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Calculating Fc from W

Miguel Bunster

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Wattage won't tell you the answer because there are various coatings and gas combinations that can result in very different brightnessness. You also need to check the CRI (color rendition index) of the lights to see how well they represent the color spectrum. The bulb manufacturers usually have photometric data available either on their website or via mail.

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A fluorescent lamp will give you between 50 and 100 lumens per watt - depending on its luminous efficiency.


But to convert either watts or lumens to ft candles is a bit like converting feet to pints. They are different units that measure different things.


Foot candles measure illuminance - how brightly something is lit. That's not the same as how much light is coming from a light source. How far away is it? Where is the light directed?


Here's a link that might be useful


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That rough guesstimation formula would be OK if you knew the absolute luminous efficacy of the tube and any reflector, which you don't. Manufacturers don't tend to publish the spec because it's too hard to measure and meaningfully apply; a fluorescent tube has an obscenely complex radiation pattern when considered mathematically.


Get a meter, and meter it!



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