There are three main fiber optic core sizes: 9/125, 50/125, and 62.5/125, measured in microns. The red represents the core, and that’s where you see your three measurements. The core is made of step-index glass, surrounded by green cladding to reflect light. The buffer and jacket protect the core and are made of lightweight Kevlar. The OM rating refers to ISO standards for data transmission speed and distance, with OM3 being the most common type.