Light trespass is a form of light pollution which impinges on other people's personal space. The most familiar example of it is a floodlight which illuminates a neighbor's yard. Many people find this pollution extremely frustrating and annoying, and it can be a source of conflict in a community. For this reason, people are encouraged to think before installing outdoor lighting; approaching a neighbor at the start can also help to encourage a good relationship which will be helpful when resolving future issues.
Essentially, light trespass is unwanted spill light. In addition to being irritating, it is also wasteful, and it contributes to the problem of light pollution. Light trespass is commonly caused by individual light fixtures on homes and other structures, but it can also be caused by poorly directed arena lighting, ambient building lighting, and other sources. This unwanted light is particularly noticeable in urban areas, where many people complain that their homes are never truly dark.
There are a number of ways in which light trespass can be prevented. The most obvious, of course, is turning lights off when they are not needed. It is also a very good idea to take the time to aim lights when installing them, and to think about where spilled light will end up. By aiming lights at the ground, people can also ensure that the lights will serve their intended purpose, which is providing illumination for safety. Many organizations also recommend the use of hoods and shields on lighting to ensure that the light is properly directed.
Many people are not aware that they are committing light trespass, and their neighbors may seethe in silence for some time before mentioning it. When a new light fixture is installed, it pays to turn it on in the evening and take a walk around the neighborhood to see if the light is visible. If you suspect that a light is spilling onto a neighbor's property or into their home, ask them to help you as you reposition the light.
If you are frustrated by an act of light trespass committed by a neighbor, it is a good idea to approach them about it as quickly as possible, and to remember that he or she may not be aware that the light is causing a problem. Try to approach the situation politely, and offer to help your neighbor aim the light more efficiently, or ask if a motion detector could be used to ensure that the light only comes on when it is needed.