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 lux, to understand the difference and relationship between the two.
The lumen (lm) is a SI unit of measurement for luminous flux, that is used to show how strong 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 lux (lx) is the SI unit of measurement for luminous intensity. It is used to measure illuminance, the brightness of an object, or how much light is shining on a surface.
Lux measures the amount of light that reaches a one meter square with one lumen of light.
Lux is typically measured in lumens per square meter.
1 lux is equal to the amount of light produced by 1 candle (which is 1 lumen) / 1 meter square
1 lx = 1 lm/m²
1 lux is the metric unit of measurement equivalent to 0.0929 footcandles, the imperial unit of measurement for illuminance.
1 lx = 1 / 10.76 fcd
10.76 lx = 1 fcd
Lumens and lux are both units of measurement used to describe light intensity, so what is the difference between lux and lumens?
So lumens tell you how bright a single light is overall, while lux tells you the amount of light that is reaching a surface from any number of light sources.
Lux is used for telling you how much light you need to achieve for a certain level of brightness on a surface or an entire room.
A lux level is recommended for the use of the space to ensure there is enough illumination to comfortably or safely perform the appropriate task in that area.
Each country and each state have its requirements but here are some typical examples of where lux measurements are used.
Lux do not define how bright a light source is but rather how bright it will be within given distances and beam angles.
Lux is therefore a more specific measurement about the use of the lumen output from a light and less useful for a comparison between light sources.
Lumens are best used to choose the appropriate light source and help compare between different light sources.
The relationship between lumens and lux is:
1 lux = 1 lumen / 1 m²
1 lumen = 1 lux x 1 m²
For example, a light source that emits 2065 lumens will produce 2065 lux spread evenly over the surface at a distance of 1 meter.
If we move that light source further away, the light will spread out over a larger area and the lux will be reduced.
At a distance of 2 meters, the same light source would produce 516 lux.
(This is calculated as 2065 lm / 2m x 2m, equivalent to 2065 / 4 = 516 lx)
At a distance of 3 meters, it would produce 229 lux.
At a distance of 4 meters, it would produce 129 lux.
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 lux.
Let’s look at recessed ceiling light and see how the lux level changes when the beam angle widens spreading the light wider, using a constant source of lumens.
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 lux, diminishes significantly compared to the lumens being delivered.
The light intensity is highest in the center of a beam of light, so if you see an LED lamp or lighting fixture specify a lux level as well as the lumens, it will be an indicative level of where it is most intense within a given distance.
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.
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