LED Light Bulbs and Tungsten Filament

A bit more on economics: an Edison bulb (and all other pre-20th century light sources) is essentially a heat source that produces a little bit of light. A “65 Watt” Edison bulb produces about 700 lumens (a lumen, “lm”, is a unit of visible light power). If you had perfect conversion of electricity into light, you would need only 1.0 Watt (“W”) of electrical power to produce 700 lm. So, an Edison bulb is 1.6% efficient. For such a widely deployed technology, this is amazingly bad. But Edison bulbs are much better than the kerosene lamps they replaced …

A comparable LED bulb* consumes 13 W and produces 730 lm, which is 5.2x better efficiency than an Edison bulb, but still only 8%. The theoretical efficiency of an LED is about 30%, but commercially available LED materials are ~16%, and making a light bulb involves a power supply, phosphors, a protective container, etc., all of which dissipate energy.

Why are LEDs better? Edison bulbs function by making a Tungsten filament very hot, and a hot object glows: radiating energy on a broad spectrum of wavelengths, only a small part of which is visible to the eye. The rest of the radiated energy is mostly heat.

LEDs function by driving electrons across a junction between two semiconductor materials in which electrons move at different energy levels. When the electrons cross the junction, they drop from the higher energy level to the lower one, and the energy each electron loses when it drops takes the form of light to a large extent. The color of the light is mathematically determined by the energy level difference. So all of the light produced by an LED is the same color, and LEDs can be designed to produce light in the visible range. This precise conversion to one color is why LEDs are referred to as “digital light”. There is some waste heat caused by inefficiencies within the LED, however, the net result is a big gain in the percentage of electricity converted to visible light.

This digital precision creates a problem, too. Our eyes prefer full-spectrum analog light: what you get from a light bulb, a fireplace, or the sun. LED bulbs use a mix of phosphors (materials that absorb light and re-emit it at different colors) to convert the single color from the LED into multiple colors that mimic the spectrum, imperfectly. I find today’s LED bulb light pleasing, but my wife thinks the same light looks too orange.

LEDs are also long lived because they do not operate at the high temperatures that degrade the filaments of Edison bulbs. The LED itself has an expected life of about 50,000 hours (six years) of continuous use. Its power supply is less robust, so LED bulbs are usually rated for 25,000 hours. How long they last in practice is TBD, however: I have not used any for 25,000 hours. Edison bulbs are good for about 2,000 hours.

Here is how the dollars work out today. This example is for flood lights of the type used in down lights (“cans”), which is most of what we have in our house (pictured above). If you use a 65 W Edison bulb heavily (6 hours per day, 2,200 hours per year), it consumes $21.50/year of electricity (@ $0.15/kWh). The LED bulb uses 80% less electricity and lasts 11 years, so the electricity savings is $17.20 per year, plus a replacement Edison bulb @ $2, resulting in $19.50 of savings. The LED bulb costs $27 at Home Depot. So you get payback in 1.5 years and expect to keep saving for quite a few years beyond that. If the bulb is less used the payback is longer but the ROI is still good.

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