In electronics, when describing a voltage or current step function, rise time is the time taken by a signal to change from a specified low value to a specified high value. These values may be expressed as ratios or, equivalently, as percentages respect to a given reference value. In analog or digital electronics, these percentages are commonly the 10 nd 90% (or equivalently and ) of the output step height: however, other values are commonly used. For applications in control theory, according to , rise time is defined as "the time required for the response to rise from to of its final value", with 0% to 100% rise time common for underdamped second order systems, 5% to 95 or critically damped and 10% to 90 or overdamped ones. According to , the term "rise time" applies to either positive or negative step response, even if a displayed negative excursion is popularly termed fall time.