A Visual Fault Locator (VFL) is a fiber optic testing tool used to identify faults and breaks in fiber optic networks. VFLs typically use a 650nm wavelength red laser that is transmitted through the fiber. This compact and lightweight tool is an essential instrument for field technicians and. The B5 Rechargeable Red Light Pen is a professional 650nm visual fault locator designed for fiber optic network maintenance, installation, and troubleshooting. To view the full specifications, download the spec sheet below. Find options with long-range detection, universal connectivity, and portable designs.