The worker must then connect one end of the fiber optic cable to a light source. How to use a fiber optic red light pen? What are the uses of fiber optic red light pens? Optical fiber red light pen (i., optical fiber fault detector, optical fiber fault test pen) is a 650nm (± 20nm) semiconductor laser as a light-emitting device, which emits stable red light through a constant. When it comes to testing fiber optic cables, a Visual Fault Locator (VFL) is an essential tool in your toolkit. It's a cost-effective and. The B5 Rechargeable Red Light Pen is a compact and reliable visual fault locator (VFL) used to quickly identify fiber breaks, bends, and connection issues. Here is how the pen helps detect errors. Tool sends visible light over a fiber strand with a 10mW power, good enough to reach distances of up to 10Km.