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Color Science, Reproduction, and Management

Ch. 14 of the Offset Lithographic Technology Book
by

Laura Roberts

on 12 June 2013

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Transcript of Color Science, Reproduction, and Management

Color Science, Reproduction, and Management
Color Proofs
Viewing Color Variables
Learning Objectives
When you have completed this presentation and the assigned activities related to this chapter, you will be able to do the following:
Explain the basic principles of color perception.
Summarize additive and subtractive color formation.
Describe the relationship between primary and secondary colors.
Discuss the basic principles of color separation.
Give examples of how variable such as vision deficiencies and external conditions affect color perception.
Define color space and dimensions of color.
Distinguish among hue, saturation, and brightness (HSB); International Commission on Illumination (CIE) XYZ; CIE xyY; and CIELAB.
Identify various color-measurement instruments.
List the various types of proofs used int he printing industry.
Explain the basic methods of color correction.
Summarize color sequencing.
Describe the color-management function.
Introduction
Color plays many significant roles in our everyday lives.

In addition to beautifying our environment, the use of color can greatly enhance communication.

As an element of design, color can add contrast and visual impact, as well as lend realism and create a specific mood.
Research indicates that well-applied color in print media is more effective than just black and white.

Even printed pages with no illustration can be more attractive if one or two colors are added to the layout.

Similar to typography, an awareness of color's properties enhances the effective use of color.

To accomplish this goal, everyone should have a basic understanding of color theory.
Understanding Color
Even a very basic understanding of color involves some physics, anatomy, and psychology.

Color is not a thing as much as it is a phenomenon.

This phenomenon is light energy generated at a source and traveling in waves, which ultimately stimulate receptors in the brain.

Therefore, the type of light source, the behavior of the light during its travel, and the color sensitivity of the viewer collectively determine perception of color.
The Interaction of Light and Pigments
Near the middle of the Electromagnetic Spectrum are wavelengths producing light.

Although most of the light is invisible, the human eye is sensitive to a narrow band known as the visible spectrum.
Light, radio waves, X rays, gamma rays, and microwaves are part of the electromagnetic spectrum because they all travel in waves.

Wavelengths are measured in nanometers, which in one billionth of a meter.
Color results from the interaction of light and pigments.

A pigment is any material
that absorbs light.

When sunlight strikes
an apple, the green and
blue colors are absorbed
and the red light
is reflected.
How Color Works
How We See Color
The light-sensitive nerve cells or the retina known as rods and cones determine our sense of light and color.

There are about 12 million rods and
7 million cones in the human eye.

The rods detect only light intensity (black and white), but the cones detect intensity (color).
Brightness is a value indicting how light or dark a color is.
The Additive Color System
People who work with light realize they can produce any desired color by combining only three colors of light - RGB. By combining all three equally it creates white light.

If they are directed onto a white surface so they create the pattern shown, three new colors - cyan, magenta, and yellow (CMY) result.

Varying the brightness of the three overlapping colors of light, creates hundreds of colors.

Producing a range of colors by combining RGB light in various ratios is known as the additive color system.

RGB are known as the primary colors - the colors from which all others can be made - because the additive system requires only these colors of light.
The Subtractive Color System
Most people who create color work with the subtractive color system because their colors are created by mixing pigments.

Pigments produce colors by absorbing (or subtracting) light.

In subtractive color system, the primary colors
are CMY. The key to the subtractive system is that
each of its primary colors - CMY- absorbs roughly
one-third or the white light striking it.

Overlapping two of the process colors absorbs
two-thirds of the white light and reflects only one-third
to the viewer.

Overlapping all three absorbs nearly all the light producing nearly black.

An area of cyan printed on a page absorbs the red wavelengths of light and reflects the blue and green wavelengths forming the color cyan.
Process Color Printing
In process color printing, only the three subtractive colors (plus black) produce hundreds of colors that are mixed visually, rather than physically.

