Glare is the result of bright and dark areas in the field of view. Light shining into our eyes can obscure night vision for up to an hour after exposure.
Glare can also result in reduced contrast, due to light scattering in the eye by excessive brightness, or to reflection of light from dark areas in the field of vision, with luminance similar to the background luminance. This kind of glare is a particular instance of disability glare, called veiling glare.
Glare can be categorized into different types. One such classification is described in a book by Bob Mizon, coordinator for the British Astronomical Association's Campaign for Dark Skies. According to this classification:
* Blinding Glare describes effects such as that caused by staring into the Sun. It is completely blinding and leaves temporary or permanent vision deficiencies.
* Disability Glare describes effects such as being blinded by an oncoming cars lights, or light scattering in fog or in the eye reduces contrast, as well as reflections from print and other dark areas that render them bright, with significant reduction in sight capabilities.
* Discomfort Glare does not typically cause a dangerous situation in itself, and is annoying and irritating at best. It can potentially cause fatigue if experienced over extended periods.
An awareness of the consequences of exterior lighting is growing among lighting specifiers, manufacturers and end users. Lighting schemes should consider luminaire and lighting designs to control sky glow and unwanted light across property lines. The two issues are called light pollution and light trespass, and they can be easily confused.
Light pollution is when the illumination from a property or a neighbourhood is directed upward into the sky. This has the effect of causing the night sky to glow over that area. The sky glow blocks out the view of the night sky for those in the area as well as surroundings. It makes the work of amateur astronomers difficult and generally stuffs up star gazing in the area. It also ruins the atmosphere of small towns and rural areas at night. Many communities feel that the darkness is an important reason why they moved to the small towns in the first place.
Light trespass is when a luminaire directs light beyond the property causing annoyance other property owners and can cause glare in surrounding properties.
Dealing with these external lighting problems is gaining in importance. The Leadership in Energy and Environmental Design (LEED) program offers a credit for controlling stray light on US projects. Greater environmental awareness is growing in the lighting design community, prompted by the International Dark Sky Association and the Illuminating Engineering Society of North America. The causes of light pollution and light trespass are not always the same things. But using luminaires with optics designed for better cut-off of light distribution can be a good start toward helping solve both problems. The Lighting Research Centre has more information.
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