Figuring out the brightness of a light fixture can be confusing. Lighting is often rated in terms of how much electricity it uses, which doesn't necessarily coordinate with how much light it gives off. But the packaging for light fixtures also presents ratings in units that measure light, usually in lumens or candle power. These two units are not synonymous but provide two types of information to help consumers determine the amount and intensity of light emitted.
Units of Light
Light can be measured with the units lumen or candle power. Candle power is an outdated term and was replaced with the term candela, or cd. One candela is roughly equivalent to the light output of one candle. Lumens, lm, and candela are both units that measure light, but they are not interchangeable. You can convert between the two units, but the ratings do not describe the same qualities of light output. Lumens measure the total light output of a fixture in any direction, so not all the output is necessarily usable light depending on its distribution. Candelas measure the beam intensity at its brightest point and aimed in one direction. Both are useful parameters for describing the light intensity of a spotlight.
Comparing Lumens and Candelas
Lumens is a measurement of the amount of light produced by a fixture. It measures the rate of flow of light which equals the light output of the device. Candelas measure the density of light over an area in terms of the intensity of the light emitted. The conversion factor between lumens and candelas is 12.57, or 4π. One candela has a light intensity of 12.57 lm. The inverse, or 1 ÷ (4π), is used to determine the intensity or candelas per lumen. One lumen has a light intensity of 0.08 cd.
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Conversely, the light intensity of 1 cd correlates to a total light output of 12.57 lm, because 1 cd = 4 × π. A spotlight with a light output of 600 lumens has a light intensity of 48 cd. A spotlight with a light intensity of 3,000,000 cd has a light output of about 37,710,000 lm.
Determining the lumen and candela ratings for a light fixture provides insight to the utility of a light for a specific task. A lumen rating for a spotlight tells how much illumination can be expected from the exposed portion of the bulb. A spotlight with a high lumen rating produces light that illuminates a wide area near the bulb. A candela rating tells more about how far the light will be visible from a distance. A spotlight with a high candela rating can shine a narrow, focused beam of light a great distance but does not illuminate the area surrounding the bulb very much.
Consumers may be more accustomed to buying light fixtures and bulbs measured in watts than lumens. However, watts are not useful for determining the brightness of a light because they measure the power usage, not light output of a bulb. The conversion of lumens to watts varies according to the type of bulb used and its level of efficiency.