Tiny, closely spaced dots of CMY ink in varying sizes absorb and reflect the different wavelengths of light to produce different colors.

Although mixing equal amounts of CMY creates a black, it still reflects light. To achieve a true black many printers use black ink as a fourth color.
Color Separations
Color separations is the process of dividing the colors of multicolored original art into the subtractive primary colors and black.

Separations can be made of the cyan, magenta, yellow, and black (CMYK) color directly to plates electronically.
Using Color-Separation Software
There are several basic considerations in creating professional color separations. These separations have the proper output resolution, sizing, dot-gain allowance, shadow details, color correction, sharpening, and output format.

The input resolution describes the ppi of the image as it was created during the scanning operation.

The output resolution describes the ppi of the image after the RGB-to-CMYK conversion, and is the resolution that moves to the prepress stage.

A photograph's output size should equal its size when printed. This resizing should occur before the actual separation is performed.

One constant of the printing process is that halftone dots become larger in diameter during the printing process. The degree of this growth, referred to as dot gain, is measured by the difference in size.

Dot gain compensation occurs in prepress by creating a smaller dot on the plate than is needed on the paper.
Using Color-Separation Software
Undercolor removal (UCR) reduces the amount of CMY applied to the shadow areas and replaces it with an appropriate amount of black. The same effect is achieved with less ink.

Gray component replacement (GCR) carries the concept of UCR to a higher level. Whereas UCR replaces CMY inks with black in only the shadow areas, GCR uses black as a substitute for calculated amounts of CMK inks wherever they are used to create a neutral gray in shadows and midtones.
Conventional
Separations




UCR





GCR
Under color addition (UCA) is a means of lightening dark areas of a reproduction and adding warmth or coolness to the dark areas.

Enhancing the perception of detail by increasing the contras of an image is known as sharpening. Usually Unsharp Mask is used for this.
Separating with a Spot Color
When CMYK separations are created, there are four separations - one for each color.

When a spot color or specialty ink is going to be used in the printing, a fifth separation must be created.

Most major page-layout and illustration programs have this capability.

All a user needs to do is select one of the colors from the available color-matching systems or create a new color and define it as spot rather than process.

The most common spot color-matching system is Pantone Matching System (PMS).
The color a person experiences is dependent on many variables. These variables include the eye's tendency to adapt and be affected by adjacent colors, as well as the fact that color perception varies among people.
The adjustment the human eye makes to light conditions is called adaption.

The colors surrounding, or adjacent to, a subject affect
color perception.
Adaptation
Not all people view colors in the same way.

If people do not have the necessary pigmentation in the cones of their retinas, some light entering the eye is not absorbed fully and not perceived.

This condition, known as color blindness, occurs in various degrees in 1% of females and 8% of males and causes people to see colors differently.
Color Blindness
As was explained earlier, light from the sun is made up of roughly equal amounts of RGB light. As a result of this balance, the sunlight is colorless.

Sources of artificial light are not as balanced, however, and they produce light with colors, depending on the color temperature of the light source.

Color temperature is important in color viewing because the same object viewed under light sources of different temperatures appears different.

The phenomenon of color shifting under different light sources is known as metamerism.

Metamerism creates problems when a printer is trying to match a customer-supplied color swatch. Metamerism can be avoided by viewing proofs and press sheets under light sources with consistent color temperature.

A common technique is to use viewing
booths to evaluate color.

A viewing booth is a viewing area with
color-balanced lighting so that anyone viewing
the same printed materials is seeing them under
the same lighting conditions.
Light-Source Temperature
Dichromatic vision
Most color-blind people have dichromatic vision and can see only yellows and blues. They confuse reds with greens and some reds or greens with some yellows.
Normal vision
Dichromatic vision
Achromatic vision
The very few people who are completely color-blind have achromatic vision and see only in shades of white, gray,
and black.
Normal vision
Achromatic vision
Vision Fatigue
Vision fatigue is caused by overuse of the vision system and can impair color judgement.
Defining Color
In addition to color-viewing variables just examined, there is also the problem of labeling colors.

