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Dot gain, or tonal value increase, is a phenomenon in
offset lithography Offset printing is a common printing technique in which the inked image is transferred (or "offset") from a plate to a rubber blanket and then to the printing surface. When used in combination with the lithographic process, which is based on ...
and some other forms of printing which causes printed material to look darker than intended. It is caused by
halftone Halftone is the reprographic Reprography (a portmanteau of ''reproduction'' and ''photography'') is the reproduction of graphics through mechanical or electrical means, such as photography or xerography. Reprography is commonly used in catal ...
dots growing in area between the original printing film and the final printed result. In practice, this means that an image that has not been adjusted to account for dot gain will appear too dark when it is printed. Dot gain calculations are often an important part of a
CMYK color model The CMYK color model (also known as process color, or four color) is a subtractive color model, based on the CMY color model, used in color printing, and is also used to describe the printing process itself. The abbreviation ''CMYK'' refers ...
.


Definition

It is defined as the increase in the area fraction (of the inked or colored region) of a halftone dot during the
prepress Prepress is the term used in the Printing and Publishing industries for the processes and procedures that occur between the creation of a print layout and the final printing. The prepress process includes the preparation of artwork for press, media ...
and printing processes. Total dot gain is the difference between the dot size on the
film negative In photography, a negative is an image, usually on a strip or sheet of transparent plastic film, in which the lightest areas of the photographed subject appear darkest and the darkest areas appear lightest. This reversed order occurs because th ...
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 Computer-to-plate (CTP) is an imaging technology used in modern printing processes. In this technology, an image created in a Desktop Publishing (DTP) application is output directly to a printing plate. This compares with the older technology, co ...
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. Mathematically, dot gain is defined as: :DG=a_-a_ where ''a''print is the ink area fraction of the print, and ''a''form is the prepress area fraction to be inked. The latter may be the fraction of opaque material on a film positive (or transparent material on a film negative), or the relative command value in a digital prepress system.


Causes

Dot gain is caused by ink spreading around halftone dots. Several factors can contribute to the increase in halftone dot area. Different paper types have different ink absorption rates; uncoated papers can absorb more ink than
coated A coating is a covering that is applied to the surface of an object, usually referred to as the substrate. The purpose of applying the coating may be decorative, functional, or both. Coatings may be applied as liquids, gases or solids e.g. Powder ...
ones, and thus can show more gain. As printing
pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...
can squeeze the ink out of its dot shape causing gain, ink
viscosity The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity quantifies the inte ...
is a contributing factor with coated papers; higher viscosity inks can resist the pressure better. Halftone dots can also be surrounded by a small circumference of ink, in an effect called "rimming". Each halftone dot has a microscopic relief, and ink will fall off the edge before being eliminated entirely by the fountain solution (in the case of offset printing). Finally,
halation An anti-halation backing is a layer found in many photographic films—and almost all film intended for motion picture cameras—usually a coating on the back of the film base, though it is sometimes incorporated between the light-sensitive emuls ...
of the printing film during exposure can contribute to dot gain.


Yule–Nielsen effect and "optical dot gain"

The Yule–Nielsen effect, sometimes known as ''optical dot gain'', is a phenomenon caused by
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and scattering of
light Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 tera ...
by the substrate. Light becomes
diffused Molecular diffusion, often simply called diffusion, is the thermal motion of all (liquid or gas) particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size (mass) of ...
around dots, darkening the apparent tone. As a result, dots absorb more light than their size would suggest. The Yule–Nielsen effect is not strictly speaking a type of dot gain, because the size of the dot does not change, just its relative absorbance. Some densitometers automatically compute the absorption of a halftone relative to the absorption of a solid print using the Murray–Davies formula.


Controlling 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
lines per inch Lines per inch (LPI) is a measurement of printing resolution. A line consists of halftones that is built up by physical ink dots made by the printer device to create different tones. Specifically LPI is a measure of how close together the lines in ...
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. Modern prepress software usually includes utility to achieve the desired dot gain values using special compensation curves for each machine.


Computing the area of a halftone pattern

The inked area (coverage) fraction of the dot may be computed using the Yule-Nielsen model. This requires the optical densities of the substrate, the solid-covered area, and the halftone tint, as well as the value of the Yule-Nielsen parameter, ''n''. Pearson has suggested a value of 1.7 be used in absence of more specific information. However, it will tend to be larger when the halftone pattern in finer and when the substrate has a wider point spread function.


