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.
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.
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.
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.
The Really Handy study guides for Kindle
- Vessel Certification