What is brighter, a 100 W white light or a 100 W blue light?

This question came up in a recent support inquiry here at DeepSea and in diving into how to talk about power and brightness in illumination it was clear that there are some common misconceptions.

Let's walk through a useful (if extreme) example here in order to help illustrate some of these differences and develop a better understanding of illumination terminology and specifications.

In this case we have two lights - one fitted with blue LEDs and the other with white LEDs. Both are using the same amount of electrical power, around 100 Watts, so the differences are just due to the color of the LEDs themselves.

For the blue light, the total luminous power is around 40 Watts and with the white LEDs the total output power is just 30 Watts, 25% less output power. It might seem logical then that the blue light is brighter, after all, it is outputting 10W more than the white LEDs.

However, if you were to look at these two lights side by side (with some eye protection) the white LEDs would appear significantly brighter! What's going on?

The difference is quite literally in the eye of the beholder. Normal human vision is non-linear and we do not have the same sensitivity to all wavelengths of visible light. Instead our eyes are most sensitive to light around about 555nm which is perceived as a yellowish-green color. The sensitivity of normal human vision to visible light is represented below by the red dashed line, called the Photopic Luminosity Function.

In this chart the spectral output distribution of our two example lights have been scaled by the photopic luminosity function to show the relative sensitivity of our vision to the spectral power emitted in each case. 

The spectral peak of the blue LED's is out at the tail end of the photopic distribution so our perception is much less sensitive to the light emitted here. However, in the case of the white LEDs, there is a lot more power emitted near the center of the photopic sensitivity curve resulting in a much higher perception of intensity. 

We hope you found this explanation useful and enlightening!