Lumen to foot-candle conversion

[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!

[LukeA]

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

[K5Guy]

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.

[James Jackson]

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

[soffiler]

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?

[maxilux]

Originally Posted by soffiler

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

Yes, you never can compare lumen and candlepower.

[K5Guy]

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

[bgbdbill67]

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????

[soffiler]

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.

[LEDninja]

http://flashlightreviews.com/features/lux.htm