The range of ship’s navigation lights

Navsregs>COLREGS>Navigation Light Range

​This post starts of with the simple facts of COLREG 22 and then delves deeper and deeper into definitions; deep into the world of lux, lumens and candelas.

Sunset in the Med

The range of Navigation lights

What Rule states the visibility of Navigation lights?

Rule 22-visibility of lights

Note: This Rule states that navigation lights shall have an intensity as specified in Section 8 of Annex I to the COLREGS.

What is the range of Navigation lights in a vessel of 50 meters or more in length?

  • Masthead light- 6 miles
  • Sidelight-3 miles
  • Sternlight-3 miles
  • Towing light-3 miles
  • A white, red, green or yellow all-round light-3 miles

What is the range of Navigation lights in a vessel of 12 meters or more in length but less than 50 meters in length?

  • Masthead light, 5 miles; except that where the length of the vessel is less than 20 meters, 3 miles
  • Sidelight- 2 miles
  • Sternlight-2 miles;
  • Towing light-2 miles;
  •  White, red, green or yellow all-round light-2 miles

What is the range of Navigation lights in a vessel of less than 12 meters in length?

  • Masthead light- 2 miles
  • Sidelight- 1 mile
  • A sternlight- 2 miles
  • Towing light-2 miles
  • White, red, green or yellow all-round light- 2 miles

What is the range of Navigation lights of an inconspicuous, partly submerged vessel or object being towed?

White all-round light- 3 miles.

Waiting for the amplitude

The intensity of Navigation lights

The intensity required to achieve the minimum range is derived from a formula given in Section 8 of Annex I to the Colregs.

The intensity formula

1=3.43 × 1 06×T × D2 × K-D

What do the parts of the Navigation light intensity formula mean?

I is luminous intensity in candelas under service conditions

The candela is the SI unit of luminous intensity.  It is the luminous power per unit solid angle emitted by a point light source in a particular direction.

Or to be more accurate, it  is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540×1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.

The steradian is a square radian, and is the SI unit of solid angle.

T is threshold factor 2 × 10-7 lux

The lux is the SI unit of illuminance and luminous emittance, measuring luminous flux per unit area.It is equal to one lumen per square metre. It is used as a measure of the intensity, as perceived by the human eye.

The lumen is  the SI unit of luminous flux, a measure of the total quantity of visible light emitted by a source.

D is range of visibility (luminous range) of the light in nautical miles

The luminous range is the maximum distance at which a light can be seen, as determined by the luminous intensity of the light, the atmospheric transmission factor and the threshold of illuminance on the eye of the observer.

Ship's at anchor off Gibraltar

 K is atmospheric transmissivity.

Atmospheric transmissivity is he ratio of the directly transmitted flux incident on a surface after passing through unit thickness of the atmosphere to the flux that would be incident on the same surface if the flux had passed through a vacuum.

The value of K shall be 0.8, corresponding to a meteorological visibility of approximately 13 nautical miles.


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