Image preparation for high-quality lithographic printing

Dot gain is a well-known phenomenon in lithographic printing. Just like a water droplet spreads when it lands on paper, the same thing is happening with ink.

Dot gain compensation

The effect of dot gain is the image will appear to darken. For instance, a neutral grey occupies 50% of the space with dots; the expansion of those dots will increase the amount of space occupied by ink and the tone will appear closer to 60%, or more. The amount of expansion is not linear across the whole range of greyscale from 0–100%, so the dot gain compensation is applied along a curve.

The amount of dot gain adjustment that is required depends on the paper. Coated paper requires less than uncoated paper. The only way to know the appropriate dot gain values is to ask the printer or paper manufacturer.

Dot gain adjustment in software like Photoshop is not difficult. The problem can be approached in many ways, some of which are discussed below. Optimal results requires adjustment of the image prior to applying the dot gain curve, paying particular attention to the shadows.

Dot gain is only part of the process of adjusting images for print. The image should have an ‘unsharp mask’ filter applied to improve the perception of detail in the printed image. (It’s a somewhat unintuitive filter name, since the effect of the filter is to sharpen the image.) The strength of the filter is somewhat proportional to the amount of dot gain, but unsharp mask cannot be applied uniformly to all images in a book – each image must be analysed independently.

These are some common techniques for applying dot gain compensation:

1. Convert to ICC profile
2. Duotone (with monotone settings)
3. Curve

Here is a quick outline of these methods.

Convert to ICC profile

In Photoshop, a greyscale image is converted to ICC profile using the Edit–>Convert to Profile option. Selecting from Photoshop’s built-in profiles, such as Dot Gain 15% or Dot Gain 20% may be appropriate. The printer should be able to provide a % value for the paper they are using.

At first, executing this function will not appear to do anything. Actually, behind the scenes, the image has been modified in such a way to compensate for the dot gain. This can be seen by using Edit->Assign Profile and then choosing Don’t Color Manage This Document. The image will appear to lighten onscreen. The image can be proofed onscreen as an approximation of the final result by using View->Proof Colors with the same Proof Setup as used in the Convert to Profile step.

Duotone

In Photoshop, a greyscale image’s mode is changed to Duotone. Within that screen, a monotone setting is applied and values relevant to the paper are inserted. This is where the printer's minimum, maximum and mid-tone numbers can be set.

The image is then changed back to greyscale mode. This is a simple and effective method for consistent results.

Curve

In Photoshop, a curve is applied to the greyscale image. The curve can be created to have similar properties to the monotone settings above, based on the printer’s specifications for the paper. While this method should generate results very similar to duotone/monotone method above, the interface is more complex which may lead to unintended results.