Rich black versus plain black
On a computer monitor, there is only one way to represent black. When there is no light coming from the monitor, the screen is black.
In print there are many different ways to represent black. The simplest is "plain black," or 100% black ink (0C, 0M, 0Y, 100K). However, you can also create a "rich black" by printing other inks along with black. There are many different possible ink combinations - the most common "rich black" contains percentages of all 4 inks: 63C, 52M, 51Y 100K. This particular variant owes it's popularity to Adobe Photoshop - when an RGB file is converted to CMYK, areas that are absolute RGB black (R0, G0, B0) will wind up with this combination, unless certain default settings have been changed. Other possible flavors of "rich black" are "Cool Black" (60C, 0M, 0Y, 100K) and "Warm Black" (0C, 60M, 30C, 100K).
No red, green, or blue (rgb) phosphors are glowing.
The problem with all these blacks is that they all look the same on the computer screen - all of them are represented as R0, G0, B0 - but they will not look the same on paper. A classic beginner's mistake is to take a photoshop image that fades into rich black on all sides, place it in a picture box in the page layout software, and assign the picture box a background of "black" ("black" in page layout software = plain black). This appears to be continuous and uniform on the computer screen. If the layout were to be printed, however, there would be a distinct difference between the areas of rich black and plain black.
The image and background look the same
Printed black looks different.
Measuring the color values
The solution to this problem is to measure the ink values in the CMYK (referencing Photoshop's info palette), and create a matching color in the page layout program. Care must be taken that the rich black in the CMYK image is actually continuous; move the cursor around and make sure that the ink values don't fluctuate. Then create a new separated CMYK color in Quark using the Edit>Colors dialog box; you can call it "Rich Black." Assign that color to the background of the picture box. Check your work by printing paper separations before sending out for film.
The special-purpose color "registration" is another potential source of black mismatches. On screen, by default, it appears as R0, G0, B0, so it seems as if it might be the same as black. It's not. Registration color prints 100% on all plates. If your layout is CMYK, then registration color is 100C, 100M, 100Y, 100K. If your layout is CMYK + Pantone 285, then registration color is 100C, 100M, 100Y, 100K, 100Pantone 285. Registration color is used for, appropriately enough, registration marks, explained elsewhere. Do not use it in place of black. Having that much ink go down presents many problems from a printing standpoint. There's no good reason to use anything other than Plain Black or a Rich Black variant.
Problems with Blacks in Photoshop
Photoshop has a number of booby traps that allow a designer to unwittingly create black mismatches. The one that ensnares the highest number of unsuspecting graphic designers is the Edit>Fill command. If you select "Fill with Black", Photoshop fills your selection with 100% of all inks, not just black. From a prepress standpoint, it would be nice if Adobe hadn't made it work that way, but oh well, that's the way it goes. If you want an area to be filled with a particular black variant, use "Fill with Foreground Color" instead, with the appropriate CMYK values assigned to the foreground color.
The "move" tool is another potential source of misery. When you move an item, the space it leaves behind is filled with the current background color. What if the current background color is plain black, and the item you're moving is surrounded by rich black? You've got yourself a black mismatch - a plain black cutout through the rich black, in the shape of the object you just moved.
If you'd like to catch such problems early on, make it a habit to cycle through the individual C,M,Y and K channels every once in a while, using the command-1, command-2, command-3, and command-4 shortcuts. Once you know what to look for, black mismatches become obvious.
Overview of black inks
|Name||CMYK||Usage or description|
|Standard black||0C, 0M, 0Y, 100K||Normal black.|
|Rich black||63C, 52M, 51Y 100K||The 'old' adobe photoshop black.|
|Cool black||60C, 0M, 0Y, 100K||Black with a bluish tone.|
|Warm black||0C, 60M, 30Y, 100K||Black with a reddish tone.|
|Registration black||100C, 100M, 100Y, 100K||Used for registration marks.|
|'Designer' black||70C, 50M, 30Y, 100K||A dark slighly cool black.|