Monthly Archives: June 2018

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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When to show Navigation lights

Navsregs>COLREGS>When to show Navigation lights
Ferry mast

Turn on the Navigation Lights

Bit of backtracking in this post, back to the fundamental question of when to turn the navigation lights on. 

What COLREG  Rule states when navigation lights should be shown?

Rule 20

​When must Navigation lights be shown?

  • Sunset to Sunrise-During the day
  • Sunrise to Sunset-During the night
  • In restricted visibility
  • May be exhibited in all other circumstances when it is deemed necessary.

What other lights can be shown from sunset to sunrise?

No other lights shall be exhibited, except such lights as:

  • Cannot be mistaken for the lights specified in the COLREGS
  • Do not impair their visibility or distinctive characterer
  • Do not Interfere with the keeping of a proper look-out

What is Restricted Visibility?

This is  any condition in which visibility is restricted by:

  • Fog
  • Mist
  • Falling snow
  • Heavy rainstorms
  • Sandstorms
  • Any other similar causes.

See COLREG Rule 3

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Navigation lights- The horizontal cut-offs

Navsregs>COLREGS>Horizontal cut-offs

A quick guide to some of the details in COLREG Annex 1

Sidelights on MV Balmoral

Following the theme of Navigation Light horizontal arcs this post now descended into the details of Annex 1.

Colreg Annex 1

What part of the Rules defines the horizontal cut-offs of Navigation Light?

Annex 1 section 9

What is the horizontal cut off for sidelights in the forward direction?

In the forward direction they should show the minimum required intensities.

The intensities shall decrease to reach practical cut-off between 1 degree and 3 degrees outside the prescribed sectors.

What is the cut-off requirements for the other sectors?

For sternlights and masthead lights and at 22.5 degrees abaft the beam for Masthead lights MV Balmoralsidelights, the minimum required intensities shall be maintained over the arc of the horizon up to 5 degrees  of the prescribed limits. 

From 5 degrees within the sectors the intensity may decrease by 50 percent up to the prescribed limits; it shall decrease steadily to reach practical cut-off at not more than 5 degrees outside the prescribed sectors.

How much can an all-round light be obscured?

Not more than 6 degrees, except anchor lights which need not be placed at an impracticable height above the hull.NUC lights

If it is impracticable to comply with these requirements by exhibiting only one all-round light, two all-round lights shall be used suitably positioned or screened so that they appear, as far as practicable, as one light at a distance of one mile.

Sidelight screens

The sidelights of vessels of 20 metres or more in length shall be fitted with inboard screens painted matt black, and meeting the horizontal sector  requirements of  Annex 1.

On vessels of less than 20 metres in length the sidelights, if necessary to meet the horizontal sector  requirements of the  Annex, shall be fitted with inboard matt black screens.

With a combined lantern with a very narrow division between the green and red sections, external screens need not be fitted.

There is another level of detail below COLREG Annex 1, and that is contained within an IMO Resolution.


Perfomance standards for Navigation lights

Within MSC 253 there are paragraphs giving more details on cut offs and intensity.


In the horizontal directions where decrease of luminous intensity to ìpractical cut-offî is required by section 9 of Annex I to COLREGs, the luminous intensity should be no more than 10% of the average luminous intensity within the prescribed sector for vessels not less than 12 m in length.

Uniformity of intensity

Within the prescribed sector in which the minimum luminous intensity is required  the horizontal intensity distribution of the light should be uniform so that the measured minimum and maximum luminous intensity values do not differ by more than a factor of 1.5 for vessels not less than 12 m in length.

This is to avoid luminous intensity changes which may result in the appearance of a flashing light

Click here for IMO MSC 253>

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