Light trespass is light falling where it is not intended or needed. Light trespass can be spill light from a streetlight or floodlight that enters a window and illuminates an indoor area causing problems, like sleep deprivation.
When unwanted light enters one’s property, for instance, by shining light over a neighbour’s fence, the phenomenon is called light trespass. A number of cities in the U.S. have developed standards for outdoor lighting to protect the rights of their citizens against light trespass. To assist them, the International Dark-Sky Association has developed a set of model lighting ordinances.
When light goes up into the sky it reduces visibility of stars. This is any light which is emitted more than 90° above nadir. By limiting light at this 90° mark they have also reduced the light output in the 80–90° range which creates most of the light trespass issues. Light trespass can be reduced by selecting light fixtures which limit the amount of light emitted more than 80° above the nadir.