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 candela, 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 candela (cd) is the unit of measurement for luminous intensity. It measures the amount of light that is emitted in a particular direction.
Candela is sometimes referred to as candlepower since it refers to the common and uniform output of light from when the candle was the main source of illumination.
The term candlepower was changed to candela in 1948.
1 candela is equal to the amount of light produced by 1 candle.
Lumens and candela are both units of measurement that can be used to describe how much light is being delivered, but they differ.
Lumens measure the total amount of light output by a source, while candela is more specific in measuring how much of that light is travelling in a particular direction.
For example, let’s look at recessed ceiling light or sometimes known as a downlight, and see how the candela (intensity) changes 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, the candela, diminishes significantly compared to the lumens being delivered.
In other words, the candela measurement is showing the light is having to spread further, becoming weaker in it’s intensity.
Note that 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 candela is:
1 lumen = 1 candela multiplied by the solid angle of the light beam.
The solid angle is a three-dimensional measurement of how much of the light is visible from a particular point. It is measured in units of steradians (sr).
1 candela is equal to 1 lumen per steradian.
For example, if a light source has a luminous intensity of 1000 cd and a beam with a solid angle of 2 sr, then that light source would have a luminous flux of 2000 lumens.
For estimation purposes:
1 candela = 12.57 lumens
Lumens are more commonly used than candela because they are easier to measure and understand.
Candela can be more difficult to measure because it requires taking into account the solid angle of the light beam and has less of a universal application to the majority of everyday lighting choices that people make.
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