Wednesday, January 26, 2011

Rich black - the problem of black in presswork

Like all process color inks, the black ink that is used in 4/C printing is transparent. As such it cannot cover ink, or black out the paper, as thoroughly as one might hope. Instead, black ink by itself actually appears to the eye as ranging from an apparent black when it's used to cover very small areas to just a dark grey when it covers large areas.
Even though it appears greyer, the box on the right is the same printed 100% black as the text on the right.

The solution to the problem of grey blacks is to boost blackness by printing other inks under the black ink. This effectively darkens the brightness of the paper over which the black ink will be applied thus making the black "blacker."

Blacker black strategies

There are several ways to making a black blacker:

1) Apply a screen tint of a process color - magenta or cyan down before the black.

2) Apply a mix of process colors - cyan, magenta, and yellow down before the black.

3) Apply two hits of black ink.

The first option is sometimes called a "rich" black, while the second is called a "super" black, and the third a "double black."

With a Rich black the other process ink is usually a 60% tone of cyan. This causes the black to appear “blacker” because the second ink color increases its density. It also makes the black "bluer" which adds to the darker appearance. A 60% magenta could be used instead of cyan to impart a warmer appearing black.
Left: 100K/60M. Center: 100K. Right: 100K/60C.

Rich blacks are typically used whenever the image is larger than 1 square inch and smaller than about 9 square inches in area.

A Super black, where 3 process colors underlie the black, is typically used when the black area is larger than about 9 square inches in area. The typical screen percentages are: 50% cyan, 40% magenta, 40% yellow, and 100% black.
Left: 100K. Right: 100K/50C/40M/40Y.

In contrast to using a single process color, this screen tint combination preserves the neutral appearance of the black. This screen ink combination also means that the maximum amount of ink in the black amounts to just 230% coverage which should not cause any on-press issues like excessive drying times.

Unfortunately a Super black can be problematic on press because it is used for large black areas while at the same time using the same inks that are used for the color-critical image areas. That can cause a conflict on press if ink densities need to be adjusted to align the image colors with the proof while keeping the Super black neutral and at the correct density. To solve that problem, printers may opt to use two hits of black ink. The first black ink is tied to the CMY of the images, while the second black is independent of the image. That allows the press operator to adjust the CMYK inks as necessary to get good color on the images with the second black ink only being applied where a large area of black is needed. The downside to this strategy is that it turns a 4/C job into a 5/C job (CMYKK) which may increase production costs.

The black booby trap

On a computer monitor, there is only one way to represent black - the screen is black when there is no light coming from the display. So a 100% black, a Rich black, a Super black, and two hits of black will all appear the same on screen.
A 100% black bar, a Rich black bar, a Super black or, two hits of black ink all appear the same on screen.

100% black bar as it appears in print if it is set to "knock out" of the background image.

100% black bar as it appears in print if it is set to overprint the background image.

A Rich black bar - 100%K/60C as it appears in print. The 60% C not only serves to darken the black but it also knocks out the background image which eliminates any "ghost" images caused when black simply overprints a background image.

Because the on-screen appearance of the black may appear the same despite being made up of different screen tint combinations it is critical that the Rich, or Super black be clearly identified as such in the custom color menus of page layout and illustration applications. Create the color and name it according to its function and make up. E.g. Black for standard process black, "RichBlack 100K60C" and "SuperBlack 100K50C40M40Y"

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