33 C.F.R. § 84.16 – Vertical sectors.

(a) The vertical sectors of electric lights as fitted, with the exception of lights on sailing vessels underway and on unmanned barges, shall ensure that:
1. (i) At least the required minimum intensity is maintained at all angles from 5 degrees above to 5 degrees below the horizontal;
2. (ii) At least 60 percent of the required minimum intensity is maintained from 7.5 degrees above to 7.5 degrees below the horizontal.
(b) In the case of sailing vessels underway, the vertical sectors of electric lights, as fitted, shall ensure that:
1. (i) At least the required minimum intensity is maintained at all angles from 5 degrees above to 5 degrees below the horizontal;
2. (ii) At least 50 percent of the required minimum intensity is maintained from 25 degrees above to 25 degrees below the horizontal.
(c) In the case of unmanned barges the minimum required intensity of electric lights as fitted shall be maintained on the horizontal.
(d) In the case of lights other than electric lights these specifications shall be met as closely as possible.