In our articles, but also on the packaging of lamps and their data sheets, you will often see the words lumen, candela and lux. Many readers will probably wonder what these terms say about light. Is it not enough to know how many watts a lamp has?
Sadly no. More watts does not necessarily mean more light. That was at most the case with old household lamps (“light bulbs”). The comparison of the wattage of household lamps with that of car lamps is a false one though. Conventional H7 halogen lamps produce a luminous flux of 1500 lumen from 55 watts. The corresponding bulb, which is not sold in Europe anymore, needs as much as 100 watts for that. A xenon lamp even gets 3000 lumen from 35 watts.
That takes us straight to the most important unit of measurement for light – the lumen. It represents luminous flux. The higher it is, the more light a light source produces. But the problem with this physical unit abbreviated as lm is that it can only be used for light sources that emit light in all directions. In headlights, but also in household reflector lamps, the light is more or less focused though. This is where candela, the luminous intensity, comes in.
The unit abbreviated as cd can be explained as follows: when light is focused, it only comes out of one part of a sphere. The smaller the part, the more candela are produced from a light source. That’s the reason why a car headlight can turn 1500 lumen into several tens of thousands of candela.
By the way, that’s Latin for candle. And one candela also roughly corresponds to the light a simple household candle produces. Actually, LEDs would always need to be specified in candela since they don’t emit light in all directions.
What is important for drivers is how much light is thrown onto the road. In this case lighting technicians talk about lux, the illuminance. Things are easier with it than with candela. 1 lx – that’s how it’s abbreviated – corresponds to one lumen per square meter.
Lumen, candela, lux and watts are internationally standardized units known as SI units. In North America, the dated unit candlepower (cp) is still commonly used for luminous intensity. It can be converted one to one using 0.98 candela.