Although there are dozens of words identifying and describing color, words are not nearly precise enough to identify the thousands of available colors.

For this reason, various systems have been developed to establish a universal system for color classification.
The Pantone Matching System Standard
In 1963, Lawrence Herbert designed the Pantone Matching System Standard, based on the major group of pigment colors (such as reds, yellows, greens, and blues) common to the printing industry.

With the Pantone Formula Guide, a customer or graphic designer can select a color from over 1000 swatches and specify the color's number to the printer, who in turn, can mix inks according to the Pantone formula to achieve the desired color.

The Pantone Matching System standard is the most widely used system in the graphic communication industry.
Dimensions of Color
It is impossible to verbally describe the appearance of a color in absolute terms. It is, however, possible to describe a color's appearance in terms of its three dimensions: hue, saturation, and brightness (HSB).

The HSB dimensions are simply terms for the characteristics of hue, chroma, and value.
The International Commission on Illumination (CIE)
In 1931, the International Commission on Illumination (CIE) made two major steps towards measuring color.

One was to establish normal human color perception.

The other was to create a three-dimensional model to numerically measure the responses of the average human eye to different wavelengths of light.
Hue
Hue is the color the eye perceives.
Saturation
Saturation is the attribute of color defining the color's degree of cleanliness, or purity. A color loses its saturation when it is mixed with a second color of a very different hue, black, white, or gray.
Brightness
Brightness is often referred to as lightness or luminosity and is defined as a value indicating how light or dark a color is.
CIE XYZ values
CIE had several people with normal vision look at a wide range of colors. From their responses, the scientists were able to establish normal human sensitivity to RGB light, which they call the standard observer.

The eye's sensitivity to RGB light is represented by X, Y, and Z. Similar to a fingerprint, the ratio of RGB light making up an individual color is unique.
The CIE chromaticity diagram
The CIE chromaticity diagram plots the wavelengths of visible spectrum in a curve bending across a grid.

Every color can be mapped on a grid framed by x- and y- axes. It pinpoints the location of a color on three axes - x, y, and Y.
To understand how the xyY coordinates work, imagine an airplane flying through the air. If you want to locate the plane precisely, you need only three pieces of data -its longitude, latitude, and altitude. These values or coordinates, define the color space, a three-dimensional representation of color as numerical data.

The CIELAB system is a method of identifying a color by the values on three axes formed by opposing colors (red-green, blue-yellow, and black-white).
Measuring Color
Color
The viewing variables just stated, color cannot be described and managed unless it can be measured numerically.
Colorimeters
The three distinct values must be measured because the CIELAB model views color as a combination of those three components.

Colorimeters measure color by generating numerical data that pinpoints sample on the three CIELAB axes described earlier: white-black, green-red, blue-yellow.
Spectrophotometers
The spectrophotometer measures light reflectance across the visible spectrum. This instrument measures a color by plotting a curve revealing the amount of light in each wavelength area reflected from a sample. This is very useful in color matching.
Densitometers
Although densitometers do not measure color, their ability to measure optical density makes them useful in maintaining color consistency throughout a pressrun.

Optical density is the light-absorption ability of an image or a surface material.

A scanning densitometer automates the measurement of press-sheet color bars.
Spectrodensitometers
Spectrodensitometers are densitometers that can measure light across the visible spectrum by using a prism or diffraction grating to spread the light and slit to isolate narrow bands of light between 1nm and 10 nm.

It combines all the functions of a densitometer and colorimeter into a single instrument. It outputs the information in numbers rather than using curves.
Measuring Color Variations
After a system to measure color has been adopted, the next step is to measure color variation. The degree of difference between the client-supplied swatch and its reproduction on a given proof or press sheet can be expressed numerically.
Color Gamuts
The range of colors that can be generated within a color system is called a color gamut, and different systems have different color-gamut sizes.
Process color, also known as four color or full color, is the faithful reproduction of color photographs or other art containing colors.
In general terms, the wide range of colors making up a color photograph can be reproduced with only four ink colors, referred to as process ink colors.
Colorants are chemical substances that give color to such materials as ink, paint, and crayons.

