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The lumen (symbol: lm) is the unit of luminous flux, a measure of the perceived power of visible light emitted by a source, in the International System of Units (SI). Luminous flux differs from power ( radiant flux ), which encompasses all electromagnetic waves emitted, including non-visible ones such as thermal radiation ( infrared ).
The relation between watts and lumens is not just a simple scaling factor. We know this already, because the 60 watt incandescent bulb and the 15 watt compact fluorescent can both provide 900 lumens. The definition tells us that 1 watt of pure green 555 nm light is "worth" 683 lumens. It does not say anything about other wavelengths.
Home Depot has a fairly lenient return policy. Most merchandise can be returned up to 90 days with proof of purchase, according to the website, and a full year if you used your Home Depot credit card.
Luminous efficacy can be normalized by the maximum possible luminous efficacy to a dimensionless quantity called luminous efficiency.The distinction between efficacy and efficiency is not always carefully maintained in published sources, so it is not uncommon to see "efficiencies" expressed in lumens per watt, or "efficacies" expressed as a percentage.
To help compare different orders of magnitude, the following list describes various source in lux, which is a lumen per square metre. Factor Multiple Value
I e is the radiant intensity in watts per steradian (W/sr), y ¯ ( λ ) {\textstyle {\overline {y}}(\lambda )} is the standard luminosity function . If more than one wavelength is present (as is usually the case), one must sum or integrate over the spectrum of wavelengths present to get the luminous intensity: