# Thread: Lumen to foot-candle conversion

1. ## Lumen to foot-candle conversion

I need to convert a lumen requirement for some lighting to foot-candles which is the unit used by the light manufacturer. The lighting of concern is overhead workspace type lighting.

I have read several articles but they are somewhat inconsistent in defining the two units and their relationship.

I have read that a foot-candle = lumen/square foot. Given a lumen rating what area is used to calculate foot-candles? Do I take the lumen rating spec and divide by the square feet of area that is to be illuminated to arrive at foot-candles required? That makes sense but it doesn't seem to take beam profile and other factors in to account.

Any help with this conversion would be much appreciated.

Thanks!

2. ## Re: Lumen to foot-candle conversion

Wikipedia says a foot-candle is equal to 10.76lux, so there's no lumen equivalent.

3. ## Re: Lumen to foot-candle conversion

Thanks for the reply. That is initally what I thought but several websites made the foot-candle seem like an english unit equivalent to the lumen.

I suppose the spec using lumens was poorly written.

4. ## Re: Lumen to foot-candle conversion

Not sure I have an answer, but wikipedia also has information on foot-candles, lumens and lux on their definition of Lux.

http://en.wikipedia.org/wiki/Lux

One lux is one lumen per square meter

It does get confusng...

Regards,

James Jackson
Oztronics

5. ## Re: Lumen to foot-candle conversion

Originally Posted by K5Guy
I need to convert a lumen requirement for some lighting to foot-candles which is the unit used by the light manufacturer. The lighting of concern is overhead workspace type lighting.

I have read several articles but they are somewhat inconsistent in defining the two units and their relationship.

I have read that a foot-candle = lumen/square foot. Given a lumen rating what area is used to calculate foot-candles? Do I take the lumen rating spec and divide by the square feet of area that is to be illuminated to arrive at foot-candles required? That makes sense but it doesn't seem to take beam profile and other factors in to account.

Any help with this conversion would be much appreciated.

Thanks!
There is no direct conversion. They are physically different concepts.

Foot-candles are a measurement of light intensity - how "bright" is the illumination striking a particular object or surface. It is a measurement that is taken at a single point. If you have a large surface, you need to take a whole bunch of individual foot-candle measurements at different points in order to get an overall average. It is common to find foot-candle recommendations in the field of ergonomics and industrial engineering, where people need to be provided with adequate light to perform certain tasks. Foot-candle measurements obey the inverse-square law: if you double the distance from the light source, you will find the intensity is now 1/4 as bright.

Lumens are a measure of total light output from a source. If you could take a foot-candle measurement at every single point in the entire beam pattern of a particular source, and multiply by the total area of the beam, you would end up with lumens. Distance from the source makes absolutely no difference.

Hope that helps. You mention you are trying to light a workspace - care to describe the situation in greater detail?

6. ## Re: Lumen to foot-candle conversion

Originally Posted by soffiler
There is no direct conversion. They are physically different concepts.
Yes, you never can compare lumen and candlepower.

7. ## Re: Lumen to foot-candle conversion

Thank you to all that replied. The spec was re-translated as lux not lumen

8. ## Re: Lumen to foot-candle conversion

The lumen (symbol: lm) is the SI unit of luminous flux, a measure of the "perceived" power of light. luminous flux is adjusted to reflect the varying sensitivity of the human eye to different wavelengths of light.

If a light source emits one candela of luminous intensity uniformly across a solid angle of one steradian, its total luminous flux emitted into that angle is one lumen.

The unit a foot-candle is defined as the amount of illumination the inside surface an "imaginary" 1-foot radius sphere would be receiving if there were a uniform point source of one candela in the exact center of the sphere.

Unless i'm nuts sounds like a 1 to 1 ratio since these measurements aren't exact science but an aproximation anyway????

9. ## Re: Lumen to foot-candle conversion

Originally Posted by bgbdbill67
Unless i'm nuts sounds like a 1 to 1 ratio since these measurements aren't exact science but an aproximation anyway????
No, it's not a 1 to 1 ratio, and yes, it is an exact science. Lumens and foot-candles are different concepts. Please refer to my post #5 above.

