Communication, The and control for Color Guide Digital ...

[Pages:52]Communication, measurement, and control for Digital Imaging and Graphic Arts

The

Color Guide

and Glossary

1 Color Communication

1

Understanding Color

4

The CIE Color Systems

14

Spectral Data vs. Tristimulus Data

19

2 Color Measurement and Control

22

Instrumentation

22

Measurement in the Graphic Arts Workflow 26

Color Specification

27

Color Management

28

Ink Formulation

35

Color Control

35

Color Verification

37

3 Glossary

41

1 Color Communication

Color communicates. Color sells. Color is the sizzle that drives the sale of virtually every consumer product in the world. It evokes a wide range of emotions that draw the buyer to the product. As design, graphics, and imaging professionals, we know that color is a crucial part of the selling process because it is such an important part of the buying decision. If we use color effectively in the manufacturing and marketing of an item, potential buyers will perceive added value in that product.

These PIA/GATF test images demonstrate colors that must be reproduced with careful precision. If the color of skin tones, sky blue, grass green, or food items are "off" by even a small margin, the appearance of the entire image will be adversely affected.

To use color effectively, it must be kept under tight control. The color workflow begins with the designer's ideas and the customer's specifications. From there, these colors must be communicated among several different individuals who will render and reproduce the colors on many different devices. At each stage of production, output from the previous step becomes the input for the next process. Every exchange brings the color into a new color space--from a photographic scene, to monitor RGB, to CMYK process proofing, and printing on a variety of systems. And every evaluation is made by a different viewer under new viewing conditions. So, how do we ensure that our original ideas and specifications will remain intact throughout this complicated process? This book is designed to answer that very question. In short, the answer is color measurement--if you can measure color, you can control it. The remainder of this booklet explains the fundamentals of color communication, measurement, and control.

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The Color Guide and Glossary

The Challenge: Color Communication Consider the many different individuals who "pass the baton" of responsibility for keeping the customer's color specifications intact:

? Client Defines message; determines image concept; provides general or

TE specific color and paper specifications.

EA CR

? Graphic Designer/Photographer Provides image, art, and page files; and printed or digital color specifications.

E

R EPA

? Pre-Press Service Provider Provides final color-separated films; color break information; printed or digital color specifications.

PR

? Materials Supplier Provide inks and paper that meet color specifications.

TE U

? Output Service Provider Provides final prints; meets color specifications.

EC

EX

Each step in the color reproduction process adds value and content to the message.

Good color specification ensures that each process provides accurate color content

based on the input received.

As we strive to create dazzling, high-quality color documents and designs, we struggle to control color at each production phase. Each viewing situation presents its own interpretation of the same color. For example:

? Our original scene contains a wide range of natural, vivid colors. ? A photograph of the scene captures much of the scenes color; however,

some of the dazzling tones are lost when the image is scanned into RGB data. Still more colors are lost or changed when the scan is displayed on a monitor--and the scene appears slightly different on different monitors.

1 Plan / Gather

CREATE

2 Design / 3 Client Capture Review

PREPARE

4 Check / 5 Prepare 6 Final

Fix Files

Files

Proof

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Color Communication

? As we move our artwork between imaging, illustration, and layout programs, the colors are specified in different ways. For example, specifying 87% magenta / 91% yellow produces a slightly different color in Adobe PhotoshopTM, Macromedia FreeHandTM, and QuarkXPress?.

? When we print our artwork, the colors get color-separated from RGB data into CMYK data. The colors are interpreted a bit differently on different devices--on our laser copier, our prepress providers proofing system, and on press.

? When we check our output, we view the colors under different lighting conditions that affect color appearance in different ways. Also, different individuals perceive colors based on their own vision skills and memory.

The common question throughout this process is: which device is telling the truth? Unfortunately, no individual viewers, programs, or devices can reveal the true identity of a color. They simply perceive the color's appearance, which can be affected by lighting and other factors.

The Solution: Color Measurement and Control Measurement is the key to total production control. Consider this: we measure size in inches or millimeters; weight in pounds and grams; and so on. These scales allow us to establish precise measurement standards that can be repeated in the production process. This ensures that all manufactured items are identical and within our quality tolerances. Using measured color data, we can do the same for color--we can monitor color at each stage of production and check the "closeness"of color matches using repeatable, standardized numerical data. So, what properties of colors allow them to be discretely identified and measured?

