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The Chromaticity Diagram

Updated: Dec 1, 2023

I once read a quote, I don't remember where or who by, that said something like "Color Science is a subject you have to sip on" - and is that ever an understatement.

This particular article is targeted at our artist and photographer clients as well as those just getting into professional art and photo printing. I'll take some liberties in hopes of making things easier to understand - please let me know in the comments all the technical things I get wrong or skip over :-)

Many of our clients will have seen this stuff before - we regularly do demonstrations of all this in-person to help our artist clients understand what goes into making beautiful prints of their work.

Color Science is different than (but related to) Color Theory

If you are very interested in human color vision, or the subject of color science generally, I highly recommend watching the Color Vision YouTube series by Craig Blackwell. It is essentially the lectures from a university human-vision 101 class and will get you a huge head start - even if you just watch the first one then come back in six months and watch the second. Color science is a subject you have to sip on afterall, it can get overwhelming quickly.

The Chromaticity Diagram

Ok this may look scary - but hold on, I'll try and explain it.

Basically, these are all of the hues that we humans can see, plotted on an x,y axis. As you get further toward the edges, the colors get more pure/saturated. So, for example, the extreme bottom right is a completely pure red, or up at the top where it says 520, that's a completely pure green.

As you move toward the curvy line in the middle, you get closer to gray and white, and the colors appear less saturated. The numbers that go around the outside represent the frequency of light waves, measured in nanometers. So 700nm is a pure red light and 520nm is a pure green light.

About that curved line

That curved line in the middle is, basically, all of the colors that we can perceive as being 'white' based on the color of the light around us. If you have a white piece of paper inside your house in a more orangey light, you will register it as being white, even though its technically reflecting orange light. Similarly if you take that paper outside, it will be reflecting sunlight which is actually very blue - but it will still read to you as being a white piece of paper and not a blue piece of paper.

This is what we call 'color temperature' in lighting or 'whitepoint' in printing. It is measured in degrees Kelvin (I'll explain why in a future article). Where it says Tc(K) on the diagram, that just means temperature in degrees Kelvin. In printing we primarily work with a temperature of 5000K as a standard reference - a very neutral color temperature right in the middle, so that things will look reasonable both inside (which is often oranger light, around 3000K) and outside (here is south Mississippi it can get over 9000K in the middle of the day). If you have ever bought light bulbs for your house or office, you will notice on the side it will list the color temperature in degrees Kelvin - usually with a graphic to indicate how orange or blue the light is - something like this:

Not really a color

Looking back up at the diagram at the beginning of the article (or just below this paragraph), you may notice there are no numbers along the bottom of the color thingy in the Chromaticity Diagram - that's because magenta isn't a real color. While we are at it, brown is really just dark orange, and not its own hue.

One more look

Hopefully looking a lot less scary now - if you are still feeling overwhelmed by this, go back and give this another read - and then again in a couple weeks. It'll sink in.

If you got all this right off - congrats!! This article will be a foundation that builds on several upcoming articles about how we use this diagram to see what colors are in an image, and to compare the colors that different papers and printers are capable of reproducing. Want to learn more about this in person? Book a free consultation through our website.

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