Dot gain
Encyclopedia
Dot gain is a phenomenon in offset lithography and some other forms of printing which causes printed material to look darker than intended. It is caused by halftone
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
.
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.
Mathematically, dot gain is defined as:
where
is the ink area fraction of the print, and 
is the pre-press 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.
and scattering of light
by the substrate
. Light becomes diffused
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.
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.
Modern prepress software usually includes utility to achieve the desired dot gain values using special compensation curve for each machine.
. 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
.
where
, 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).
, proposed a different model, one in which dots with larger perimeters tended to exhibit greater dot gain than those with smaller perimeters.
where
is the dot gain when the input area fraction is one-half; the highlight critical printing area, 
, is computed as:
and the shadow critical printing area,
, is computed according to
Note that, unless
, either the highlight critical printing fraction, 
, will be non-zero, or the shadow critical printing fraction, 
will be not unity, depending on the sign of 
. 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.
, and empirical modeling is used instead. To a certain extent, the models described above are empirical
, as their parameters cannot be accurately determined from physical aspects of image microstructure and first principles
. However, polynomial
s, cubic splines, and interpolation
are completely empirical, and do not involve any image-related parameters. Such models were used by Pearson and Pobboravsky, for example, in their program to compute dot area fractions needed to produce a particular color
in lithography
.
Halftone
Halftone is the reprographic technique that simulates continuous tone imagery through the use of dots, varying either in size, in shape or in spacing...
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
CMYK color model
The CMYK color model is a subtractive color model, used in color printing, and is also used to describe the printing process itself. CMYK refers to the four inks used in some color printing: cyan, magenta, yellow, and key...
.
Definition
It is defined as the increase in the diameter of a halftone dot during the prepress and 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.
Mathematically, dot gain is defined as:
where
is the ink area fraction of the print, and is the pre-press 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 ones, and thus can show more gain. As printing pressure can squeeze the ink out of its dot shape causing gain, ink viscosity 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 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 absorptionAbsorption (electromagnetic radiation)
In physics, absorption of electromagnetic radiation is the way by which the energy of a photon is taken up by matter, typically the electrons of an atom. Thus, the electromagnetic energy is transformed to other forms of energy for example, to heat. The absorption of light during wave propagation is...
and scattering of light
Visible spectrum
The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 390 to 750 nm. In terms of...
by the substrate
Substrate (printing)
Substrate is a term used in converting process such as printing and Lamination or coating as a more general term to describe the base material onto which e.g. images will be printed and to be laminated as per the packing specification required for the product...
. Light becomes diffused
Diffuse reflection
Diffuse reflection is the reflection of light from a surface such that an incident ray is reflected at many angles rather than at just one angle as in the case of specular reflection...
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 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.
Modern prepress software usually includes utility to achieve the desired dot gain values using special compensation curve for each machine.
Computing the fractional coverage (area) of a halftone pattern
The inked area 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,. 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 substrateSubstrate (printing)
Substrate is a term used in converting process such as printing and Lamination or coating as a more general term to describe the base material onto which e.g. images will be printed and to be laminated as per the packing specification required for the product...
has a wider Point Spread Function
Point spread function
The point spread function describes the response of an imaging system to a point source or point object. A more general term for the PSF is a system's impulse response, the PSF being the impulse response of a focused optical system. The PSF in many contexts can be thought of as the extended blob...
.
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 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 waswhere
, 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 MunichMunich
Munich The city's motto is "" . Before 2006, it was "Weltstadt mit Herz" . Its native name, , is derived from the Old High German Munichen, meaning "by the monks' place". The city's name derives from the monks of the Benedictine order who founded the city; hence the monk depicted on the city's coat...
, proposed a different model, one in which dots with larger perimeters tended to exhibit greater dot gain than those with smaller perimeters.
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:where
is the dot gain when the input area fraction is one-half; the highlight critical printing area, , is computed as:and the shadow critical printing area,
, is computed according toNote that, unless
, either the highlight critical printing fraction, , will be non-zero, or the shadow critical printing fraction, will be not unity, depending on the sign of . 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 geometryGeometry
Geometry arose as the field of knowledge dealing with spatial relationships. Geometry was one of the two fields of pre-modern mathematics, the other being the study of numbers ....
, and empirical modeling is used instead. To a certain extent, the models described above are empirical
Empirical
The word empirical denotes information gained by means of observation or experimentation. Empirical data are data produced by an experiment or observation....
, as their parameters cannot be accurately determined from physical aspects of image microstructure and first principles
First principles
In philosophy, a first principle is a basic, foundational proposition or assumption that cannot be deduced from any other proposition or assumption. In mathematics, first principles are referred to as axioms or postulates...
. However, polynomial
Polynomial
In mathematics, a polynomial is an expression of finite length constructed from variables and constants, using only the operations of addition, subtraction, multiplication, and non-negative integer exponents...
s, cubic splines, and interpolation
Interpolation
In the mathematical field of numerical analysis, interpolation is a method of constructing new data points within the range of a discrete set of known data points....
are completely empirical, and do not involve any image-related parameters. 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
Color or colour is the visual perceptual property corresponding in humans to the categories called red, green, blue and others. Color derives from the spectrum of light interacting in the eye with the spectral sensitivities of the light receptors...
in lithography
Lithography
Lithography is a method for printing using a stone or a metal plate with a completely smooth surface...
.