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Navigation Light Intensity
Here's the section from the USCG Navigation Rules (COLREGS), page 142 explaining how to calculate the brightness of a navigation light. You can use this procedure to see if the LED replacement buls you bought are bright enough for the application.

8. Intensity of lights

(a) The minimum luminous intensity of lights shall be calculated by using the formula:

        I = 3.43 x 106 x T x D2 x K-D

 

where:

I is luminous intensity in candelas under service conditions.

T is threshold factor 2 x 10-7 lux.

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

K is atmospheric transmissivity. For prescribed lights the value of K shall be 0.8, corresponding to a meteorological visibility of approximately 13 nautical miles.

Example

Your boat, at 38', is less than 12m, which requires an anchor light that's visible for 2 nm. Using the above formula, the result is a light with a brightness of 1.1 candelas. To convert from candelas to lumens, use the formula:

Lumens = Candela * 2 * π * (1 - cos(θ/2))    (θ in radians)

The anchor light is an all-around white light (360 or 2π radians) light. In this situation, the 1.1 candelas is 13.8 lumens. Compare this number to the lumens value on the LED package to see if it is bright enough for your needs.

Remember, this method isn't USCG approved, so caveat emptor.

 

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