LED Lighting: What is Lux, Lumen and Candela ?

When we talk about LED Lightings, there are several important units that we often see: Lux, Lumen and Candela.

These units are often used interchangeably. This gives confusion about the quality and application of lighting. It is also difficult to make a good comparison. We are happy to explain the difference between these units and what to look for when purchasing new LED lamps.

Lumen

The lumen (symbol: lm) is the unit for luminous flux. It measures the total amount of light emitted by a light source in all directions. Luminous flux takes into account the sensitivity of the eye to the visible part of the electromagnetic radiation. Lumen is therefore the unit to measure the brightness of a light source independently of the direction of the light beam.

Luminous flux is a measure of the total amount of light in a light beam. Light intensity is a measure of the light density. If one sees a part of a light beam, that part has a smaller luminous flux (in lumen) but (in principle) the same luminous intensity (in candela) as the whole beam. The luminous flux depends on the power of the source and on the color of the light. An incandescent lamp gives a luminous flux of approximately 12 lm / W. The smaller the angle of radiation in which the light is emitted, the higher the luminous intensity (with constant luminous flux).

Lux

The lux (symbol lx), latin for light, is a unit of illumination: 1 lux is the illuminance produced by 1 candela on a surface perpendicular to the light rays at a distance of 1 meter from the source.

The lux thus corresponds to the illuminance that is obtained when each square meter of the considered surface receives a luminous flux of one lumen. The number of lux is thus found as the quotient of the total received luminous flux, expressed in lumens, and the size of the illuminated area expressed in square meters; therefore 1 lux = 1 lumen / m².

Lux can be measured from different distances. Be careful when making a comparison between two lamps. A lamp that gives the same number of lux at three meters distance than a lamp measured at one meter is much more powerful than the one measured on a meter.

With a light meter you can measure the lux value of any surface or space. When measuring, make sure that the light meter is placed at working height. As the distance from the light source to the object to be illuminated increases, the illuminance decreases in the square.

Candela

Brightness is indicated by the candela (symbol cd). The light intensity indicates how much light is in each piece of a light beam. The word candela means candle in Latin. One candela corresponds approximately to the light intensity of a normal candle.

The candela is related to the lumen and lux units. The unit lumen is used for the total luminous flux in a light beam. A light beam with a strength of 1 candela and a space angle of 1 steradian (for a cone-shaped beam corresponds to an opening angle of 65.5 °) has a total luminous flux of 1 lumen. 1 candela is thus equal to 1 lumen per steradian. When a beam with a strength of 1 lumen falls on a surface of 1 square meter, this gives an illuminance of 1 lux. 1 lux is therefore equal to 1 lumen per square meter.