The unit of luminous flux that is part of the international system is called lumen . A lumen is equivalent to the luminous flux emitted by a specific uniform source located at the vertex of a solid angle of one steradian with an intensity of one candela .
As you can see, to understand what a lumen is - whose symbol is lm - it is necessary to analyze several concepts. The luminous flux , in this framework, is the magnitude that refers to the luminous energy that a body emits or receives in a unit of time.
A solid angle, meanwhile, is a portion of space that is limited by a conical surface. Its unit in the international system is the steradian ( sr ). The candela ( cd ) is another unit, in this case responsible for measuring luminous intensity: that is, the luminous flux emitted in a specific direction per unit of solid angle.
The lumen, in short, is defined from its relationship with the steradian (the unit of solid angle) and with the candela (the unit of luminous intensity). The formula is the following:
1 lm = 1 cd x sr
In other words, when a light source emits a luminous intensity of one candela uniformly at a solid angle of one steradian, the total luminous flux of its emission at that angle is equivalent to one lumen.
There is a concept that is often confused with "lumen" although it does not mean the same thing: it is "lux", which is used to determine the amount of light that is projected on a surface . There is a relationship between the two, as one lux is equal to one lumen per square meter.
In this framework, it is necessary to take into account the orientation of the light source, since lux does not consider the number of light sources from which the light flow originates, nor its structure or color . If we talk about a lamp, for example, the value of its lux is usually equivalent to the amount of light that is located in the center of the angle, that is, where the intensity reaches its maximum peak.
We can say that lux is different from lumen because it takes into account the area on which the light flow is projected; If a flux of 2000 lumens is projected onto a surface of one square meter, then we can say that it has an illuminance of 2000 lux. If, on the other hand, the area were ten square meters, then the value in lux would drop to 200, given the relationship that exists between both units.
Continuing with artificial lighting based on electricity, on the box of any lamp that we can purchase in a store there are different technical data, and among them is its light output expressed in lumens. For example, a 23W fluorescent lamp typically emits between 1400 and 1600 lumens.
This is usually required by law. Since September 1, 2010, the European Union put into force the requirement to label the luminous flux in lumens for any commercial lighting equipment, replacing the value of "electrical energy power." In a specific example, a 60 W bulb must have a light output equal to or greater than 700 lumens.
In the field of biology , on the other hand, the internal area of a tubular-shaped structure , such as blood vessels or the intestine, is called lumen. The space inside a vein through which blood circulates and the space inside the gastrointestinal tract, for example, can be called the lumen.