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We have all seen commercials for longer life light bulbs. Typically, someone is either in a situation where bulb burnout is really bad news or is trying to change a bulb that is very difficult to reach. The ad goes on to say that light bulbs are available that last twice as long and don’t cost any more. So what’s the catch? Why haven’t standard bulbs disappeared entirely? There is indeed a tradeoff to be made and the extra life does not come for free.
The bulbs we are talking about are properly called incandescent lamps. An incandescent filament is a tungsten wire that is an electrical resistor. As current passes through it, it gets white hot, or “incandesces.” As a thermal emitter, the filament has an emission spectrum that physicists refer to as a “blackbody.” Such a spectrum has two particular properties that are important to us here.
First, the output spectrum has a long sweeping tail on the red side that extends well into the infrared. For an incandescent lamp, most of the output actually falls in the infrared and ends up as just heat. In fact, only about 10% of the input electrical energy is converted into useful light, light that is in the visible part of the spectrum. The second fact is that this fraction is a very sensitive function of temperature; if the filament could be operated just a little hotter it would be considerably more efficient. Tungsten lamps are in fact operated as close to the melting point as practical and running them even a few degrees hotter would cause them to fail quickly.
To make a long-life bulb, the manufacturer takes advantage of what we just said. By designing the filament so that it is a little heavier, it operates at a slightly lower temperature and lasts much longer. The tradeoff is that it also produces less light.
Light output is measured in a unit called Lumens. The human eye is not equally sensitive to all wavelengths, being most sensitive to a particular green color that is, not coincidentally, where the sun emits the strongest light. For all other wavelengths (colors of light) both longer (redder) and shorter (bluer) it is less sensitive. The output spectrum of the light source is therefore weighted against the sensitivity curve for the eye and the resulting unit, the lumen, is a measure of “useful” light.
Checking my local home store I compared two bulbs. Both were made by the same manufacturer, both were 100-watt soft white bulbs and both cost the same. Reading the label tells the story.
The standard bulb was rated at 750 hours and produced 1710 lumens of light. The long life bulb was rated at 1500 hours but produced only 1580 lumens. The long life bulb lasts twice as long but produces only about 92% as much light. The lumen numbers will be different for bulbs with higher or lower wattage but the pattern is the same.
The standard bulb has long been optimized for “cost of light.” This expression accounts for the fact that electricity to operate a bulb over its life costs much more than the bulb itself. We can easily estimate this for our standard 100 watt bulb. Assuming electricity cost 9 cents per kilowatt-hour (about the U.S average), our bulb would use (.1 kW) x (.09 dollars/kW-hour) x (750 hours) = $6.75 over its rated life. In optimizing, the usual calculation therefore assumes that you would need additional bulbs to get the job done and that the additional power required would make long life bulbs unattractive.
If I were lighting an auditorium, for example, I would need 100 long life bulbs to get the same lighting as 92 standard ones. If these were 100 watt bulbs, I would be consuming an additional 800 watts whenever they were on and getting nothing for it. Such calculations commonly appear in analyses of this trade-off and always show that the standard bulb is optimal. For an industrial application, cost of light is carefully accounted for and where incandescent lamps are called for, standard ones are generally used.
In a typical home application, however, one has to question the usefulness of such a calculation. No one is going to use 100 bulbs instead of 92 or even 10 instead of 9. What you do in practice is put a single long-life bulb in the lamp and just live with the reduced output. Whether you’re better off with long life than standard then depends entirely on you.
For cases where a bulb is hard to reach and difficult to change, the convenience outweighs the reduced light output. For other applications, we should keep in mind that most people use a larger bulb than they really need to start with. Depending on the fixture, the lampshade if any, the application (general lighting or task lighting), and how sensitive you are to lighting levels you may not even notice the difference. If you try a long life bulb and don't notice the difference or don't mind the slight reduction in light, it is certainly a better deal since both use the same amount of power and the long life has to be replaced less frequently. If you really need the full 1710 lumens then use the shorter lived standard bulb and console yourself that you’re actually doing better with cost of light.
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