# Calculating Fc from W

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### #1 Miguel Bunster

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Posted 01 November 2005 - 03:18 PM

Hi is there any formula to calculate the Fc of a given light? I am looking into some fluorecents and cant find there Fc info all I know is that they are 40w. Is there a formula, or any way to get an idea?
Thanks!
Miguel
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### #2 Mitch Gross

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Posted 01 November 2005 - 05:51 PM

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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### #3 Ozgur Baltaoglu

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Posted 01 November 2005 - 06:26 PM

the following is an approximation:

1 lux= 0,0929 fc
1watt= 58 cd (candels)
1 fc =(app.) 10 lux

for instance

1K @ 10 meters @ ISO 500 gives little over

50 lux = T 4.2 / T 4 (@ 24fps / 170 Shutter ANgle)

Ofcourse the formulae don't guarantee good cinematography
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### #4 Dominic Case

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Posted 01 November 2005 - 06:58 PM

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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### #5 Phil Rhodes

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Posted 01 November 2005 - 10:00 PM

Hi,

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!

Phil
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### #6 Laurent Andrieux

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Posted 02 November 2005 - 09:46 AM

Thanks Dominic, BTW.

This link gives me better translations for these terms than the ones I had...
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### #7 Miguel Bunster

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Posted 02 November 2005 - 09:49 PM

Thanks for the info..I will meter it. It was just a tought but the info is great!!!
Thanks!
Miguel
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