The term "gain" alone is ambiguous, and can refer to the ratio of output to input voltage ("voltage gain"), current ("current gain") or electric power ("power gain"). In the field of audio and general purpose amplifiers, especially operational amplifiers, the term usually refers to voltage gain, but in radio frequency amplifiers it usually refers to power gain. Furthermore, the term gain is also applied in systems such as sensors where the input and output have different units; in such cases the gain units must be specified, as in "5 microvolts per photon" for the responsivity of a photosensor. The "gain" of a bipolar transistor normally refers to forward current transfer ratio, either "h"FE ("beta", the static ratio of "I""c" divided by "I"b at some operating point), or sometimes "h"fe (the small-signal current gain, the slope of the graph of "I""c" against "I""b" at a point).