A VFL is used to detect faults, breaks, or bends in fiber optic cables by emitting a bright red light that is visible even through the fiber's jacket. It's a cost-effective and. Fiber optic color coding is an essential part of managing and working with fiber optic cables and components. By adopting the TIA/EIA‑598C standard, you gain a universal “language” of colors that speeds identification, reduces miswiring, and enhances safety. Several reasons can cause the LOS light to blink red: Fiber Optic Cable Damage: A physical cut or bend in the fiber optic cable can disrupt the signal. Service Outage: ISP maintenance or a local service outage can interrupt your connection.