A compact fluorescent lamp (CFL), also known as a compact fluorescent light bulb is a type of fluorescent lamp. Many CFLs are designed to replace an incandescent lamp and can fit in the existing light fixtures formerly used for incandescents.

Compared to general service incandescent lamps giving the same amount of visible light, CFLs use less power and have a longer rated life, but generally have a higher purchase price. In the UK, a CFL can save over £60 in electricity costs over the lamp's lifetime compared to an incandescent lamp, and save 2000 times its own weight in greenhouse gases.

Like all fluorescent lamps, CFLs contain mercury; this complicates the disposal of fluorescent lamps. CFLs radiate a different light spectrum from that of incandescent lamps. Improved phosphor formulations have improved the subjective color of the light emitted by CFLs such that the best 'soft white' CFLs available in 2008 are subjectively similar in color to standard incandescent lamps.

The incandescent light bulb is gradually going out of existence, and over the next few years it will be outlawed altogether in the UK. As of January 2008, 150 watt and 200 watt incandescent light bulbs ceased to be manufactured. Any existing stock held by suppliers or retailers may be used up, but after that they will not be legal.

As of January 2009, 100 watt incandescent light bulbs are going the same way. Followed the year after by 60 watt versions. Slowly but surely, energy saving replacements are being manufactured for every type of current incandescent and halogen light bulb.

Some are proving more difficult than others, and as yet there is no energy saving equivalent to the common low voltage halogen spotlight. There are 240v GU10 versions available and courtesy of Megaman, dimmable energy saving bulbs are now on the market.

The average rated life of a CFL is between 8 and 15 times that of incandescents. CFLs typically have a rated lifespan of between 6,000 and 15,000 hours, whereas incandescent lamps are usually manufactured to have a lifespan of 750 hours or 1,000 hours. Some incandescent bulbs with long rated lifespans of 20,000 hours have reduced light output.

The lifetime of any lamp depends on many factors including operating voltage, manufacturing defects, exposure to voltage spikes, mechanical shock, frequency of cycling on and off, lamp orientation and ambient operating temperature, among other factors.

The life of a CFL is significantly shorter if it is only turned on for a few minutes at a time: In the case of a 5-minute on/off cycle the lifespan of a CFL can be up to 85% shorter, reducing its lifespan to the level of an incandescent lamp.

While the purchase price of a CFL is typically 3 to 10 times greater than that of an equivalent incandescent lamp, the extended lifetime (fewer lamps to replace and reduced labour) and lower energy use will compensate for the higher initial cost in many applications.

CFLs are extremely cost-effective in commercial buildings. A CFL replacing a 75 watt incandescent fixture offers an average yearly savings of £45 considering direct energy saving, reduced HVAC cost, and reduced labour to change lamps.

Incandescents give light almost immediately upon the application of voltage. CFLs take a perceptible time to achieve full brightness, and can take much longer in very cold temperatures.

Certain styles of lamp using a mercury amalgam can take up to three minutes to reach full output. Coupling this with the shorter life of CFLs when turned on and off for short amounts of time may make incandescent bulbs more attractive for applications such as outdoor or motion-activated lighting.

Although the majority of CFL’s are produced in the standard ‘White’ colour tone, ‘Cool White’ and ‘Warm White’ are other options. Recently the ‘Daylight’ colour tone has proved popular as this emits a light level equivalent to an S.A.D. bulb which is used to combat Seasonal Affective Disorder.

The majority of CFL’s in the UK can be found in BC (B22 Bayonet Cap) or ES (E27 Edison Screw). Other bulbs ie. candles, golf balls, can be found in the SBC (B15 Small Bayonet Cap) and SES (E14 Small Edison Screw) variety, in addition to the standard BC and ES fittings.

Over the last few months, an energy saving replacement for the R50, R63 and R80 spotlights has been introduced, aswell as a Par 38 type, used most commonly in garden spike and wall fittings.