Three ways to make light
Electric lamps in common use have relied mainly on three different physical processes. Understanding the process explains most of the differences in efficiency, heat and lifespan, and it explains why one technology replaced another.
Incandescent: light from heat
An incandescent bulb heats a filament until it glows. Any hot object gives off light, but most of the radiation from a filament at lamp temperatures is infrared, which we feel as heat but do not see. Raising the temperature increases the share of visible light, but the filament would melt or evaporate faster. Tungsten, with its very high melting point, allows a hotter filament than carbon, and a coiled filament in an inert gas improves life and brightness further.
Even so, a typical household incandescent bulb converts only a small fraction of its electricity into visible light, with the rest leaving as heat. Quoted figures are often in the range of about 10 to 17 lumens of light per watt, though they vary with design. A lumen is a measure of visible light output, so lumens per watt (luminous efficacy) is a convenient way of comparing lamps. Typical rated life was around 1,000 hours, with filament evaporation and eventual breakage setting the limit.
The strengths were low cost, a warm color of light, instant full brightness and simple, dimmable operation. Those strengths kept the filament bulb in homes for around a century.
Fluorescent: light from excited gas
A fluorescent lamp works very differently. Electric current passes through low-pressure mercury vapor inside a glass tube, exciting the atoms so that they give off ultraviolet radiation. A phosphor coating on the inside of the tube, a material of the same general kind used in cathode ray tubes, absorbs this ultraviolet and re-emits it as visible light. Because the process does not rely on heating a metal wire to a high temperature, it can be several times more efficient than a filament lamp.
Experimenters had made gas discharge tubes in the 1800s, and mercury vapor lamps appeared around the start of the 1900s. Fluorescent lamps with phosphor coatings were introduced commercially in the late 1930s. Fluorescent tubes became common in offices, shops and factories long before they reached many homes, partly because they need a ballast to regulate current and the early light color was often described as harsh. Typical figures given for fluorescent lighting are often in the range of about 50 to 100 lumens per watt, and lifetimes of several thousand hours. Disadvantages include the mercury content, which requires careful disposal, and slower warm-up in some designs.
LED: light from a semiconductor
A light-emitting diode produces light when electrons recombine with holes at a junction in a semiconductor, releasing energy as photons. Nothing needs to be heated to a glowing temperature, so far less energy goes into heat at the light source. The color depends on the semiconductor material. White LED lamps usually combine a blue LED with a phosphor that converts part of the blue light to yellow, giving a white mix.
Modern LED lamps are often rated at well over 80 lumens per watt, and some designs exceed 100, with lifetimes commonly stated as 15,000 hours or more. LEDs still produce some heat, which must be drawn away from the chip, and the electronics that run them can limit the life of a lamp more than the LED itself does.
A simple comparison
Using rough, typical figures and a bulb producing about 800 lumens, which is often the output of a traditional 60-watt household incandescent lamp:
| Technology | Typical efficacy | Power for 800 lumens | Typical rated life |
|---|---|---|---|
| Incandescent | about 13 lumens per watt | about 60 W | around 1,000 hours |
| Compact fluorescent | about 55 to 65 lumens per watt | about 13 to 15 W | several thousand hours |
| LED | about 80 to 100 lumens per watt | about 8 to 10 W | often 15,000 hours or more |
The arithmetic: 800 lumens divided by 60 watts is about 13 lumens per watt, and 800 divided by 10 watts is 80. These are illustrative numbers, since individual products vary widely.
Why LEDs won
Each technology had a period when it was the best choice. Incandescent lamps were the simplest and cheapest to make. Fluorescent lamps saved energy in large buildings that were lit for long hours. LEDs became the best all-around choice once the blue LED made white light possible, and costs fell as production scaled up in the 2000s and 2010s.
Other factors helped. LEDs are small, so they can be designed into many shapes. They also serve as backlights for flat-screen displays and as the lamps in traffic signals. They switch on instantly, work well with dimmers and controls once the electronics are matched, and contain no mercury. They use electronic circuits and in that sense belong to the same family as silicon chips.
Where the older types remain
Incandescent bulbs still have a role where simple, cheap, heat-producing lamps are useful, such as in some specialist lighting and appliance lamps. Fluorescent tubes remain in many existing buildings. Color quality varies between all three types, and buyers have often preferred the warm glow of filament lamps, which is one reason makers developed LEDs with warmer color options. The three technologies are best seen as successive answers to the same question: how to turn electricity into useful light while wasting as little of it as possible.