The 8151A, in combination with the 81511A/81512A Optical Heads (Not included), is a response measuring instrument for the level characterization of fiber optic components, modules and systems. In addition to performing average power measurements, the 8151A enables the user to accurately determine upper and lower peak power levels - important in digital applications. This feature is also useful for apps where non-repetitive signals need to be evaluated, or where threshold levels are to be determined. The instrument's versatility is further enhanced by capabilities which allow the user to measure the amplitude, mesial power and extinction ratio of a signal. The 8151A's transducer has a freq. range of 250 MHz, and outputs an electrical signal which corresponds directly to the optical input waveform.