Communication, Measurement, and Control for Digital Imaging and Graphic Arts The COLOR GUIDE and Glossary Communication, Measurement, and Control for Digital Imaging and Graphic Arts TABLE OF CONTENTS 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 Color Formulation 35 Color Control 35 Color Verification 37 3 Glossary 41 www.x-rite.com © X-Rite, Incorporated 1998 ALL RIGHTS RESERVED X-Rite® , X-RiteColor®, the X-RiteColor logo, Digital Swatchbook®, X-Scan®, and QuickInk® are registered trademarks of X-Rite, Incorporated. Adobe, Adobe PageMaker, and PostScript are registered trademarks and Adobe Photoshop and Adobe Illustrator are trademarks of Adobe Systems Incorporated. Encapsulated PostScript (EPS) is a trademark of Altsys Corporation. FreeHand is a trademark of Macromedia, Inc. QuarkXPress is a registered trademark of Quark, Inc. All other brand or product names are trademarks or registered trademarks of their respective holders. Mention of third-party products is for informational purposes only and constitutes neither an endorsement nor a recommendation. X-Rite, Incorporated assumes no responsibility for the performance or use of these third-party products. Apple and Macintosh are trademarks of Apple Computer, Inc., registered in the United States and other countries. Mac and ColorSync are trademarks of Apple Computer, Inc. 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 market- ing of an item, potential buyers will perceive added value in that product. These GATF test images demonstrate colors that must be repro- duced 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 photographic film 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. 1 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: • Content Specifier/Client Defines message; determines image concept; provides general or specific color and paper specifications. • Graphic Designer/Photographer Provides image, art, and page files; and printed or digital color specifications. • Pre-Press Service Provider Provides final color-separated films; color break information; printed or digital color specifications. • Printing Ink Supplier Provides inks that meet color specifications; considers paper specification. • Printing Company Provides final printed piece; meets color specifications. 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 scene’s 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. • As we move our artwork between imaging, illustration, and layout programs, the colors are specified in different ways. For example, specifying 87% magenta / input 2 Color Communication 91% yellow produces a slightly different color in Photoshop™, FreeHand™, 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 trade shop’s 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). 3 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-millionth of a millimeter. 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 electro- magnetic 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, the senors 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. 4 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.