The absolute best way to gain an understanding of how separations work is to spend a lot time around a press and see how it all comes together. Since most of us donÆt have that luxury, Photoshop is the next best thing. PhotoshopÆs channels palette has many hidden powers but for now it just serve as a visual example. Open any CMYK file you have, preferably high res. By clicking on the name of one channel at a time you will get a grey scale preview of what that channels printed plate will look like. When you print a separation, or plate, you only ever print a black and white image, or black and clear to be more specific. No coloring actually happens till ink is pressed through a plate, so if you press the right color of ink through the right plate in the right order you get a nice beautiful image. So why Cyan, Magenta, Yellow and Black? These colors have been found to work for a full spectrum of color when mixed at different densities.
ThatÆs all pretty basic but the real science is in how you get high quality, clean and tight prints. When applying ink, there is no way to ækindaÆ lay it down. You either cover the area or you donÆt. As you know, yellow and blue make green, but some greens are a little more blue (cyan) than yellow. So how do you mix in only a little yellow if you have to either coat the area or not? Well, you actually print small solid dots far enough away from each other so that the color ôappearsö lighter, thus creating a lesser mix and creating the green you wanted. The distance away the dots are is called the frequency or line screen. You cannot however lay these dots down in a perfect grid, if you do the areas of small dots start to create areas of large dots within areas of larger dots and so on. This is called a morree pattern (pronounced mor û A). To avoid this you can lay the dots down at different ôAnglesö to prevent any patterns from showing up. Scan in a full color image from a magazine at 300dpi and zoom in really close and youÆll see what I mean. Since that scanned image already has a dot pattern (angle/frequency) you cannot send that scan to press again because the old pattern and the new pattern would conflict, this is called re-screening and is a no no.
So how do you know what angle and frequency to use? Most printers have their own print servers or Rips, which are entire computers dedicated to crunching the numbers before æimagingÆ the plate on the film. These Rips usually come from the manufacturer with preset defaults for what angles to use. There is a pretty standard set of angles used in CMYK printing and you will find that both Illustrator and Photoshop as well as I believe Quark, Pagemaker and In-Design all have those angles defaulted in the separation setup. The frequency is usually dependant on what you are printing on. Newspaper prints at 85 lpi or lines per inch. Most high end magazines print at 175 lpi, but 100, and 133 are also used quite often. Printers know what they like and will let you know if you are outputting film for them.
The two big pitfalls in 4 color process are Spreading and Trapping. Any press can set up a print and produce one really incredible print, but after a 1000 or so prints things start to misalign and little imperfections start showing up. Look at last Sundays paper and look at the edges of some of the photos, not every copy of that paper printed that badly, your just witnessing what happens after thousands of prints. Todays printers have gotten it down to a pretty small margin of error but thereÆs still about .8 points give on todays presses. How do you adjust for this on the PRE-press side? If you have a large area of color that butts up against another area of color, its likely a few of the prints will reveal a sliver of white between them unless you ôTrapö it. You only have to do this in the case that Black is one of those areas of color. Simply overlap the lighter area by .8 points or close to that. This can usually be done by just adding a stroke to the shape and putting it on top. Another problem with black is that it is actually pretty faded when printed by itself. To darken up a large area of black just add 30% Cyan, 40% yellow and 40% magenta to your black to ensure a nice rich black on the press. DonÆt ever use a four color black for type though, one is enough. A rich black or four color black is hard to print white type on because the white is actually the paper showing through and it does so right in the middle of a big pool of 4 layers of ink. The result is usually too much ôGainö or seepage when the ink hits the paper and its starts to spill over into the area intended for the text. The way to protect against this is to ôSpreadö your type. This is as simple as making a copy of the block of white type, and pasting it behind the original and give it a stroke of 100% black to the backround copy. Now there is sort of a moat around the type that will keep the four color black from seeping in. These two tricks will ensure clean prints over and over. HOWEVER! Some of todays more advanced print houses have RIPS with automatic trapping and spreading programs that will take care of all this for you. So be sure to check before you waist any time doing this to your files.