Skyglow is the brightening of the night sky over inhabited areas, which reduces the contrast of stars or other celestial objects against the dark sky background. Some stars will blend into the background and become indistinguishable.
Clutter is the bright, confusing and excessive groupings of light sources, such as many lit advertisements signs in one place.
At night, too many lights may generate confusion, distract from obstacles (including those that they may be intended to illuminate), and potentially cause accidents.
Clutter is particularly noticeable on roads where the street lights are badly designed, or where brightly lit advertisements surrounds the roadways. Depending on the motives of the person or organization that installed the lights, their placement and design can even be intended to distract drivers, and can contribute to accidents.
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.
Glare is excessive brightness that causes visual discomfort, just like when a car blinds you with its headlights at night.
Glare from unshielded lighting is a public-health hazard—especially the older you become. Glare light scattering in the eye causes loss of contrast, sometimes blinds you temporarily and leads to unsafe driving conditions, for instance.
Glare can be disabling or simply uncomfortable. It is subjective, and sensitivity to glare can vary widely. Older people are usually more sensitive to glare due to the aging characteristics of the eye. Disability glare is the reduction in visibility caused by intense light sources in the field of view, while discomfort glare is the sensation of annoyance or even pain induced by overly bright sources (Rea 2000). Reducing glare is an effective way to improve the lighting.