Utilizing a high-intensity 650nm Class IIIA laser diode, this pen-style tester effectively illuminates fiber breaks, micro-bends, and damaged connectors by leaking visible red light at the point of failure. VFLs use red lasers to trace single-mode or multimode fiber links. They detect bends and breaks in 900µm, 3mm jacketed cables and bare fibers in splice trays. When there are breaks, bends, or poor connections in the fiber, the red light leaks out at. Luxbond LBTEK Fiber Optic Red Light Pen (also known as a pen-style visual fault locator or fiber optic fault detector) uses a 650 nm semiconductor laser as the light source. They are perfect complementary.