Dot gain

Dot gain A factor in print production

Dot gain is the difference between the intended ink dot size in the digital file and the actual ink dot size when printed. Ink dots get larger in print due to either a mechanical or optical factors. Dots can grow between 15–30% in size, depending on the paper, ink and print method used. This is especially prevalent in images with continuous tone, like a scanned photo. It is helpful to understand what dot gain is and how it works so you can diagnose problems in print, and to control the process of preparing digital images for print.

Dot gain is managed by the dot gain curve that is part of an ICC profile. It reduces the size of the printed dot by the amount it is expected to gain. If this seems complicated, remember that it is automated when applying an ICC profile. It is yet another reason why colour management is part of a good design and print workflow.

Dot gain is a normal result of the printing process. It should be taken into consideration when choosing:

Paper for the design. For example, understanding how much detail can be held by the substrate (paper). It may mean tiny text or fine detail can't be accurately reproduced.
The printing process and speed. Some print processes have more dot gain than others.
Inks. Some inks will spread more than others.

Intended dot size in the digital file (left) and the actual dot as it is printed (right).

How dot gain impacts on images in print

[Left] Intended tonal reproduction. [Right] Incorrect tonal reproduction due to dot gain (Pritchard 2010).

Illustration comparison between intended and actual dot (Larsen 2018). While the actual print is 'muddier' on newsprint, it arguably looks more organic, it's cheap to print and prints fast. In this example, dot gain would have been taken into account. A cleaner, crisper print is not always a better print!

If dot gain is not taken into account, the typical result is a printed image that looks darker than intended. This also means a loss of detail in the shadows and midtones.

You may also get an effect on the image called banding. This is because the dots at the midpoint gain more than the big dots (shadows) and little dots (highlights). There is more information on the rate of gain below.

A gradient without banding. This is how it is intended to be printed.

A gradient with banding. This is the typical result of a printed gradient if dot gain is not factored into production.

There are also other techniques for minimising banding in gradients, especially in digital illustrations. A better print result can be attained by adding a 3% gaussian noise in the Noise filter in Photoshop (Photoshop > Filters > Noise: 3% gaussian noise). This added noise looks more pronounced on screen than it does in print.

Mechanical dot gain

Mechanical dot gain (aka physical gain) is caused by the printing process. It can be when paper soaks up the ink, increasing the dot size. It can also be the result of the ink dot being squashed by rollers during the printing process, increasing the size of the dot.

Roller pressure changes the shape of a printed dot a bit like using a using pressure on dough.

Factors in mechanical dot gain:

The paper that is used,
the ink's properties,
dot shape (we have only looked at round dots, but other shapes are available, like ovals and squares)
the printing press,
the roller pressure,
press speed.

Uncoated papers like newsprint have a higher dot gain than coated papers.

Optical dot gain

Optical dot gain is the dot casting a shadow into the paper, making the dot appear slightly bigger than just the mechanical gain. When combined with mechanical gain, it accounts for a couple of percent of the total gain, depending on the substrate (e.g.15% mechanical + 2% optical = 17% dot gain).

[L] Intended dot. [M] Mechanical dot gain [R] Optical dot gain.

Dot Spread

Another problem in printing dots occurs when two or more dots meet. This is also determined by dot shape; round dots are the most common, but oval and square dots are also used in printing.

Unjoined dots of liquid with a round shape.

The ideal of what would happen when two dots merge would be for each to maintain their shape, resulting in sharp angles. This is not what happens!

When dots overlap, they clump and change their shape. The corners round out, and also gain like a regular dot. This partially explains why the rate of gain depends on the size of the dot in a regular AM screen.

(Adapted from Cookson n.d.)

Dot gain curve

To counteract dot gain, a dot gain curve reduces the size of the dot by the amount of gain.

For example, if the dot gain is 15%, the dot size is reduced by 15%.

This curve is part of the colour profile (ICC profile). It is typically applied after image capture and during editing.

Applying a dot gain curve to an image is a destructive edit (a permanent change), so it is best done on a copy of the original digital file.

This curve is applied when the image's mode is changed from additive colour to subtractive colour, or when the ICC profile is applied. Different curves are needed for different substrates. Typical levels of 15% (coated paper) to 30% (uncoated paper) are used, depending on the ink and paper quality.

Web presses normally produce a higher dot gain than sheet-fed presses, so different colour profiles are needed. See What's the Difference Between Sheet-Fed and Web Printing? for more info.

Not every dot grows by the same amount. It depends on the size and shape of the dot, the speed and the print method.

The amount of dot gain is not the same for every ink due to ink impurities. In process printing, the dot gain curve is different for each ink (C, M, Y and K).

Medium-sized dots (midtones) gain more than very large dots (shadows) or very little dots (highlights). This is why the function to reduce the dot size is a curve, not a straight line.

This graph shows dot gain increasing up to the midpoint and then decreasing as the dots continue to grow and overlap.

Can dot gain be eliminated from a print?

Dot gain can be minimised with the right tools, settings and expertise. It can not be eliminated because it is inherent in the printing process. As a graphic designer, you do not have to do much other than use the required ICC profile! You should always:

Colour manage your files. Ask if there's an ICC profile you should use. If no-one knows, use Europe Prepress 3 or common sense (e.g. a profile with newsprint in the name if it is printed on newsprint).
Take into account the substrate and print method when scanning, designing and printing.
Use the settings and design recommendations of the press operator. For example, they may recommend a minimum type size, stroke weight and use of colour.

Reference list

Cookson, G n.d., ‘Printing “Dot Gain” | Shutha’, shutha.org, viewed 24 August 2020, <http://shutha.org/node/817>.

Larsen, E 2018, ‘There’s something aesthetically appealing about the look of newsprint. Yes, it’s cleaner and more vibrant on better paper but there’s an aesthetic warm quality that’s missing. Maybe it’s just me.’, @ErikJLarsen, 8 June, viewed 25 August 2020, <https://twitter.com/ErikJLarsen/status/1004759284595318784?s=20>.

Pritchard, G 2010, ‘The principle of dot gain compensation plate curves’, The Print Guide, 14 February, viewed 24 August 2020, <http://the-print-guide.blogspot.com/2010/02/principle-of-dot-gain-compensation.html>.

Created By

Leigh Milward

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Credits:

Created with an image by Nadya Spetnitskaya - "dough and hands"