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RGB Color - The Absolute Basics


Many readers of this blog will be familiar with the idea that our eyes contain rods and cones. Lets take a first, high-level look at this.



Cones and Rods


The rods in our eyes are mostly used in low light and peripheral vision. They don't contribute much, if at all, directly to color perception, so they will only get a passing mention here. They are called rods because the cells look kind of... rod-like. This picture from Wikipedia is just to give some visal reference.


Contrast that to cones, which have, get this, cone-shaped ends.


RGB


The cones come in three varieties. Red, Green, and Blue - each being excited by light in three different ranges of wavelength, more on that below.


People who are color blind can be missing, or missing functionality of, one or more of these three varieties, something we will look at in a later article.


In these blog articles, as in most color science textbooks and literature, we will only be looking at "typical" vision. All working in color science have to always keep in mind that color vision is ultimately subjective, and sometimes varies considerably from person to person.


Ok, so what? Mom, I'm bored.


All the theory in the world is only good if its practical - so how can we use this information?


You are seeing it in action right now. If you were to look at your screen under magnification, it would look like this:


Compters, TVs, tablets, and cell phones all work by having tiny red, green, and blue lights that corespond to the red, green, and blue cones in our eyes. The lights are grouped into sets of red, green, blue lights we call "pixels".


If lighting technology were perfect, in theory, one could combine these red, green, and blue lights to show all colors that humans are able to perceive. High end screens that use LEDs, plasma, LCDs, and such can be pretty good at representing a large set of colors we can see, but it likely wont be until we have accessible laser-based displays that we can actually achieve this.


Wait, so how we do we get yellow or purple if we only see red, green, and blue?


Color-mixing. Here is a little video showing it in action:





If are a bit science-oriented, lets look at this science-y graph showing the ranges of wavelength our eyes respond to, and then compare that to the Chromaticity Diagram. If you are less science-inclined you can safely skip down to the Bonus section at the bottom :-)




The Red, Green and Blue cones here are called L, M, and S for "Long wavelngth, Medium wavelength, and Short wavelength).


Here is a little refresher image about the Chromaticity Diagram - if this looks unfamiliar or confusing hop over to this earlier article and then come back.



The numbers around the outside of the Chromaticity Diagram corespond to the numbers along the bottom of the cone response graph above. As you can see, the Chromaticity Diagram is kind of like a triangle, with Red, Green and Blue at each corner. From here its possible to plot any position as a percentage of Red Green and Blue. We will look more at this when we cover RGB Color Spaces in a future article.


Bonus: A little sidetrack about Yellow


Some people also have Yellow cones, but this has only ever been confirmed in one person ever. Something like a partially functional yellow cone is theoriezed to exist in up to 50% of females and 8% of males. This would likely give the person more ability to discern subtle color differences in the blue-green, yellow, and orange color ranges. A few years ago several TV manufacturers tried making RGBY televisions, but found it didn't make enough of a difference for enough people.




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