Let's find out by examining these properties--how color happens in nature and in our minds; how it is reproduced on screen and on paper; and how color can be communicated as reflectance values (spectral data) and as three-dimensional values (tristimulus data).

7 Make Plates

EXECUTE

8 Set Up 9 Run

Press

Job

10 Finish / Fulfill

From idea to press: The graphic arts workflow begins and ends with the customer. The challenge we face is to present the customer with consistent color results at every stage.

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The Color Guide and Glossary

UNDERSTANDING COLOR To help you clearly understand how color is measured, we should first study the fundamentals of color's physical and physiological properties. Color results from an interaction between light, object, and the viewer. It is light that has been modified by an object in such a manner that the viewer--such as the human visual system--perceives the modified light as a distinct color. All three elements must be present for color as we know it to exist. Let's examine color's origins in more detail by first studying light. Light--Wavelengths and the Visible Spectrum Light is the visible part of the electromagnetic spectrum. Light is often described as consisting of waves. Each wave is described by its wavelength - the length from wave crest to adjacent wave crest. Wavelengths are measured in nanometers (nm). A nanometer is one-billionth of a meter. The region of the electromagnetic spectrum visible to the human eye ranges from about 400 to 700 nanometers. This amounts to a mere slice of the massive electromagnetic spectrum. Although we can't see them, we use many of the invisible waves beyond the visible spectrum in other ways--from short-wavelength x-rays to the broad wavelengths that are picked up by our radios and televisions.

Visible Spectrum

Our eyes have light sensors that are sensitive to the visible spectrum's wavelengths. When light waves strike these sensors, the sensors send signals to the brain. Then, these signals are perceived by the brain as a particular color. Exactly which color is perceived depends on the composition of wavelengths in the light waves. For example, if the sensors detect all visible wavelengths at once, the brain perceives white light. If no wavelengths are detected, there is no light present and the brain perceives black.

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Color Communication

Now we know how our eyes and brain respond to the presence of all visible wavelengths or no wavelengths. Next, let's examine how our vision system responds to each individual wavelength. Passing a beam of white light through a prism disperses the light so that we can see how our eyes respond to each individual wavelength. This experiment demonstrates that different wavelengths cause us to see different colors. We can recognize the visible spectrum's dominant regions of red, orange, yellow, green, blue, indigo, and violet; and the "rainbow" of other colors blending seamlessly in between.

When our visual system detects a wavelength around 700nm, we see "red;" when a wavelength around 450-500nm is detected, we see "blues;" a 400nm wavelength gives us "violet;" and so on. These responses are the basis for the billions of different colors that our vision system detects every day. However, we rarely see all wavelengths at once (pure white light), or just one wavelength at once. Our world of color is more complex than that. You see, color is not simply a part of light--it is light. When we see color, we are seeing light that has been modified into a new composition of many wavelengths. For example, when we see a red object, we are detecting light that contains mostly "red" wavelengths. This is how all objects get their color--by modifying light. We see a world full of colorful objects because each object sends to our eyes a unique composition of wavelengths. Next, let's examine how objects affect light.

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The Color Guide and Glossary

Objects--Manipulating Wavelengths When light waves strike an object, the object's surface absorbs some of the spectrum's energy, while other parts of the spectrum are reflected back from the object. The modified light that is reflected from the object has an entirely new composition of wavelengths. Different surfaces containing various pigments, dyes, and inks generate different, unique wavelength compositions. Light can be modified by striking a reflective object such as paper; or by passing through a transmissive object such as film or a transparency. The light sources themselves - emissive objects such as artificial lighting or a computer monitor also have their own unique wavelength composition.

Reflected, transmitted, or emitted light is, in the purest of terms, "the color of the object." There are as many different colors as there are different object surfaces-- each object affects light in its own unique way. The pattern of wavelengths that leaves an object is the object's spectral data, which is often called the color's "fingerprint." Spectral data results from a close examination--or measurement--of each wavelength. This examination determines the percentage of the wavelength that is reflected back to the viewer--its reflectance intensity. You can visually examine a color's spectral properties by plotting its measurement data as a spectral curve. This type of data can be gathered only by using a spectrophotometer such as X-Rite's model 939 Spectrophotometer, 530, DTP41, or Auto-Tracking Spectrophotometer (ATS) system.

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