Comparing two units of light output can be confusing when they are measured in differing units. Here we look at a common photometric comparison of lumens with footcandles, to understand the difference and relationship between the two.
The lumen (lm) is a unit of measurement for luminous flux, that is used to show how strong or intense a light source is. It measures the total quantity of visible light emitted from a light source regardless of its direction.
So lumens measure the amount of light that is produced by a light bulb or lighting fixture. The more lumens, the brighter the light will be.
The footcandle (fcd or fc) is the unit of measurement for luminous intensity. It is typically used to measure illuminance, how much light is shining on a surface.
A foot-candle measures the amount of light that reaches a one-foot square with one lumen of light.
1 footcandle is equal to the amount of light produced by 1 candle (which is 1 lumen) / 1 ft square
1 fcd = 1 lm/ft²
1 footcandle is the imperial unit of illuminance measurement equivalent to 10.76 lux, the metric unit of illuminance measurement.
1 fcd = 10.76 lx
Lumens and footcandles are both units of measurement used to describe light intensity, but they differ.
Lumens measure the total light output of a light source, regardless of direction. Footcandles measure the amount of light that reaches a surface.
So lumens tell you how bright a light is overall, while footcandles tell you the illuminance on a surface.
Let’s look at recessed ceiling light or known as a downlight in Europe, and see how the footcandles (intensity on a surface) change when the beam angle widens.
In this instance, the lumens initially created from the internal LED chip begin at 1138lm.
So the results show that as the beam angle of the light widens from 26 to 58 degrees, the intensity of the light on a surface, the footcandles, diminishes significantly compared to the lumens being delivered.
Note: the delivered lumens in this example only vary a little between each beam angle because of the type of glass lens used to create each angle.
The relationship between lumens and footcandles is:
1 footcandle = 1 lumen / 1 ft²
1 lumen = 1 footcandle x 1 ft²
For example, a light source that emits 1000 lumens spread evenly over a surface will produce 1000 lumens / 1 ft² = 1000 footcandles on a surface one foot away.
The same light source will produce 250 footcandles on a surface two feet away, because the light is now being spread over a larger area twice the distance away.
(This is calculated as 1000 lm / 2ft x 2ft, equivalent to 1000 / 4 = 250 fcd)
At 4 feet away it will produce 62.5 fcd
This demonstrates the inverse square law which states that “the illuminance from a point source is inversely proportional to the square of the distance from the source”.
So, doubling the distance from a light source creating the lumens will result in ¼ of the original foot candles.
Footcandles do not define how bright a light source generally is, but rather how bright it will be within given distances and beam angles.
So foot-candles are therefore a more specific measurement and less useful for a quick comparison between light sources, such as when selecting a flashlight or torch.
Lumens are more commonly used because they are easier to measure, understand and compare.
Looking at a common photometric comparison of lumens with footcandles, light output vs illuminance, see the difference and relationship between the two.
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