noise, any signal that does not convey useful information. Electrical noise consists of electrical currents or voltages that interfere with the operation of electronic systems. Electrical noise limits the sensitivity of radio receiving systems and, when present at high enough levels, may cause false outputs from digital circuits. In radio receivers it is important that the noise produced by amplifiers, especially early-stage amplifiers, be kept as low as possible. The signal-to-noise (S/N) ratio is an important factor when evaluating much electronic equipment. Random noise originates when a current flows through a conductor that has resistance and is above absolute zero in temperature. It also arises in electron tubes and semiconductor devices, as well as from atmospheric disturbances and radiation from space (see static). Nonrandom noise originates from the operation of other systems, e.g., automotive ignition systems, and from interfering signals. Noise also affects optical detection systems where light is treated by the particle, or quantum, theory. The output voltage of an optical detector is proportional to the intensity of the incident light. The noise can be from the detectors themselves, the electrical amplifiers that amplify the detector outputs, or thermal noise, which is caused by the vibration of atoms and molecules. Noise can also be inherent in the radiation being detected.