posted 10-08-2010 11:20 PM ET (US)            

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The specifications for the light shown from a vessel is given in the technical annex to the navigation rules. See:

http://www.navcen.uscg.gov/index.php?pageName=33CFR84#13

for details of the chromaticity of the light. The navigation rules also specify the intensity of the light. See:

http://www.navcen.uscg.gov/index.php?pageName=33CFR84#15

There are no federal rules about what miniature electric light bulbs must be used. If you have an approved lamp, you must replace the electric light bulb with the one specified by the manufacturer, otherwise the lamp will no longer be considered to be an approved lamp. Consult with the manufacturer of your lamp to find the details of the electric light bulb they specify. Or, make an inference about the proper miniature electric light bulb to use based on a close visual inspection of the existing miniature electric light bulb in the lamp. Typically a miniature electric light bulb will have a numerical code printed on the glass envelope or on the brass base; this number can be used to locate a similar replacement.