Colorants do not dissolve but are distributed as tiny solid particles through a vehicle are called pigments.
After the original color image has been separated into its CMKY components, proofs are prepared to check the quality of the separation.

A proof is a preview of the printed job, and it allows the printer and the customer to see what the job will look like after the job comes off the press and provides the opportunity to make alterations before the actual press run.
Proofs make from film and dyes or electronic data are prepress proofs.

Those made from the actual plates after they have been mounted on the press are press proofs.

Besides confirming the layout, fonts, and other design elements were not lost or deformed before reaching the output stage, proofs should be checked for color accuracy, resolution, and registration.

Registration is the overall agreement in the position and alignment of printing detail on a press sheet.
Analog Proofs
These are film generated proofs which include Matchprint proofs, bluelines (diazo), the Cromalin system, and Color Key proofs.
Matchprint proofs are four-color proofs produced from the film that will be used to create the printed pages.
bluelines (diazo)
Digital Proofs
When a printing company adopts a CTP work flow, it eliminates the film from which plates and analog proofs have traditionally made.

Without film, proofs are generated directly from digital code, a process called digital printing.
Press Proofs
For some jobs, offset plates are made and run on four-color proof presses to create press proofs.

A proof press is a printing machines used to produce press proofs, which are proofs printed from plates prepared from film.
Color Correction
Some of the biggest challenges in the printing industry are getting color to reproduce the way it should and even predicting what the printed colors would look like.
Color correction is now performed almost exclusively electronically with photo-editing software, such as Photoshop.
Color Sequencing on Press
In process color printing, the color sequence, laydown sequence, or printing sequence is the order in which the colors of ink are printed.

In offset lithography, the most common printing sequence on a four-color press is black, cyan, magenta, and yellow.
Color Management
Maintaining the original color throughout production is one of the major challenges within the printing industry.

Electronic prepress has eliminated film, and replaced them with scanners, software, monitors, proofers, and platesetters.

Each of these components of prepress production is a common cause of color drift. This drift is any variation in hue, saturation, or lightness that occurs when a color image moves from one electronic-prepress device to another.
Color management software (CMS) is designed to coordinate each device involved in printing color images.

The devices include scanners, computer monitors, proofers, imagesetters or platesetters, and the press itself.

Without CMS, the different color responses of these devices result in an inconsistent display of the image. The software detects idiosyncrasies and makes allowances for them.
Color Management Software
Calibration and Profiling
Industry Specs
In an effort to help enhance and standardize color printing, three divisions of the printing industry have developed standards for prepress and on-press operations to follow when CMYK is printed on substrates typically used in their respective sectors of the industry.
Color Sequence on Proof
Film-generated and digital proofs do not operate on the same principle as press proofs.

Therefore, it is not necessary, and sometimes not even possible, to use the same color sequence that will be used on press.
To calibrate an electronic device is to ensure its settings are at the manufacturer's specs.

To profile a device or software is to measure its color output and compare the output with a device independent color mode, such as CIE XYZ or CIELAB profiles.

Calibrating and profiling the monitor
should be the first step to make sure
it is RGB balanced.

Profiling the scanner after the monitor
is the next step.

The lastly would be to profile output devices
such as platesetters and printers.
Specifications for Web Offset Publications (SWOP)
SWOP specs include specs for the preparation of digital files and CTP operations.
Specifications for Newsprint Advertising Production (SNAP)
SNAP specs are tailored to printing on newsprint.
General Requirements for Applications in Commercial Offset Lithography (GRACoL)
GRACoL specs provide guidelines for making color separations, creating digital files, maintaining color on press, and specifying paper and ink.
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