Core Diameter can be defined as in the cross section of a realizable optical fiber, ideally circular, but assumed to a first approximation to be elliptical, the average of the diameters of the smallest circle that can be circumscribed about the core-cladding boundary, and the largest circle that can be inscribed within the core-cladding boundary
The inside diameter measurement of the cardboard core at the centre of a ribbon or media roll
In the cross section of a realizable optical fiber, ideally circular, but assumed to a first approximation to be elliptical, the average of the diameters of the smallest circle that can be circumscribed about the core-cladding boundary, and the largest circle that can be inscribed within the core-cladding boundary