What are the units of measurement of light?

Apart from Kilogram, Celsius, Liter, Watt, Calorie, etc. there are many other units of measurement unknown to a layperson, mainly because they are not used in our everyday life. Experts have fixed more than 125 clearly defined units. Half a dozen of them are only for measuring the intensity of light, including Candlepower and Lumen among many others.

First of all, though, let’s understand the unit called Foot-candle. One Foot-candle of light is the amount of light that a birthday cake candle generates one foot away. If you say you have a lamp that produces 100 Foot-candles of light, that means at one foot from the lamp, you will receive the amount of light equivalent to that of 100 birthday candles.

A lumen is a unit of measuring light that we might have heard of when buying flashlights or other forms of LED lights. It measures light much the same way as Foot-Candle. Remember, Foot-candle is how bright the light is one foot away from the source. Lumen is a way of measuring how much light gets to what you want to light! Lumen is equal to one Foot-candle falling on one square foot of area. You can better understand this by taking your candle and ruler, and placing a book at the opposite end of the candle. You will have a bit of light if you put the book right next to the candle. If that book happens to be one foot by one foot, it is one square foot. Now, all the light falling on that book, one foot away from your candle equals both, 1 Foot-candle and one Lumen!

Candlepower is a way of measuring how much light is produced by a light bulb. Years ago, Britain gave a definition that the light emitted by a 7/8 inch-thick candle made of sperm whale’s pure wax is equal to 1 Candlepower. Afterward, America gave a new unit called Candela, which is not much different from Candlepower. Nowadays, the term Candela is used instead of Candlepower.

Candlepower, or Candela, is a measure of how much light the bulb produces. It’s measured at the bulb, rather than how much falls upon the thing you want to light up. The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 × 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.

In short, Candlepower is a rating of light output at the source, using English measurements. Foot-candles are a measurement of light at an illuminated object. Lumen is a metric equivalent to Foot-candle in that it is measured at an object you want to illuminate. Divide the number of Lumens you have produced, or are capable of producing, by 12.57 and you get the Candlepower equivalent of that light source.

We use light for just about every task in our lives, so it’s always good to have some backup sources.

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Why does a candle flame defy gravity and burn upward?