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 common men, because they are not used in our every-day life. Experts have fixed more than 125 units clearly defined. There are half a dozen of them only for measuring intensity of light, in which included are Candlepower and Lumen.
But first of all let’s understand the unit called Foot-candle. One Foot-candle of light is the amount of light that 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 100 Foot-candles of light.

Now Lumen. Lumen is a unit of measurement of light. It measures light much the same way. 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. So, take your candle and ruler, place a book at the opposite end from the candle. You will have a bit of a light up 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. Candlepower is a way of measuring how much light is produced by a light bulb. Is it a measure of how much light falls upon an object some distance away? No. That’s luminance. Is it a measure of how well we see an object that is illuminated by that light source? No. That’s something altogether different. Years ago Britain gave a definition that the light emitted by 7/8 inch thick candle made of sperm whale’s pure wax is equal to 1 Candlepower! Afterwards, 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, measured at the bulb, rather than how much falls upon the thing you want to light up. Definition of Candela: 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.

More Reading:
Foot-candle (Wikipedia)
Lumen (Wikipedia)
Candela (Wikipedia)
Candlepower (Wikipedia)

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