Light made understandable, part 1, Candela, Luminous flux, Efficacy.

In this series I will try to explain lighting terms (properties) a bit more in detail. Many new people (companies) are entering the lighting business that do not always know the basics. Do you have to be an expert, no, but you have to understand the basics. So lets take a look at the lumen.

Many people discuss about the lumens emitted from a light source or the lumens from a fixture. Lets explore lumens a bit more. The lumen is the dimension of luminous flux. The luminous flux is the total light coming from a source corrected for light sensitivity of the human eye. The eye is not equally sensitive for the whole spectrum.

So the total light coming from a light source is measured in lumen. Lumen is derived from the candela.

So what is a candela? It started out as the light coming from a defined candle and was the start om measuring light. The candela is now very clearly defined, no need to go into those details, but the basic idea is that it is intensity measured.


The lumen or the luminous flux is the total light coming from a light source. While the candela is view from one side to the source. I will not bother you with the formula but the flux can be calculated back to the light source. If the light is homogeneous, 1 candela will supply a flux of 12.6 lumen.

The flux is therefore the total light being produced by a light source independent of the direction it is going in. It does give you information on how much light you have at the source but for the rest it does not give a lot of information.

Another issue directly related to the flux is the efficiency of a light source. You hear the strangest things but in the end it is not so difficult to calculate. You simply look at the flux coming from the light source and divide that through the energy that you put in. Clean and simple but beware.

Maximum efficiency is theoretically around 680 lumens per watt. Everybody that claims a higher efficacy has broken the laws of physics, which you can never exclude but will be highly unlikely.

Efficacy can be for a LED on LED Level, on LED package level, on LED Module level, on LED module/driver level, or f.e. on a complete luminaire level. In the past I have seen many example where the efficacy was given at LED level, under lab conditions at ideal temperatures etc so not realistic in real life conditions. Efficacy goes down when temperature goes up, when components are added in the form of electrical components but of course also in the form of lenses, reflectors etc. Always be sure when you talk about efficacy which one you actual are discussing.

The best commercial available LED that I have seen so far reaches up to just over 200 lm/W. in the application level this will be a lot less. So there is still a lot of room for improvement since we are just about to go to 30% efficiency in converting electric energy to light. Lets not consider the losses that occur due to the conversion of oil to electricity and the transport of the electricity because if you take that in the calculation as well the numbers are really incredibly low.

Also on the electric power there is a lot to say but that will have to wait till another part in the series.

-Luminous flux is the total light coming from a source irrespective of the direction
-The Candela is a defined standard unit (SI) that defines light intensity
-The efficacy of a source is the relation ship between the energy put into the system and the actual light being produced.
-The efficiency is the percentage of power that is actually converted in light. Normally in lighting Efficacy is used in stead of efficiency.

Candela: a unit (basic SI) of luminous intensity equal to 1/60 of the luminous intensity per square centimeter of a blackbody radiating at the temperature of solidification of platinum. Also called candle. Symbol cd

Lumen: the derived unit of luminous flux; the flux emitted in a solid angle of 1 steradian by a point source having a uniform intensity of 1 candela (SI). Symbol: lm

Watt: a derived unit of power, equivalent to one joule per second and equal to the power in a circuit in which a current of one ampere (SI) flows through a resistive load with a potential difference of one volt. Symbol: W