visors, louvers, or other devices.
Care should be taken in specifying shielding requirements to identify appropriate exceptions.
Because of the cost of replacing street lighting, application of shielding requirements to existing
lighting may be regarded as prohibitively expensive, leading to the conclusion that the requirements
should apply only to new or retrofit lighting. It is also likely impractical to apply shielding
requirements to door lights or other low intensity lighting, particularly in residential areas.
Legislation and bylaws typically exempt light fixtures producing less than a specified intensity. A
more difficult problem is application of shielding requirements to illuminated signs. The Indiana
Framework applies directional control requirements to commercial signs. The Royal Astronomical
Society draft bylaw, on the other hand, exempts certain types of signs. It can be argued that
illuminated signs should be subject to general intensity rules and prohibitions, but that detailed
regulation of signs belongs in zoning bylaws.
Light trespass — Light trespass occurs when unwanted light enters property from a light source
outside the property, for instance, a light shining over a neighbor’s fence. Most bylaws contain
provisions directly addressing light trespass. For example, the Indiana Framework provides: