When we’re kids, in school, we’re taught that the primary colours are red, yellow and blue. But this isn’t entirely accurate when it comes to light. Pure white sunlight is made up of a whole spectrum of colours, with the primaries actually being red, green and blue. Our cameras with Bayer filter arrays on the sensor see RGB. Our monitors also display RGB.
But have you ever wondered how we’re able to get so many different colours from just three? And why just blasting red, green and blue LEDs at an object doesn’t always give you true white light? This fascinating video from Technology Connections isn’t really specific to photography, but light in general, and how red, green and blue affects our (and our camera’s) perception of colour.
When I first stumbled across this video, I was instantly fascinated. And while I sort of understood much of this already, I still couldn’t take my eyes off it. Understanding something and actually seeing it demonstrated right there on the screen are two very different things.
Also fascinating was that while red, green and blue can pretty much make out just about any colour we can potentially see, our cameras don’t always see them the same way. Our brains are extremely clever and can fill in a lot of the gaps in missing colour information so that we think we’re seeing what we’re seeing. Cameras aren’t quite so intelligent, and they’re much more demanding of the requirements of light sources.
This is why things like Colour Rendering Index (CRI) exist for lights. The sun and inefficient incandescent light bulbs put out a true, pure light covering every colour of the spectrum. You can see that it puts out true white light when you run it through a prism – an experiment most of us did back in school. But other light sources, like the LED panels so common today, don’t put out pure white light with the complete colour spectrum. So, our cameras can see things differently to the way our eyes do.
Colour Rendering Index is an objective measure of how much of the colour spectrum a given LED panel, or another light source, puts out. The higher the CRI, the more of the visible light spectrum it covers and the more accurate colours appear to the camera. At least in theory. CRI doesn’t always explain exactly where those gaps appear, though.
You might have two light sources with the same CRI that look very different. While they may both offer the same percentage of the overall visible light spectrum, those gaps in the spectrum aren’t necessarily in the same spots on both lights. So, to the camera, they both appear to render slightly different colours to each other. Of course, whether you or the camera notices this difference will depend on the colours in the scene you have your camera pointed it. This is why many lights now also carry a Television Lighting Consistency Index (TLCI) rating, too.
The video is 22 minutes long, and while it goes very in-depth into the colour of light and its spectrum, it might leave you more confused after you’ve watched it than you were before you started. But it’s definitely worth watching. Possibly multiple times.