What Is 40 Lumens?
Lumens indicate how much visible light is being produced by a light source. The more lumens a light source has, the brighter the light will appear to the human eye.
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History
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Electrical Meter Tracking Watts Used Historically, consumers compared watts to understand the brightness of a light source they were buying. A watt is a measure of electrical consumption--how much electricity it takes to power something. You will notice that your electrical company, for instance, measures the total number of watts your home has consumed, and bills you by the watt.
Watts Measuring Lighting
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More watts typically meant more light because consumer lighting choices used to be much more limited. With incandescent light bulbs the predominant available style, and with few varieties of these, the watt (a measure of power consumed by the bulb) did correlate with intensity of the light produced. More energy into the light bulb meant a brighter light.
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Consumer Variety
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Halogen, and fluorescent technologies both provide more visible light per watt than incandescent bulbs. But the variety of technologies affecting the quality of light even among the incandescent lights strongly affect the lumens created per watt among just these more traditional bulbs. Power consumption is no longer a useful proxy for the light emitted.
Comparing Watts to Lumens
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Technology Differences A quick trip to a few retailers in January of 2010 shows how little relationship there can be between watts consumed and light emitted. A 40 watt incandescent light bulb might produce anywhere from 325 to 465 lumens. With compact fluorescent light bulbs, nine watts can yield 450 or even 490 lumens, but with just 48 watts, a halogen recessed track light could provide up to 850 lumens.
For the Scientists
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Technically, a lumen is the metric system's unit for luminous flux. Luminous flux is not a measure of the whole radiant capacity of a light source, but rather that portion of the capacity that is visible to the human eye. It is an output measure, rather than an impact measure. The term has a different meaning in astronomy, where one lumen represents a rate of 40 photons per second incident upon the eye.
Looking Forward
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By late 2009 many manufacturers had begun to list lumens produced on their consumer packaging. However, not all manufacturers -- and not even all products from the same manufacturer -- list lumens for consumer comparison. Though you may expect this trend to grow, the lumen is only one aspect of lighting to consider. Watts consumed will remain important for economic and ecological reasons, and ultimately, the home consumer will also want to consider the quality of the light produced.
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References
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- Photo Credit Image by Flickr.com, courtesy of Egor Gribanov Image by Flickr.com, courtesy of Christian Senger Image by Flickr.com, courtesy of Cambodia4kids.org Beth Kanter
