Sector light

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 (paper and electronic) give all the required information.^[2]

Sectors of colored glass (or plastic) 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.^[3] 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:

white - this sector is in the middle of the safe channel
red - indicates the port edge of the channel for vessels approaching the light source
green - indicates the starboard edge of the channel for vessels approaching the light source.

A ship that is sailing in safe water and then sees the red (or green) color of the light has to make an alteration in course.

The world has different navigation standards managed by IALA (International Association of Lighthouse Authorities). For example, the United States uses a signalling standard which is the opposite of Europe. In USA, the red light indicates the starboard side of the channel for harbour bound vessels, while the green light indicates the port side of the same channel. An expression to remind of this is "red on right returning".

^ Baltic Pilot. Vol. 1 (2 ed.). Washington DC: United States Hydrographic Office. 1920. pp. 292–293. OCLC 37374157.
^ ^a ^b Myers, J. A. L., ed. (1991). Symbols and Abbreviations Used on Admiralty Charts (1995 ed.). Taunton: Admiralty Hydrographic Office. pp. 45, 47. OCLC 980318575.
^ Hobbs, Richard R (1974). Marine Navigation 1. Vol. Piloting (1981 ed.). Annapolis, MD: US Naval Institute. pp. 84–5. ISBN 978-94-011-7372-8.

[1] Baltic Pilot. Vol. 1 (2 ed.). Washington DC: United States Hydrographic Office. 1920. pp. 292–293. OCLC 37374157.

[UKHO-2] Myers, J. A. L., ed. (1991). Symbols and Abbreviations Used on Admiralty Charts (1995 ed.). Taunton: Admiralty Hydrographic Office. pp. 45, 47. OCLC 980318575.

[Hobbs-3] Hobbs, Richard R (1974). Marine Navigation 1. Vol. Piloting (1981 ed.). Annapolis, MD: US Naval Institute. pp. 84–5. ISBN 978-94-011-7372-8.

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