The OFI-400 Series are rugged, hand-held and easy-to-use fiber optic test instruments designed to detect and measure the core power levels of optical signals on single-mode optical fiber without disrupting traffic on that fiber. The OFI-400 is designed and calibrated for use with 250 µm, 900 µm, ribbon, 2 mm and 3 mm jacketed optical fibers. The instrument clamps onto a fiber and displays presence and direction of traffic, continuous test signals, and modulated test tones; it also provides a built-in power meter and Set Reference feature.