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# Picture This: Season of Colors

Calibration goes beyond CMYK and RGB. 12/01/2006 7:00 AM Eastern

# Picture This: Season of Colors

Dec 1, 2006 12:00 PM, By Jeff Sauer

Calibration goes beyond CMYK and RGB.

This CIE color chart is a two-dimensional representation of the color gamut of the human eye.

I'm sure most of you remember learning about colors around kindergarten time, and how red, blue, and yellow can be mixed to make all other colors. It's something I remember learning quite well myself nearly four decades ago, partly because of how it worked and partly because of how it didn't.

I have a lingering memory of making a mistake on a kindergarten color-by-number assignment. I honestly don't remember what the picture was, but our job was to color in blank spaces with colors that matched the numbers. That's straightforward enough, of course. We've all seen that sort of childhood exercise before.

The trouble for me was that I'd seen and done color-by-numbers many times before by that point in my young life, and I dismissed the task as embarrassingly easy. I plowed right in with the crayons without really giving it much thought. Then horror struck. I realized that I had put blue in an area that was marked for green.

Fortunately, I was a logical kid and I knew my colors. I figured that if I drew on top of the blue crayon with a yellow crayon, I would be able to change it to green, and I'd be saved from the humiliation of such an egregious (for a kindergartner) error. My plan worked, but only in a rudimentary way, and the teachers still noticed my mistake — although they noted the recovery, too. The problem was that the crayon colors did not blend as gracefully as the theory of mixing primaries suggested they would. One could still see blue and yellow lines more than actual green. Even the green itself wasn't quite what I would have expected.

## LIGHT SIDE OF COLOR

When my own kindergartner came home one day talking about red, blue, and yellow as primary colors, I thought to myself, “Do I tell him it's a lie, that ‘red’ and ‘blue’ are not actually the real subtractive primary colors, but rather oversimplifications from almost universally misinformed childhood educators?” Or that color, by definition, is something our brains perceive, and is subject to the light in which we see it, rather than what's written on a crayon wrapper?

He's a kindergartner, though, so I decided not to get into it (for now). After all, to him, green is green.

But, of course, as graphic artists and color printer specialists have all learned since kindergarten, “red” and “blue” are really just sloppy kindergarten words for magenta (bluish-red) and cyan (greenish-blue). And while you can't actually make every color by mixing cyan, magenta, and yellow, those are the real primary colors when it comes to offset printing, mixing paint, and even blending crayons. (Actually, standard modern printing uses a “four-color” CMYK process that adds “K” for black, as to not be confused with “B” for blue. In theory, as we learned in kindergarten, blending all colors makes black, but in practice it really makes muddy brown. Adding straight black ink to the printing process helps a lot.)

Thus, while I did not confuse my kindergartner with subtle shades of subtractive color, I couldn't resist talking to him about the projectors he knows I test, in order to show him that “primary colors” aren't as absolute as his teachers were saying.

“Red, green, and blue are also primary colors,” I told him, and I set up a little experiment to prove it. I put red, blue, and green gels in front of two projectors pointed at the same screen, and we saw, more or less, that red and green mixed together actually make yellow and orange. And, instead of all colors together making “black,” all colors of light mixed together make white.

It was an exercise in additive color versus subtractive color, of course. That “light blue” cyan on a piece of paper is really subtracting, or filtering out, the red light that would otherwise reflect back into our eyes. Magenta similarly filters out any visual perception of green light, and yellow filters the blue light.

All this is rather esoteric for a six-year-old, I admit, and I didn't delve into the part about how our eyes work. But I know that I had a hard time breaking from the kindergarten absolute of red, blue, and yellow when I first learned about additive color. Maybe this little experiment will open his mind a little more than mine was at that age.

## COLOR MODELS

Opening your eyes to good color is particularly important in the professional AV world. That means more than just understanding and displaying “primaries.” Both CMYK and RGB really are nothing more than color models that make an attempt to represent all of the colors our eyes can see in the real world. And both ultimately come up quite short when compared to nature.

The CIE color chart (pictured on p. 16), taken from Datacolor ColorFacts color calibration software, is a two-dimensional representation of the color gamut of the human eye, effectively disregarding any variation in luminance. The inner triangle represents the RGB color gamut with red, green, and blue primaries at the three corners, and the cyan, magenta, and yellow secondary represented by nodes between the corners. The arc in the middle of this version of the CIE chart plots color temperature values of white that represent different possible “white” blends of red, green, and blue light mixed together. D65 (6500 degrees Kelvin) represents equal parts of each R, G, and B. The chart also shows that there is plenty of room for improvement over the traditional RGB color space when it comes to displaying the breadth of colors we see with our eyes.

Yet, how would display manufacturers get there? Go to a paint store and you'll see close to a dozen pigments that can be added to differently colored “white” base paints to produce exact colors. That may be harder to do with visual displays, but it's not as farfetched as it might sound on the surface.

For example, Texas Instruments has experimented with different color wheel configurations for DLP projection systems. Adding a yellow segment to a RGB+white color wheel can theoretically reproduce more accurate “skin tones” and other yellows and oranges, although in practice it has tended to be used just as much to boost brightness. TI's BrilliantColor technology offers support for up to six different colors (RGBCYM) in a single color wheel revolution. Similarly, Genoa Color Technologies' Keshet family of integrated circuits tout a “multi-primary” approach using four, five, or six “primary” colors to widen the potential color gamut of displays.

The caveat with these multi-color approaches is that they necessarily decrease brightness by either, in the case of LCDs, filtering out more light, or in the case of a reflective technology like DLP, reducing the amount of time light is being reflected toward the screen. Fortunately, today's increasingly brighter light sources are reducing the resistance.

There's a lot of color in the world, whether it's in the light of the holiday season or the subtle variations of green in the spring. And there's a lot of room to display that color more accurately.

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