Perfectos was a radio device used byRoyal Air Force'snight fighters during theSecond World War to detect German aircraft. It worked by triggeringLuftwaffe'sFuG 25a Erstlingidentification friend or foe (IFF) system and then using the response signal to determine the enemy aircraft's direction and range. This allowed RAFinterceptors to track the Germans without the need for a radar system of their own, in contrast to the earlierSerrate radar detector that lacked range information and thus required a radar of their own for the final approach.

The resulting rapid ramp-up of night fighter losses in late 1944 alerted the Germans that the RAF was deploying a system to track them, and suspicion immediately fell on the Erstling. Pilots were told to leave their Erstling units turned off until they approached friendly airbases, where it was needed in order for theirflak units to avoid firing on them. This resulted in further chaos as crews often forgot to turn them back on, and flak units became increasingly paralyzed asfriendly fire incidents mounted.

• Brown, Louis (1999).Technical and Military Imperatives: A Radar History of World War 2. CRC Press.ISBN 9781420050660.