Signal lamps, also called Aldis lamps, were visual communication equipment used extensively by both sides during the battle of the Atlantic.

This example is German. Signallers sent messages in Morse code by sighting the target through the eyepiece (upper left) and turning the light on and off, either with a trigger or by opening and closing shutters over the lens. Unlike radio transmissions, visual signals could not be detected at long ranges. They were a preferred method of communication because they reduced the risk of disclosing a vessel's location.

Signal Lamp, German
CWM 19460004-033