Why dots?

The image on a photograph is made up of pigment layers of varying thickness. Dark areas have thick layers, light areas, thinner. Offset printing does not allow ink to be added on layers of varying thickness. Images are therefore, screened. Screening involves dividing a surface into a succession of microscopically small dots of equal ink thickness. These dots give the impression of a single, continuous image. The images, therefore, have to be ‘screened’ or divided into to be usable for offset.

What is Screen Ruling?

Raster density, which is often called screen ruling, indicates how many lines/dots there are in a unit of printed surface. Screen ruling is given as the number of lines/dots per inch or centimeter, depending on the practice in the country in question. With a high screen ruling, there are more ink dots per measuring unit. To the naked eye, halftones are continuous and therefore, the illusion is given that a photographic image is being reproduced. The screen ruling must be adjusted to suit the printing method, as well as the surface roughness and absorption properties of the paper.

Large billboards are often made with a screen ruling of ca 45Lpi (18L/cm). They are viewed from a reasonably long distance, so that the rather large ink dots are not directly visible. For newspapers, the lineation of 85Lpi (35L/cm) is standard. Books, brochures and advertising material usually have a screen ruling of 133-175Lpi (54-70L/cm). More exclusive printed matter often uses a higher screen ruling up to 175-200Lpi (70-80L/cm). Dry offset (waterless offset) facilitates the use of very high screen rulings of up to 300Lpi (120L/cm).

Stochastic screen uses dots of the same size, but which vary in number: many dots in dark areas, few in light areas. Stochastic screening is therefore, described in terms of dot size rather than screen ruling. 21 microns (21 thousands of a millimeter) is a common size. Positive film is recommended when using stochastic screening.

What is Dot Gain or Dot Expansion?

For coated paper, the coating ensures that the ink is directly absorbed by the paper during printing. This prevents the ink from spreading as much as on an uncoated paper. The open surface of the uncoated paper causes a larger dot gain, or dot enlargement.

The different phases where dot gain occurs in offset printing are:

On the film the dots are sharply defined

1) While exposing the offset plate the dot becomes slightly distorted

2) When the ink comes onto the offset plate the dot swells on the edges

3) When the ink is transferred from the printing plate onto the rubber blanket the dots get squeezed

4) Finally, the ink dot reaches the uncoated paper and the dot sweeps out because of the fibrous, open surface

All these steps in the printing process cause dot gain. Dot gain appears in each printing procedure, although it happens more often with uncoated paper than coated. Therefore, you always need to keep the surface smoothness and the absorption ability of the paper in mind.

Dot Percentage versus Dot Gain

In actual fact, dot gain is the growth in diameter of a dot. As is shown in illustration 3, the dots are smallest in the light and shadow parts and largest in the mid-range tones. The gain of a dot is therefore, the largest in the mid-range tones (50%) because there, the circumference of the dot is at its maximum. In 100% coverage (completely black), or in 0% coverage (completely white) there isn’t any dot gain at all.

What is U.C.R.?

In colour reproduction, the shades are composed from three different process colours: yellow, magenta and cyan. Theoretically, these three colours are sufficient to form all colours, but because of the insufficient absorption quality of the ink, a fourth print run is necessary to obtain a neutral black. This black provides more depth and structure to the image. All the colours used in the reproduction are formed by the three process colours , plus a quantity of black.

U.C.R. scanning ensures that neutral grey values built from the three primary colours can be partly replaced by black. Because of this, the number of ink layers decreases considerably so that the paper dries more quickly and warps less.

Dot gain is the change (increase) in size from that on the original film to the final dot printed on the paper. Dot gain is not a fault, but is part of the lithographic printing process. Dot gain must however, be controlled to allow for accurate representations of the original. The areas where dot gain can occur are:

  • At the contact exposure between film and plate
  • As the ink is transferred from the plate to the blanket
  • As the ink from the blanket to the paper
  • As the ink dries on and within the paper

Dot gain is measured with a densitometer, by measuring the control patches. Known percentage dot patches are added to the films and exposed onto the plates and the densitometer is used to measure the final dot area from these control patches on the paper. An original 50% patch may measure, on the final print 62%, thus giving a dot gain of 12%.

Assuming correct exposure of the plate and the correct press settings, the factors that would influence the dot gain are the rheology of the ink (specifically its viscosity), the hardness of the litho blanket and the surface of the paper along with its ink absorption characteristics. A more fluid ink will tend to spread more on the sheet and so the dots tend to become larger. Soft blankets will also increase the dot size, as will higher blanket pressures.

Rough papers, especially embossed, textured and felt finished products can require compressible blankets and higher blanket pressures (and sometimes lower viscosity inks) to allow the inks to ‘bottom out’ to produce an even and complete solid. All these factors will increase dot gain. Uncoated papers, especially open sheets, can cause dot gain due to their higher ink absorption which allows the ink to flow laterally to a greater extent.

Excessive dot gain will cause a loss of shadow detail as the half tone dots which should remain open start to fill in to a solid as they increase in size. Highlights also become toned and this gives an impression of flatness or lack of contrast. As dot gain can be controlled, it can be compensated for. With dot gain compensation, the films are produced with smaller half tone dots than required i.e. the films are ‘opened up’. This improves shadow detail and contrast and is particularly important with embossed/textured grades, rough laids or felt finished products – or, generally, uncoated grades that have an uneven surface.

The actual dot gain value for a specific product cannot be quoted here as the ink/press factors have such a major influence on the final dot gain obtained.