Candela vs Lumen

Candela and lumen are two units used to measure some properties of light. Candela is used to measure the luminosity intensity detected by the human eye. Lumen is used to measure the luminous flux. Both of these units are very important in studying light and other electromagnetic waves. A proper understanding in candela and lumen is required in fields such as classical optics, modern optics, electromagnetic theory and various other fields in physics. In this article, we are going to discuss what candela and lumen are, what quantities are measured by these units, their similarities and finally the difference between candela and lumen.


Candela is a base SI unit. Candela is used to measure the luminous intensity of visible light that is emitted by a particular source in a given direction. The luminous intensity is measured by multiplying the power of emission in a particular direction multiplied by the luminosity function. Luminosity function, which is also known as the luminous efficiency function, is a function that describes the optical sensitivity of the eye for a given wavelength.

The symbol for candela is cd. It is a base SI unit. In general, a candle emits 1 candela. The name root of candela leads to the meaning “candle”. The candela is defined as “the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540×1012 hertz and that has a radiant intensity in that direction of 1⁄683 watt per steradian” by the 16th General Conference on Weights and Measures in 1979.


Lumen is a SI derived unit. Lumen is a unit that is used to measure luminous flux. Luminous flux is a measurement of the amount of visible light incident on the human eye. Luminous flux also depends on the luminosity function. Luminous flux can be derived from the luminous intensity, which is measured by candela.

Luminous flux is formulated as the product of luminous intensity and the solid angle measured from considering the source as the center. 1 lumen is equal to 1 candela steradian. The symbol of lumen is lm. Lumen is not a SI base unit. The term “lumen” is evolved from the word luminous, which describes how bright an object looks.

Both lumen and candela are dependent on the sensitivity of the human eye.