Fluorescent lights are the most energy saving. A fluorescent lamp is a glass tube, filled with argon, or argon/krypton gas, and mercury. When turned on, the mercury vapors get ionized and emit ultra-violet (UV) radiation, striking the phosphor coating on the tube, causing it to glow — or fluoresce. That is the process that produces light. About 22% of the energy used by the lamp is converted to light.

As with incandescents, there are tradeoffs with fluorescents. For example, all fluorescents contain mercury and require an appropriate ballast. Additionally, regular exposure to fluorescent lighting has been known to cause or contribute to health problems — especially where there is more flicker and hum.
Compact Fluorescent Lights (CFLs) are miniature versions of full-sized fluorescents. Those qualified under the Energy Star program are 3-4 times more energy efficient than standard incandescent bulbs and last up to 10 times longer. Tradeoffs are present with CFLs too. They are a more diffuse light than incandescent and not appropriate for all applications. They do not produce enough light for many reflector fixtures nor for some ambient ceiling lighting applications.
All ballasts generate noise and carry sound level ratings A, B, C, or D. An “A” rated ballast typically hums softly; a “D” rated ballast typically buzzes rather loudly. Electronic ballasts produce less flicker and emit less humming noise — generally quieter than required for an “A” sound rating.
Fluorescent Mercury Vapors: When broken, fluorescent lighting can release mercury vapors into the atmosphere, affecting those nearby. Under Federal hazardous waste regulations, businesses can no longer dispose of spent fluorescent light bulbs, lamps, ballasts or other lights containing mercury as unregulated refuse. When specifying fluorescent bulbs, specify low and non-leachable mercury bulbs. When coordinating deconstruction work, reference certified mercury and ballast recyclers
Color Rendering: Fluorescent bulbs emit light in a few narrow bands of color created by the argon and mercury vapors inside, then play off a broader spectrum produced by the phosphors. This affects their color rendering. The perceived white color of a fluorescent is the result of phosphors selected to produce the illusion of white light, causing color distortion. Some claim that our vision corrects for this, and we don’t notice a distortion, except in certain color-critical applications. Color rendering of newer CFLs is claimed to be equal to incandescent light bulbs, and able to produce full-spectrum lighting.