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Annex I (to Protocol Additional I to the Geneva Conventions of 1949) : Regulations concerning identification, 6 June 1977
-- Light signal
1. The light signal, consisting of a flashing blue light, is established for the use of medical aircraft to signal their identity. No other aircraft shall use this signal. The recommended blue colour is obtained by using, as trichromatic co-ordinates:
green boundary y = 0.065 + 0.805x
white boundary y = 0.400 - x
purple boundary x = 0.133 + 0.600y
The recommended flashing rate of the blue light is between sixty and one hundred flashes per minute.
2. Medical aircraft should be equipped with such lights as may be necessary to make the light signal visible in as many directions as possible.
3. In the absence of a special agreement between the Parties to the conflict reserving the use of flashing blue lights for the identification of medical vehicles and ships and craft, the use of such signals for other vehicles or ships is not prohibited.
Commentary of 1987