optical fiber, multiplied by the refractive index of the material in Contact with the exit or entrance face. Note: This usage is approximate and imprecise, but is often encountered.