Clause 5 Screens for sidelights
Maritime Rules Part 22: Collision Prevention
(1) The sidelights of vessels of 20 metres or more in length must be fitted with inboard screens painted matt black, and meet the requirements of Appendix 1.9.
(2) On vessels of less than 20 metres in length, the sidelights, if necessary to meet the requirements of Appendix 1.9, must be fitted with inboard matt black screens. With a combined lantern, that has a single vertical filament and a very narrow division between the green and red sections, external screens need not be fitted.