Lumens are a measure of light energy, which could also be expressed in watts if you prefer. Lumens however are weighted using a standard curve which represents the average response of the human eye. Not an approximation. The curve may not precisely match YOUR eyes but the curve itself is standardized, no approximation involved.

Foot-candles are a measure of light intensity. You could take 1 lumen and concentrate it into a very small, laser-like beam, and you'd have a relatively high foot-candle measurement because the light in that tight beam is very intense. Likewise you could take 1 lumen and spread it out over a larger area, resulting in a relatively much lower foot-candle measurement because you've spread it out, it's less intense.

11. ## Re: Lumen to foot-candle conversion

Originally Posted by K5Guy
I need to convert a lumen requirement for some lighting to foot-candles which is the unit used by the light manufacturer. The lighting of concern is overhead workspace type lighting.

I have read several articles but they are somewhat inconsistent in defining the two units and their relationship.

I have read that a foot-candle = lumen/square foot. Given a lumen rating what area is used to calculate foot-candles? Do I take the lumen rating spec and divide by the square feet of area that is to be illuminated to arrive at foot-candles required? That makes sense but it doesn't seem to take beam profile and other factors in to account.

Any help with this conversion would be much appreciated.

Thanks!

Yes, a foot-candle is expressed in lumens/square foot, while a lux is expressed in lumens/square meter. A foot-candle is equal to 10.76 lux, and this is derived from 1 lumen/square meter = 1 lumen/10.76 square feet. (One square meter = 10.76 square feet).

Yes, by lumen method of calculating illumination intensities, you can divide the area to be illuminated by the lumen rating of the lamp. But you have to take other considerations. The formula used is: Illumination = (Lamp lumen rating x Coefficient of Utilization x Lamp light loss Factor)/Area to be illuminated. The Coefficient of Utilization (CU) takes care of the beam profile and how much lighting actually goes into the area that will be lighted. This is provided by the manufacturer. Manufacturers normally base this CU at 10 feet height. The Light Loss Factor is also provided by th manufacturer.

I hope this helps.

12. ## Re: Lumen to foot-candle conversion

Originally Posted by soffiler
No, it's not a 1 to 1 ratio, and yes, it is an exact science. Lumens and foot-candles are different concepts. Please refer to my post #5 above.

Lumens are a measure of light energy, which could also be expressed in watts if you prefer. Lumens however are weighted using a standard curve which represents the average response of the human eye. Not an approximation. The curve may not precisely match YOUR eyes but the curve itself is standardized, no approximation involved.

Foot-candles are a measure of light intensity. You could take 1 lumen and concentrate it into a very small, laser-like beam, and you'd have a relatively high foot-candle measurement because the light in that tight beam is very intense. Likewise you could take 1 lumen and spread it out over a larger area, resulting in a relatively much lower foot-candle measurement because you've spread it out, it's less intense.
This forum sounds so academic but still no calculation formula to determine foot candles. Is there a formula to calculate the lumen output needed to obtain say 30 ft-candles at desk level in an office? If there isn't then this is all pretty much theoretical and not practical. Real world says that I need to be able to provide for a customer the requirements needed for sufficient lighting. I mean after all there are foot candle meters used. That had to be manufactured with a formula in mind to make it practical. Other than a light meter is there a decent formula out there that would calculate the ft-candles for a given lumen output.

13. ## Re: Lumen to foot-candle conversion

Originally Posted by jocin
This forum sounds so academic but still no calculation formula to determine foot candles. Is there a formula to calculate the lumen output needed to obtain say 30 ft-candles at desk level in an office? If there isn't then this is all pretty much theoretical and not practical. Real world says that I need to be able to provide for a customer the requirements needed for sufficient lighting. I mean after all there are foot candle meters used. That had to be manufactured with a formula in mind to make it practical. Other than a light meter is there a decent formula out there that would calculate the ft-candles for a given lumen output.
The post right before yours by ArtMo tells you exactly how to do the calculation.

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