An instrument for measuring the reflectance of a surface. In telecommunication, an optical time domain reflectometer (OTDR) is an optoelectronic instrument used to characterize an optical fiber.

An OTDR injects a series of optical pulses into the fiber under test. It also extracts, from the same end of the fiber, light that is scattered back and reflected back from points in the fiber where the index of refraction changes. (This is equivalent to the way that an electronic TDR measures reflections caused by changes in the impedance of the cable under test.) The intensity of the return pulses is measured and integrated as a function of time, and is plotted as a function of fiber length. An OTDR may be used for estimating the fiber's length and overall attenuation, including splice and mated-connector losses. It may also be used to locate faults, such as breaks