Both candlepower and lumens refer to how much light a light source produces, but candlepower deals in radiance and lumens deal in illumination. Lumens is the more practical term and is often used for measuring the capabilities of LED lights or other devices, since what matters in these cases is how much light is shone on other objects, not how much light is produced from the source. Both terms are used when calculating the power and effect of lights meant for industrial purposes.
Candlepower is a scientific measurement of light at its source or how much light is produce by an object itself. If a lamp has 200 candlepower, then it produces--at its source--the equivalent radiance of 200 candles. Candles themselves are unpredictable in terms of radiance, so the actually value was replaced long ago with an exact metric definition that roughly equals the light an average candle puts out. Technically, this definition says that one candlepower, or candela, is the same as a monochromatic radiation of frequency 540 x 1012 hertz and which has a radiant intensity in that direction of 1/683 watt per steradian. This essentially transforms energy and frequency ratings into light being produced.
Lumens, on the other hand, are a measurement of illumination. Radiance refers to the light produced, not its effect on external objects. Illumination refers to how well the light reveals objects, and the lumen is the primary unit of measurement in this field. This is a more practical term that shows precisely how well the light illuminates other objects based on distance. One candela, for instance, is equal to about 12.57 lumens. Lumens are measured by looking at how much light is radiated out from a single source in the centre of a theoretical sphere to the boundaries of that sphere.
Lumens should not be confused with foot-candles. Foot-candles are similar but have no area. Instead, they measure how much light a source generates based on feet away. One foot candle is the light a candle produces one foot away from the source. Lumens, on the other hand, are measured in area and are metric in nature instead of English.
Watts are used in figuring out the power of lumens and candlepower, but refer only to the energy being used, not the light. By looking at how much energy a light source uses and how it is used, scientists can determine how much light is being given off by the source. This is especially useful for calculating candlepower.
LUX is a term used frequently when discussing the power of lights to illuminate, and it means lumens per square meter. This shows how well a light source illuminates at a distance, and the more lumens that light gives one square meter of space the farther it will reach.
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