Introduction to Color Management
RGB, 24-bit color, printer profiles. What are these and why do you care? What is color management and how does it relate to your digital pictures? Let’s define terms so we’re all looking at the same picture.
The basic definition of color is what a human can see. Color theory was first introduced by Isaac Newton in the 17th Century. The first color photograph was demonstrated by James Clerk Maxwell in 1861. You can see the picture here.
People see a spectrum of colors that range from red to violet. The problem is that cameras, computer displays, and printers can not “see” the same range as humans. And computers only deal with numbers. So how do they represent what we can see?
A color model is a set of rules for defining colors. One model you are familiar with is the Red, Green, Blue – RGB – model. The RGB model is an additive model; red, green and blue are added in proportion to create other colors. This is what Maxwell demonstrated.
Another model is the CMYK model: Cyan, Magenta, Yellow and blacK. This model is based on the absorbtion of light by inks. The black stems from the fact that when CMY are combined the result should be black, all light absorbed, but usually turns out a less than pure black. Thus many printers use black ink in addition to the CMY inks.
A third model is the CIE L*a*b model. Adobe shortens the name to Lab. CIE is the international standard organization. In the Lab model, the L represents Luminance. The a represents the part of the light in the spectrum from green to red and b represents the spectrum from blue to yellow.
The L*a*b color model can represent the entire spectrum people can see. The RGB model covers less of the spectrum and the CMYK model is the smallest of the three models.
All of these models are independent of physical devices. The real-world is not, however, device independent; devices work in a color space. Color spaces are defined within a particular model. Color space, in the RGB model, is a representation of a color by a triplet of numbers (R,G,B).
For example, red would be represented by (255,0,0); green by (0,255,0) and blue by (0,0,255). Other colors would be represented by a mixture of numbers: 102,133,79 would be a greenish color, for example.
If all three numbers in a triplet are the same, e.g. (100,100, 100) the resulting color is a shade of gray. Black is (0,0,0) and white is (255,255,255)
—–You may wonder why the numbers range from 0 to 255. This is because I’m assuming an “8-bit” color model. The range of numbers that can be represented by eight binary digits (bits) is 0 to 255. You may hear someone refer to a “24-bit color space.” That’s just three times eight bits to represent the RGB number: eight bits for each color, three colors. If you work in “16-bit” color, then representing a color triplet takes 48-bits.——
Common color spaces include sRGB: used as a standard on the web; Adobe RGB: supported by many cameras and ProPhotoRGB: a wide gamut color space that many photographers use. A gamut is the subset of colors that can be represented in a color space.
For the color spaces above, ProPhoto has the
largest gamut, followed by Adobe RGB with sRGB having the smallest gamut.
Not all colors can be accurately represented in all color spaces. Adding to the challenge is the fact that a given triplet will not result in the same color in different color spaces. Or, to put it another way, a different triplet must be used to show the same color in a different color space.
One decision that you make is what color space to work in. If you are still shooting JPEG( and why are you doing that?), the color space is chosen in the camera; you can, of course, convert to another color space once the picture is imported to your computer. When shooting RAW, the decision is made in the RAW converter on your computer. Whatever decision you make, be assured your printer doesn’t use the same color space.
A (digital) photograph moves from camera to computer to printer, all of which may use different color spaces. And that is why, color management is important. Color management is simply controlling the workflow from camera to computer to printer and assuring that the colors are as accurately reproduced as possible at every step.