How does a compact fluorescent light-bulb save energy?

1. How does a fluorescent lamp (fluorescent tube) work?
2. How does a compact fluorescent light-bulb (CFL) work?
3. How much energy does a CFL save?
4. How long does a CFL typically last?
5. Do CFLs have any health issues?
6. Since fluorescent lamps and CFLs emit ultraviolet light, why don't I get sunburn?
7. What is the world trend in the choice of light bulbs?

1. How does a fluorescent lamp (fluorescent tube) work?

   In a modern fluorescent lamp, a ballast is used to produce a high voltage that causes an electric discharge across the electrodes.  The discharge excites the mercury vapour inside the lamp, giving out ultraviolet light.  The ultraviolet light hits on the phosphor coatings on the glass, which produces visible light.  The ballast then maintains the current, working at a lower voltage to sustain the discharge after the gas inside the lamp has been ionized. Fluorescent lamps in the older days, using starters and "magnetic ballasts", flickered when started and might take seconds or even minutes to stabilize.  Such inconvenience has gone with the advent of "electronic ballasts", which have also made possible lamps of smaller size.

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2. How does a compact fluorescent light-bulb (CFL) work?

   CFL works in the same way as a fluorescent lamp, except that the glass tube is bent.  Both ends of the tube are affixed onto a base which also holds a ballast and can fit an ordinary light-bulb socket. Photo: Courtesy of H.Y. Chiu

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3. How much energy does a CFL save?

   An ordinary light bulb uses only 10 to 12% of the input energy to generate light (the rest is mostly emitted as heat).  A halogen lamp is about the same.  Please refer to "How to make light bulbs last longer".  In comparison, a CFL is about 4 times as efficient, i.e. it uses about 40 to 50% of the energy.  Thus, a 25-watt CFL is as bright as a 100-watt light bulb.

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4. How long does a CFL typically last?

   A CFL typically can last for about 8,000 hours.  In comparison, an ordinary light bulb lasts about 900 hours only. Taking account of its energy efficiency and longer life, a CFL outperforms an ordinary light bulb by more than 30 times.

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5. Do CFLs have any health issues?

   CFLs have the same health issues as fluorescent lamps:- They need to be disposed of carefully to avoid mercury contamination. Some consumers find the light too harsh.  This is because the phosphor coatings on the glass do not emit all the lights in the visible-light spectrum.  The human eye, on the other hand, would like to see all of them.  Some people are sensitive to ultraviolet light.

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6. Since fluorescent lamps and CFLs emit ultraviolet light, why don't I get sunburn?

   Research indicates that 8 hours of exposure under office fluorescent lighting is about the same as 1 minute under the sun.  So, no sunburn.

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7. What is the world trend in the choice of light bulbs?

   Under a new law in the United States, by the early 2010s manufacturers of ordinary light bulbs are required to make them generally 30% more efficient.  Europe has also made its own rules. Since CFLs are already better by more than 30 times in terms of energy efficiency and expected lifetime combined, it is expected that more consumers will turn to CFLs.

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Reference:

1. "The switch is on", Scientific American, p.84, March 2008.
2. "What Einstein told his barbar", R.L. Wolke, Dell Publishing, 2000.