Definition
An alternating light shows different colours in succession from the same direction, with no eclipse between them, written for example Al WR or Al Bu Y. The IALA emergency wreck marking buoy uses Al Bu Y 3s, alternating blue and yellow with a one second flash of each and a half second eclipse. Alternating lights are visually distinctive but blue is poorly perceived by the dark-adapted eye at long range, limiting practical detection distance.
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