Section 9 in The Merchant Shipping (Prevention of Collisions at Sea) Regulations, 1975
9. Horizontal Sectors.
(a)(i)In the forward direction, sidelights as fitted on the vessel must show the minimum required intensities. The intensities must decrease to reach practical cut-off between 1 degree and 3 degree outside the prescribed sectors.(ii)For sternlights and masthead lights and at 22.5 degrees abaft the beam for sidelights, the minimum required intensities shall be maintained over the arc of the horizon up to 5 degree within the prescribed sectors the intensities may decrease by 50 per cent up to the prescribed limits; it shall decrease steadily to reach practical cut-off at not more than 5 degrees outside the prescribed limits.(b)All-round lights shall be so located as not to be obscured by masts, topmasts or structures within angular sectors of more than 6 degrees except anchor lights, which need not be placed at an impracticable height above the hull.
