There are different units for the same thing (Fahrenheit and Celsius temperature scales), and there are similar units for slightly different things (troy ounces and avoirdupois ounces). Measuring light is no different, because there have been many methods of measuring it proposed over the years and different groups will use differing units or methods. I'll try to give you a basic understanding of the differences for the common measurements of light, how they are used, and what they actually mean.
Candle-power or Candela
One candle-power is the amount of light put out by a single standard candle. The “standard” candle was a pure whale wax (Spermaceti) candle weighing 1/6th of a pound, burning at a rate of 120 grains (there are 7000 grains in a pound) per hour. Any normal candle will put out about the same amount of light, so don't worry about the details too much.
Lumen
The total output of light in the
visible part of the spectrum. Used for measuring the total amount of
light produced by a source, most often used when comparing flood- or
lantern-type lights. Household light bulbs will have their output
listed in Lumens since they radiate light in a spherical manner. The
light produced is what is measured in Lumens, but the light falling
on a surface is measured in Lux or Foot-candles.
Lux
The metric unit of measure for
illumination of a surface. Most commonly used when comparing
spot-type light sources. One lux is equal to one lumen per square
meter. A good example is a flashlight with a variable focus, like a
Mag-Lite. The bulb puts out a steady amount of light (Lumens) but by
changing the focus you can concentrate that light into a smaller
area, making it brighter (Lux).
Foot-Candle
A foot-candle is actually one lumen of
light density per square foot of a target one foot from the source of
light, and is the American (SI) version of the Lux. Since most of
what is sold in the US is made in countries that use the metric
system, it's not likely that you'll see foot-candles listed on new
lights.
Watt
Some lights are advertised by the wattage of their LED or bulb. Watts do not relate directly to the light output level. As used for light sources, Watts defines the rate of energy consumption by an electrical device when it is in operation. Differences in the efficiency of the LED or bulb as well as in the amount of light produced that is visible to the human eye make it difficult to relate Watts of power usage to light output.
I hope that clears up some of the
confusion about the different units. I managed to keep most of that
pesky math out of it while I did it, too.
Footnotes
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