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Interesting Fact
Very few artificial light-sources, such as a military flash-bomb, have the high candlepower rating of a narrow beam spotlight, while simultaneously having a very high lumen rating as well.
Be it for a nighttime trek through the jungles of Amazon, or just for a casual exploration of the haunted house in your neighborhood, the importance of having a capable flashlight for after-dark adventures simply cannot be undermined. But, from among the profusion of different flashlights out in the market, how do you pick out the best one? The answer to this question can be found in the luminous output of each individual flashlight.

Each light-source is designed to output light having a certain degree of luminosity. It is a direct indicator of the amount of light that the source can produce. Manufacturers express luminosity in terms of candlepower, lumen, or both. However, these two quantities don't mean the same thing, and therefore, cannot be used interchangeably. A small conversion factor is involved in the process of changing one to another. But before we find out what it is, let's get a clear understanding of what each of the terms 'lumen' and 'candlepower' imply.

Candlepower

Candlepower is one of the oldest units used to measure the intensity of light from a source. Though it is mostly obsolete now, it is still used by flashlight and spotlight manufacturers to specify the luminosity of their products.

From its name itself, one can infer that candlepower expresses the luminosity of a source in terms of the luminosity of an actual candle having standard dimensions and construction. It is important to note that candlepower measures the intensity of light falling on the target, rather than the total intensity of light emitted by the source. As such, the more tightly focused a beam of light is, the greater its candlepower shall be. For flashlights, candlepower measures the brightest part of the beam of light.

Lumen

Lumen is the international standard (S.I.) unit of measurement of luminosity. It measures the total amount of visible light that is being emitted by a source in all directions. Almost every light source, including a flashlight and a spotlight, has an associated lumen rating provided by the manufacturer.

The lumen measurement only takes into account the total amount of light that a source is emitting, without considering how much light is falling upon the target. Thus, the lumen rating of two sources can be the same, even if one outputs a focused beam while the other disperses light equally in all directions.

Difference Between Candlepower and Lumen

Though both lumen as well as candlepower are units used for measuring the intensity of light or luminosity, they differ from each other in terms of the actual positions where each of the measurement takes place.

Lumen measures the overall light intensity emitted at the source, while candlepower measures the intensity of the brightest part of the beam of light. Thus, it is possible to have a light-source with a high lumen rating, but a low candlepower rating, and vice versa.

To get your head around this concept, take the example of a laser source. A laser source emits an extremely focused beam of light (laser). A powerful laser could have a light intensity that is so high that it could burn a hole in the target. However, even the most powerful laser would hardly be of any use in lighting up a dark room.

A laser displays these contradictory characteristics, because even though a laser has a high light intensity in one direction, it has very little light intensity in the remaining ones. The narrower the beam of light, the higher its candlepower-to-lumen ratio will be. In other words, a laser has a high candlepower rating, but a low lumen rating.

Conversely, if you take the example of a normal light bulb which emits equally in all directions, you will find that it has a low candlepower-to-lumen ratio, meaning that, even though its lumen rating is high, its candlepower rating is low.

How to Convert Lumen to Candlepower and Vice Versa

From the above discussion, it is clear that lumen and candlepower measure two different things, and therefore, accurately converting one to another is not possible. However, by applying mathematics, a close-enough approximation can be obtained.

For all practical purposes, candlepower equates directly to candela. From its definition, a source of light emitting one candela of luminous intensity uniformly over a solid angle of one steradian is said to be emitting one lumen.

Thus, a lumen can be expressed mathematically as,

1 lm = 1 cd·sr

where,

lm = lumen
cd = candela
sr = steradian

The solid angle in a full sphere is equal to 4 Π steradians. Therefore, a light source which uniformly radiates an intensity of one candela in all directions is given by following,

1 cd·sr· 4 Π

= 12.57 cd·sr

therefore, 1lm = 12.57 cd·sr

Hence, the formula to covert lumen to candlepower can be obtained as following,

1 lumen = 12.57 candlepower

while the formula to convert candlepower to lumen is given as,

1 candlepower = 0.079 lumen

Thus, to convert from candlepower to lumen, you can multiply the candlepower measurement with 12.57, and to convert from lumen to candlepower, you can simply divide the lumen measurement by 12.57.

For example, a 40-watt lamp rated at 500 lumen has a candlepower of (500 ÷ 12.57) = 39.8 cd.

You can also use the following lumen-to-candlepower and vice versa conversion calculator to convert one quantity to another.

1 lumen(lu) = 12.57 candlepower(cd)


0 lu

Lumen and candlepower are used for measuring two different characteristics of a light source. If any one is known, the other can be approximated using the above method. This can allow you to create a lumen-to-candlepower conversion chart for different flashlights and spotlights, which you can then use to correctly pick the one most suitable for your application.