Light Trespass & Spill Light

Light trespass is light falling where it is not wanted — flood lighting spilling into bedroom windows, port lighting washing over a neighbouring nature reserve, sky glow from poorly aimed fixtures. It is increasingly regulated: CIE 150 and EN 12464-2 set limits on vertical illuminance at windows, upward light ratio and luminaire intensity toward observers, and many municipalities enforce curfew hours with stricter night-time limits. Control is an optics and aiming exercise: full cut-off distributions that emit nothing above the horizontal, back-light shields on street lights at the residential edge of a site, and flood lights tilted low with asymmetric beams that throw light forward without pointing the aperture at the horizon. Addressing it at the design stage costs little; retrofitting shields after a complaint costs more.