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.

Color Spectrum Chart

Now that we have shown you our two color spaces -- additive (RBG) and subtractive (CMY) -- let us take a look at them both on a color spectrum chart. This way we can visualize the differences between the two.

The first thing to notice is the relative size of the CMYK color space to the RGB color space. Neither one can reproduce the same amount of colors our eyes can see, but more importantly, the CMYK color space cannot reproduce the same colors as the RGB color space.

When we convert our RGB images to the CMYK color space for offset printing, we loose some color. Specifically, we loose the most saturated (purest) Red, Green and Blue colors at the tip of the RGB color space.

Halftone Line Screen

Once we have our images converted over to CMYK, every prepress department within a commercial offset print shop will apply a line screen to each of the four colors. Smaller dots makeup lighter color values, and larger dots produce more color. Each color screen is rotated to allow the companion colors to show thru. When combined they create a dot pattern. This process is used to create the optical effect of a full color image, with only using the process colors on press.