How LED Bulbs Make Light and Why They Last So Long
LED bulbs turn electric current into light far more efficiently than old filament bulbs. Learn how white light is made, how long LEDs last, and why heat matters.
In this article
An LED, short for light emitting diode, makes light from an electric current passing through a small chip rather than by heating a wire until it glows. That difference is why LED bulbs waste far less energy as heat and last far longer than the filament bulbs they replaced. This article covers how LEDs produce white light, what their lifespan figures mean and why temperature is the thing that wears them out.
A different way to make light
A traditional incandescent bulb passes current through a thin filament that gets hot enough to glow. The US Department of Energy's consumer pages say such bulbs release about 90 percent of their energy as heat, so only a small share becomes visible light. An LED works differently. According to the Department of Energy, an electrical current passes through a microchip, which lights tiny light sources, and the heat that is produced is drawn away by a heat sink. The department says LED products produce light up to 90 percent more efficiently than incandescent bulbs. Wikipedia's article on the LED gives the underlying physics: electrons recombine with electron holes inside a semiconductor and release the energy as photons, and the semiconductor's band gap sets the colour.
Why LEDs start out coloured
An LED does not naturally emit white light. The Department of Energy explains that LEDs emit nearly monochromatic light, meaning light of essentially one colour. That suits uses such as traffic lights and exit signs. To get white light for a room, makers use one of three approaches:
- Phosphor conversion: a phosphor coating on or near the LED converts the coloured light into white.
- Colour mixing: red, green and blue LEDs are combined so that together they look white.
- Hybrid: phosphor-converted LEDs are combined with single-colour LEDs.
A phosphor is a material that absorbs light of one colour and gives out light of other colours. The Department of Energy's overview names these three methods but does not rank them for any particular product, so it is not possible to say from that source which method a given bulb uses. The packaging or manufacturer's data sheet is the place to look. Wikipedia adds that phosphor conversion, typically a blue or ultraviolet LED coated with phosphor, is the more common method because it is simpler and cheaper to make, while red-green-blue mixing is seldom used for white lighting because the colour balance can shift with viewing angle.
Efficacy: how much light per watt
Lighting efficiency is often given as efficacy, measured in lumens per watt. A lumen is a unit of visible light output, and a watt is the electrical power used. The Department of Energy says LED efficacy reaches up to about 150 lumens per watt in some specialised cases, and it sets long-term research goals of 250 to 325 lumens per watt for warm-white LED packages and 250 to 275 for complete lighting products. Those are goals, not the performance of bulbs on shop shelves today.
The department also points out that with many conventional fixture types, only 50 to 60 percent of the light a lamp produces actually leaves the fixture. LEDs are directional, which means they emit light in one direction rather than in all directions as incandescent and compact fluorescent bulbs do. In some fittings that helps, because less light is trapped inside the fixture.
How long they last, and what that means
The Department of Energy expects good-quality white LEDs to last 30,000 to 50,000 hours. For comparison it gives about 1,000 hours for incandescent lamps, 8,000 to 10,000 hours for compact fluorescents and more than 30,000 hours for the best linear fluorescent tubes.
These numbers describe something different from an ordinary burn-out. The department explains that LEDs typically do not burn out suddenly. Instead they go through lumen depreciation, which means the light slowly dims. The rated life is measured as the point where light output has fallen to 70 percent of its starting level. One Energy Saver page describes the same measure as a 30 percent drop. A bulb past its rated life may therefore still shine, only more dimly.
Heat is the main enemy
Although LEDs waste far less energy than filament bulbs, they still produce some heat, and it is concentrated in a small chip. The Department of Energy says heat is the biggest factor in an LED's lifespan: the hotter it runs, the faster it degrades. That is why LED bulbs carry heat sinks, the metal fins or bodies that carry heat away from the chip. It is also a reason that enclosed fittings with poor airflow can shorten life, though the exact effect depends on the product.
Colour quality
Brightness is not the only property of a light. Colour rendering index, or CRI, is a score for how faithfully a light shows colours compared with a reference. The Department of Energy recommends a minimum CRI of 80 for interior lighting and says a CRI of 90 or higher indicates excellent colour fidelity.
Price
The Department of Energy notes that some LED lamps cost only 1 to 2 US dollars, with prices varying by performance features. That figure is from the department's page and prices differ by country and over time, so treat it as an indication only. This article is general information, not a buying recommendation.
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