Tuesday, August 4, 2009

Evaluating Color by the Numbers | Newsletter

Have you ever adjusted at an image based on how it looks on your computer screen only to find that it prints differently on press? You can always tell how well an image will print on press by looking at it’s numbers. What numbers? The numbers that I’m referring to here are the percent values for each color that are measured using most any image editing program.

Evaluating Color by the Numbers for Press

For decades color separators and scanner operators have been evaluating color by the numbers to determine it’s correctness. With the advent of desktop scanners and automated image editing software the art of color evaluation is being lost. Creating an image is easier, but knowing what to look for to establish color accuracy is disappearing.

Evaluating color by the numbers is a far more reliable and consistent method for making decisions on how well an image will print. We are going to take a look at some important color concepts, number values, and relationships to help improve our understanding. Once you know what to look for, evaluating your images will become a whole lot easier.

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Color Theory Basics for Press

To help us understand how process color imaging are handled in offset printing let us review some basic concepts of colors and how they are reproduced.

Additive and Subtractive Colors

There are two different color gamuts, or spaces. First being the visible light spectrum, RGB (Red, Green and Blue). This color space is made up of light, and when all three RBG colors are added together they result in white. This makes up what we call the Additive Color Space.

The problem with light is that you can not print it on paper. Light is used extensively in photography, scanning, computer monitors, and television. But it is never used on press, because combining Red, Green and Blue inks on press only result in black. Therefore the opposite ink colors Cyan, Magenta and Yellow are used instead of Red, Green and Blue. When combined in various combinations they produce Red, Green and Blue, but more importantly when all are subtracted they produce white. Thus the name Subtractive Color Space.

Now if you add black to CMY to help define details and support the other color you end up with the basis of all full color image reproduction in offset lithography. These four colors go by different names, but they are all the same; process color, four-color, four-color process, and CMYK. The letter K is used to represent black, not to be confused with B which represents Blue.

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