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Lumens, Illuminance, Foot-candles and bright shiny beads….
In defining how bright something is, we have two things to consider.
1. How bright it is at the source- How Bright is that light?
2. How much light is falling on something a certain distance away from the light.
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.……
We're in America, so we are going to talk about units of measurement that concern distance in feet and inches. So, we will use some terms that folks in Europe don't use. We're going to talk about "foot-candles".
This one's simple. Get a birthday cake candle. Get a ruler. Stick the candle on one end of the ruler. Light the candle. Turn out the lights. One foot-candle of light is the amount of light that birthday cake candle generates one foot away.
That's a neat unit of measurement. Why? Say you have a lamp. You are told it produces 100 foot candles of light. That means at one foot from the lamp, you will receive 100 foot candles of light.
But here's where it gets tricky. The further away you move the light from what you want to illuminate, the less bright the light seems! If you measure it at the light, it's just as bright. But when you measure at the object you want illuminated, there is less light!
A Physics teacher is going to tell you that light measured on an object is INVERSELY PROPORTIONAL to the distance the object is from the light source. That's a very scientific and math rich way of saying, the closer you are to the light bulb, the brighter that bulb is. Or, think of it this way. You can't change how much light comes out of your light bulb. So, to make more light on an object, you have to either move the light closer, or add more lights.
Now, lets get to LUMENS.
A LUMEN is a unit of measurement of light. It measures light much the same way. Remember, a foot-candle is how bright the light is one foot away from the source. A lumen is a way of measuring how much light gets to what you want to light! A LUMEN is equal to one foot-candle falling on one square foot of area.
So, if we take your candle and ruler, lets place a book at the opposite end from the candle. We'd have a bit of a light up if we put the book right next to the candle, you know. If that book happens to be one foot by one foot, it's one square foot. Ok, got the math done there. Now, all the light falling on that book, one foot away from your candle equals both…….1 foot candle AND one LUMEN!
Let's split off from this and talk about the difference between RADIANCE and ILLUMINANCE.
RADIANCE is another way of saying how much energy is released from that light source. Again, you measure it at the source.
ILLUMINANCE is what results from the use of light. You turn your flashlight on in a dark room, and you light something up. That's ILLUMINANCE.
Illuminance is the intensity or degree to which something is illuminated and is therefore not the amount of light produced by the light source. This is measured in foot-candles again!
Pretend you're an old photographer, like O. Winston Link, or Ansel Adams. These two gods of black and white photography (and a print made by either can fetch quite a hefty sum of money these days) used a device called a light meter to help them judge their exposure.
All this brings back two points.
The first point is if we measure the output of a light at the source that gives us one thing.
The second point is that we use an entirely different unit of measure if we are measuring the results of that light's output.
We've measured two different things. We have a unit of measure for how much light is produced called Foot-candle
More Confusion! Candlepower!
Candlepower is a way of measuring how much light is produced by a light bulb, LED or by striking an arc in a Carbon-Arc spotlight. Is it a measure of how much light falls upon an object some distance away? No. That's illuminance. Is it a measure of how well we see an object that is illuminated by that light source? No. That's something all together different, and we are not going there!
Nowadays we use the term CANDELA 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.
Further confusing the matter is beam focus. That's how much candlepower can be focused using a reflector/lens assembly. Obviously, if you project all your light bulbs intensity at a given spot, or towards something, it will be more intense, and the illuminance will be higher.
And here comes the confuser! A candlepower as a unit of measure is not the same as a foot-candle. A candlepower is a measurement of the light at the source, not at the object you light up.
complete read & source The LEDlight.com
And a candela is the metric equivalent of the light output of that one candle, based on metric calculations. And since using a candle is rather imprecise, the definition was amended to replace a light source using carbon filaments with a very specific light source, see the following:
The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.
The above from the National Institute of Standards Reference on Constants, Units, and Uncertainty.
Candlepower is a measure of light taken at the source-not at the target. Foot-candles tell us how much of that light is directed at an object we want to illuminate.
Summing it all up:
Candlepower is a rating of light output at the source, using English measurements.
Foot-candles are a measurement of light at an illuminated object.
Lumens are a metric equivalent to foot-candles in that they are 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.