Countryal
UKC Forum Member
Registered: Dec 2003
Location: Eclectic Alabama
Posts: 131 |
Here's a pretty technicale answer. My understanding of how bright an object is lit has more than one thing to consider not just lumen and candle power but beam focus also. Hey and I'm not as smart as the fellow that wrote this! I just copied it! I think I understand the basics. Larry
*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.
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.
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.
*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.
*LUX is an abbreviation for Lumens per square meter.
Foot-candles equal the amount of Lumens per square feet of area.
So, that one candlepower equivalent equals 12.57 lumens.
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Last edited by Countryal on 12-26-2010 at 01:42 AM
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