Light trespass is light from a property that falls where it is not wanted, typically across the property line onto neighboring homes, windows or streets.
Parking lots and garage roofs are common sources because they use tall poles and bright fixtures near property edges. Many cities limit light trespass in their zoning or lighting ordinances, usually as a maximum illuminance measured at the property line, and some follow the Model Lighting Ordinance developed by the Illuminating Engineering Society and DarkSky International.
How to prevent it
- Full-cutoff or shielded fixtures with low backlight ratings. See cutoff luminaires.
- Poles set back from property lines, with house-side shields where needed.
- Lower mounting heights near residential neighbors.
- Photometric plans that show values at and beyond the property line.
- Dimming or switching after hours where security allows. See adaptive lighting.
The Parkonomics view
Light trespass is the lighting problem neighbors notice, and it is almost always avoidable at design. Ask for the photometric plan to extend past the property line, not stop at it.
Related terms: Light Pollution, Glare, Cutoff Luminaires, Illuminance, DarkSky International