When an otherwise natural area is contaminated with something that is harmful to it, it becomes polluted. The item introduced may be certain substances or factors. For example, most often when people think of pollution they think of the pollution in the air or water by chemicals. There are other types of pollution, however, such as pollution caused by light. Light pollution, also known as luminous pollution, is defined as the excessive introduction of artificial lighting to the outdoor environment. People who are unfamiliar with this problem often do not fully understand how light can become a pollutant. Around the world, street lights, stadium, park, and other lighting are illuminating the sky to such a magnitude that it creates a number of problems that are detrimental in terms of energy use, the environment and even to people. While light serves an undeniably important purpose, understanding light pollution can help people reduce its negative impact.