Instead, the inside of the glass is coated with phosphors, chemicals that give off light when hit by ultraviolet radiation. Electricity makes an arc between two electrodes in the lamp, heating the mercury, which gives off UV radiation. Flourescent lights are the coolest, most energy efficient type of light and the most common commercial light source in North America. Longest life is best achieved, however, if the light is left on for long periods without being turned off and on frequently. The color of flourescent light varies a lot depending on the type of phosphors used in the lamp.
Some lamps make red and green look brown.
An HID lamp is very energy efficient and excellent for lighting indoor and outdoor arenas. The color of the light depends on the gas within the bulb. Metal halide lamps have the most natural light, mercury lamps produce a blue-green light, sodium lights produce a golden light. HID lights are the most expensive to buy but the most efficient to run. They last from 5,000 to 27,000 hrs.