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A sector light is a man-made pilotage and position fixing aid that consists of strictly delineated horizontal angle light beams to guide water-borne traffic through a safe channel at night in reasonable visibility. Sector lights are most often used for safe passage through shallow or dangerous waters. This may be when leaving or entering harbour. Nautical charts give all the required information.
Sectors of colored glass are placed in the lanterns of these lights. The light will then show these colors when observed certain bearings. Bearings referring to a sector are given in degrees true, as observed from sea. Though the colors of the light will change, the characteristics will not. The change of color is not abrupt. The transition is made through an arc of uncertainty of 2° or greater.
The colors that are used, are conform to the IALA Maritime Buoyage system that is designed by the International Association of Lighthouse Authorities:
A ship that is sailing in safe water and then sees the red color of the light has to make an alteration in course.