Light Pollution Basics
The International Dark-Sky Association (IDA) describes light pollution as any adverse effect of artificial light.
Established in 1988, the IDA was the first organization to call attention to the hazards of light pollution and it is dedicated to protecting and preserving the nighttime environment through quality outdoor lighting. The IDA promotes one simple idea: Light what you need, when you need it.
The IDA defines the four components of light pollution as:
Urban Sky Glow: the brightening of the night sky over inhabited areas
Light trespass: light falling where it is not intended, wanted or needed
Glare: excessive brightness that causes visual discomfort (high levels of glare can decrease visibility)
Clutter: bright, confusing and excessive groupings of light sources, commonly found in over-lit urban areas; the proliferation of clutter contributes to urban sky glow, trespass and glare