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First developed for naval use in the 1860s, signal lamps are still used today for communications during emergencies or when radio silence must be maintained. The handheld 5" Aldis Lamp was invented in 1941 and used throughout the Second World War and for many years afterwards for both naval and aerial signalling. 

SOURCES:

https://web.archive.org/web/20121027031528/http://www.faa.gov/air_traffic/publications/atpubs/aim/aim0403.html
https://www.atc100years.org/the-aldis-lamp/
http://www.deadmedia.org/notes/24/241.html
http://www.jproc.ca/rrp/rrp2/visual_lights.html