Optical Time Domain Reflectometer, OTDR for short, is an electronic-optical instrument that is used to characterize optical fibers. It locates defects and faults, and determines the amount of signal loss at any point in an optical fiber. The OTDR only needs to have access to one end of a fiber to make its measurements. An OTDR takes thousands of measurements along a fiber. The measurement data points are displayed on the screen as a line sloping down from left to right, with distance along the horizontal scale and signal level on the vertical scale. By selecting any two data points with movable cursors, you can read the distance and relative signal levels between them.