Models for dot gain

Another factor upon which dot gain depends is the dot's area fraction. Dots with relatively large perimeters will tend to have greater dot gain than dots with smaller perimeters. This makes it useful to have a
model A model is an informative representation of an object, person or system. The term originally denoted the Plan_(drawing), plans of a building in late 16th-century English, and derived via French and Italian ultimately from Latin ''modulus'', a mea ...
for the amount of dot gain as a function of prepress dot area fraction.


An early model

Tollenaar and Ernst tacitly suggested a model in their 1963 IARIGAI paper. It was :\mathrm_=a_ \,\left(1 - a_\right) where ''avf'', the shadow critical area fraction, is the area fraction on the form at which the halftone pattern just appears solid on the print. This model, while simple, has dots with relatively small perimeter (in the shadows) exhibiting greater gain than dots with relatively larger perimeter (in the midtones).


Haller's model

Karl Haller, of FOGRA in
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, proposed a different model, one in which dots with larger perimeters tended to exhibit greater dot gain than those with smaller perimeters. One result derivable from his work is that dot gains depend on the shape of the halftone dots.


The GRL model

Viggiano suggested an alternate model, based on the radius (or other fundamental dimension) of the dot growing in relative proportion to the perimeter of the dot, with empirical correction the duplicated areas which result when the corners of adjacent dots join. Mathematically, his model is: :\mathrm_=\begin a_-a_, & \mathrm\ a_\leq a_\\ pt2\,\Delta_\sqrt, & \mathrm\ a_pta_-a_, & \mathrm\ a_\geq a_\end where Δ0,50 is the dot gain when the input area fraction is ; the highlight critical printing area, ''awf'', is computed as: :a_=\begin \dfrac, & \mathrm\ \Delta_<0\\ pt0, & \mathrm\ \Delta_\geq0\end and the shadow critical printing area, ''avf'', is computed according to :a_=\begin 1, & \mathrm\ \Delta_\leq0\\ pt\dfrac, & \mathrm\ \Delta_>0\end Note that, unless Δ0,50 = 0, either the highlight critical printing fraction, ''awf'', will be nonzero, or the shadow critical printing fraction, ''avf'' will not be 1, depending on the sign of Δ0,50. In instances in which both critical printing fractions are non-trivial, Viggiano recommended that a cascade of two (or possibly more) applications of the dot gain model be applied.


Empirical models

Sometimes the exact form of a dot gain curve is difficult to model on the basis of
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, and empirical modeling is used instead. To a certain extent, the models described above are
empirical Empirical evidence for a proposition is evidence, i.e. what supports or counters this proposition, that is constituted by or accessible to sense experience or experimental procedure. Empirical evidence is of central importance to the sciences and ...
, as their parameters cannot be accurately determined from physical aspects of image microstructure and
first principles In philosophy and science, a first principle is a basic proposition or assumption that cannot be deduced from any other proposition or assumption. First principles in philosophy are from First Cause attitudes and taught by Aristotelians, and nuan ...
. However,
polynomial In mathematics, a polynomial is an expression consisting of indeterminates (also called variables) and coefficients, that involves only the operations of addition, subtraction, multiplication, and positive-integer powers of variables. An exa ...
s,
cubic spline In numerical analysis, a cubic Hermite spline or cubic Hermite interpolator is a spline where each piece is a third-degree polynomial specified in Hermite form, that is, by its values and first derivatives at the end points of the corresponding ...
s, and
interpolation In the mathematical field of numerical analysis, interpolation is a type of estimation, a method of constructing (finding) new data points based on the range of a discrete set of known data points. In engineering and science, one often has a n ...
are completely empirical, and do not involve any image-related
parameter A parameter (), generally, is any characteristic that can help in defining or classifying a particular system (meaning an event, project, object, situation, etc.). That is, a parameter is an element of a system that is useful, or critical, when ...
s. Such models were used by Pearson and Pobboravsky, for example, in their program to compute dot area fractions needed to produce a particular
color Color (American English) or colour (British English) is the visual perceptual property deriving from the spectrum of light interacting with the photoreceptor cells of the eyes. Color categories and physical specifications of color are associ ...
in
lithography Lithography () is a planographic method of printing originally based on the immiscibility of oil and water. The printing is from a stone (lithographic limestone) or a metal plate with a smooth surface. It was invented in 1796 by the German a ...
.


References


External links


Understanding Yule–Nielsen factors

Free dot gain compensation calculator
{{DEFAULTSORT:Dot Gain Quality issues in printing Printing terminology Vision