Lux Figures & Colours

If you’ve paid attention to the lumens figures for multicoloured lights, you will have seen that the efficacy of the colours appears to be much lower than a standard white LED. So why is this?

The short, and not very helpful, answer is because they are colours. But let’s dig a little deeper and provide a better answer. To start, let’s consider what measurement you’re actually looking at. Chances are you are probably looking at the lumens figure, or perhaps you’ve entered an IES file into your design and are looking at the delivered lux. Either way is good but going further we’re going to look closely at lux (lumens is actually a calculated sum of all the lux from a light).

So a lux is the SI unit for illuminance. But unlike other SI units like the metre, gram or Watt (which are all objective measurements), lux (which is a subjective measurement) is scaled based on the human eye response, or the Luminosity function. You can see this function on the graph below, along with the standard wavelength of LED colours marked.

Now from this, you can very clearly see that a royal blue light (450nm) will have a much lower lux level than say a green light (530nm) for the same radiometric power (or irradiance, measured in W/m2). In fact, it will be almost 20 times lower. It is for this reason that LED manufacturers (by which I mean companies like Cree & Lumileds) actually specify the light output of LEDs at the outer limits of the visible spectrum (royal blue, deep red etc) in radiometric power, and not in lumens.

Is this an issue? Not really. You just need to be mindful when doing a lighting design that you will not achieve, and should not be trying to achieve, the lux or lumens levels you would achieve using a white light.