At night, vision of coastlines and navigational hazards is often extremely limited. This is the time when sector lights make sense. A sector light is a conventional light in the sense that it will mark, at sea or from land, a particular point marked on the nautical chart.
A sector light is a lighted beacon used to guide vessels through a channel surrounded by dangers to navigation. The colour of the light (red, green, white) perceived from the vessel will vary according to the azimuth of the light and will indicate to the mariner whether or not the vessel is in the safe zone of the channel, illuminated in white.
Colour changing to show the route to be followed
Depending on the traffic lights, you may even find a red and green sector framing the white. An observer who sees red will know that it is too far to port of the safe area ( and that he must therefore turn to starboard ), he who sees green will know that there is too much green to starboard ( and that he must therefore turn to port ). In long channels, the sector light generally replaces the installation of an expensive suite of channel side buoys.
At the boundary between healthy and dangerous areas, there may be an oscillation, which will indicate to the mariner that he is proceeding too far to port ( the oscillation will then be red / white ) or too starboard ( the oscillation will then be green/white ) and invites him, therefore, to correct his course to return to the safety area of this channel.
On the nautical chart, the different sectors are indicated by dashes from the light and the colour of each is indicated by standardised letters, G for green, W for white, R for red. Finally, one and the same light can carry several sequences, be used to mark several channels or simply a virtual channel in the middle of which this light would be located.
Official definition
Sectoral fires are defined and regulated by IMHA ( International Association of Marine Aids to Navigation and Lighthouse Authorities ) in the brochure number 1041 which presents the set of solutions ( leds, light bulbs.. ) and operating procedures to provide mariners with the services of these special lights.
