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Dot Gain
           
   

Definition of Dot Gain

It is defined as the increase in the diameter of a halftone dot during the printing processes. Total dot gain is the difference between the dot size on the film negative and the corresponding printed dot size. For example, a dot pattern that covers 30% of the image area on film, but covers 50% when printed, is said to show a total dot gain of 20%.
However, with today's computer-to-plate imaging systems, which eliminates film completely, the measure of "film" is the original digital source "dot." Therefore, dot gain is now measured as the original digital dot versus the actual measured ink dot on paper.

Causes of Dot Gain

All printers create dot gain. Ink-based printers tend to create more dot gain than toner-based printers. When ink absorbs into the paper, it spreads outward, making each dot spread out and grow larger. Halftones that are not adjusted to allow for dot gain may appear too dark on any ink-based print device. It also helps to choose the right paper. Both coated and uncoated finishes are acceptable. Papers with a coated finish can be better for printing photos because its coated finish reduces dot gain.
Uncoated finish may be better for heavier ink coverage (reverse backgrounds or heavy graphics) absorbs the ink into the fiber of the paper, which minimizes set-off or “bleed-through.” It also has a softer finish so it creates higher dot gain.
Not all halftone dots show the same amount of gain. The area of greatest gain is in midtones (40-60%); above this, as the dots contact one another, the perimeter available for dot gain is reduced. Dot gain becomes more noticeable with finer screen ruling, and is one of the factors affecting the choice of screen.
Dot gain can be measured using a densitometer and color bars in absolute percentages. Dot gain is usually measured with 40% and 80% tones as reference values. A common value for dot gain is around 23% in the 40% tone for a 150 lpi screen and coated paper. Thus a dot gain of 19% means that a tint area of 40% will result in a 59% tone in the actual print.

The significance of the printing blanket as a quality factor in the printing process is often underestimated, although it transfers the dot from plate to the paper. The printing blanket transfers not only the ink from the offset printing plate to the material being printed, it also absorbs vibrations from the printing machine and compensates for different paper grades and qualities. In the printing process, the blanket affects a series of key variables and parameters which reflect the dot gain value of the printing machine, which in turn is the basis for the ICC profile* of the machine and therefore complete digital workflow.

*ICC profile

(International Color Consortium profile) An ISO-approved color management standard for specifying the attributes of imaging devices such as scanners, digital cameras, monitors and printers so that the color of an image remains true from source to destination. A profile can be embedded within the image itself. For more information, visit the International Color Consortium Web site at www.color.org

Solution

Modern prepress software usually includes a profile to achieve the desired dot gain values, ie Photoshop grayscale profile of 20% dot gain
Photoshop knows all about dot gain and bulges out the tone curve to lighten the ink coverage by the expected amount that it will darken due to the dot gain. Hence if you set up for bigger dot gains than you actually get, you end up light and vice versa. This only happens if you tell Photoshop about it, by turning the colour management on and obeying the rules. Monochrome dot gain settings are just like colour space selection. Incorrectly handling a file by moving it, in an uncontrolled way, to another profile will create a change in saturation; doing the same thing with a monochrome by moving it, in an uncontrolled way, from one dot gain to another, will produce a change in the tone values.


IMPORTANT: Ask your printer for his dot gain values.

TJ International uses, for grayscale bookwork

max 95% shadow and a min 5% highlight

when scanning grayscales embed the Photoshop profile for 20% dot gain our KBA presses have a 15%-17% dot gain.

 

  dot gain sample
 
 
Effects of Dot Gain
 
     
 
Digital File View
 
 
dot gain
 
 
Printed result!!
 
 
color settings
 
 
In Photoshop go to Edit>Color Settings to access this here you can set the profile you need for your dot gain.
 
 
custom profile
 
 
In Photoshop go to Edit>Color Settings to access this here to set up a custom profile you might need for your dot gain.
 
 

 

 
 
On the right is a sample of an image with no dot gain compensation built in. Notice how it gets darker by about 15% when printed although it looked acceptable on the computer screen.
 
 
good dot
 
 
What was printed with correct dot gain adjusted in Photoshop
 
     
 

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