The part of the cross-sectional area of an optical Fiber within which the Refractive Index is everywhere greater than that of the innermost homogeneous cladding, by a specified fraction of the difference between the maximum Refractive Index of the Core and the refractive index of the innermost cladding.

Note 1: Artifacts of the manufacturing process, such as refractive index dip, are ignored in computing the points (refractive indices) of demarcation.

Note 2: The core area is the cross-sectional area within which the refractive index is given by where N 1 is the maximum refractive index of the core, n 2 is the refractive index of the homogeneous Cladding adjacent to the core, n 3 is the defining refractive index, and M is a fraction, usually not greater than 0.05.