W O R K F L O W T E S T F O R M S

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1 W O R K F L O W T E S T F O R M S E N D - T O - E N D W O R K F L O W I M A G E Q U A L I T Y A S S U R A N C E U S E R G U I D E Isis Imaging Corporation

2 Isis Imaging Corporation Workflow Test Forms Suite User Guide Version 1.0 Isis Workflow Test Forms Suite User Guidei

3 Copyrights Workflow Test Forms User Guide Copyright 2003 Isis Imaging Corporation. All rights reserved. Analytic Test Form Copyright 2003 Isis Imaging Corporation. All rights reserved. Grayscale Test Form Copyright 2003 Isis Imaging Corporation. All rights reserved. GATF Color Management Test Form* Copyright Graphic Arts Technical Foundation. All rights reserved. Nature Scene Photograph Copyright 2002 Edward Burtynsky. All rights reserved. People Photographs Copyright 2002 Edward Burtynsky. All rights reserved. Adobe InDesign, Adobe Acrobat and Adobe Photoshop are registered trademarks of Adobe Systems Inc. Icefields is a registered trademark of Isis Imaging Corporation. *Available only with the GATF option No part of this publication (whether in hardcopy or electronic form) may be reproduced or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written consent of Isis Imaging Corporation. The content of this user guide is furnished for informational use only, is subject to change without notice, and should not be construed as a commitment by Isis Imaging Corporation. No responsibility or liability for any errors or inaccuracies that may appear in this user guide is assumed by Isis Imaging Corporation. Isis Workflow Test Forms Suite User Guide Copyrights i

4 The Isis Imaging Workflow Test Forms Suite (IIWTS) Your End-User Limited License Agreement for Use The IIWTS described in this license is provided under license and may only be used or copied in accordance with the terms of such license. License Agreement: Please read this Agreement carefully. By using the contents of this IIWTS, you acknowledge that you have read this Agreement, understand it, and agree to be bound by the terms of this License Agreement. The IIWTS is licensed, not sold, to you for use only under the terms of this Agreement, and Isis Imaging Corporation reserves any rights not expressly granted to you. You own only the binary data of IIWTS, but Isis Imaging retains title to and ownership of all copies of IIWTS. You may use the photographs, GATF test forms and Isis Imaging analytical test forms in a single computer by the registered end-user. You may make one copy of IIWTS to serve as a backup. This license grants you a nonexclusive license to use IIWTS with one host computer only. Otherwise you agree to use this IIWTS like a book, meaning the IIWTS may be moved from one computer to another so long as there is no possibility of it being read/used in two different locations. Prohibited Uses: You may not rent, lease, loan, resell for profit, distribute, network or create derivative works based upon the IIWTS or any part of it. You may not use IIWTS in advertising, marketing, communications, presentations, internet site and multimedia projects. Limited Warranty and Disclaimers: Isis Imaging Corporation has made no express warranties to you regarding the IIWTS and that the IIWTS is provided to you as is without warranty of any kind. Isis Imaging disclaims all warranties with regard to the IIWTS; express or implied; including without limitation; any implied warranties of fitness for a particular purpose; merchantability; merchantable quality or non-infringement of third party rights with respect to the IIWTS and accompanying electronic and printed materials. (Some states or jurisdictions do not allow the exclusion of implied warranties; so the above limitations may not apply to you. In no event shall Isis Imaging s total liability to you for all damages, losses, and causes of action (whether in contract, tort [including negligence] or otherwise) exceed the amount paid by you for the IIWTS.) No Liability for Consequential Damages: In no event will Isis Imaging be liable to you for any damages whatsoever (including, without limitation, damages for loss of business profits, business interruption, loss of business information, loss of use, or any other direct, indirect, special, incidental, or consequential pecuniary damages of any kind) arising out of the use of, or inability to use, this IIWTS, or as a result of any defect in the IIWTS or the manual, regardless of the form of action whether in contact, tort (including negligence), strict product liability or otherwise, even if Isis Imaging has been advised of the possibility of such damages, or in the event of a computer viral infection. (Some states or jurisdictions do not allow the exclusion or limitation of incidental or consequential damages; so the above limitation or exclusion may not apply to you.) Isis Imaging s entire liability and your exclusive remedy shall be, at Isis Imaging s option, either return of the price paid, or replacement of the IIWTS that does not meet Isis Imaging s limited warranty. This limited warranty is void if failure of the IIWTS has resulted from accident, abuse, or misapplication. Any replacement IIWTS will be warranted for the remainder of the original 30-day warranty period. Isis Imaging disclaims all other warranties, express, implied or statutory, including but not limited to the implied warranties of merchantability and fitness for a particular purpose, with respect to the IIWTS and accompanying electronic and printed materials. This limited warranty gives you specific legal rights. You may have others that vary according to jurisdiction. This Agreement is governed by the laws of British Columbia, Canada. Isis Imaging Corporation support@isisimaging.com ii End-user license Isis Workflow Test Forms Suite User Guide

5 Table of Contents Copyrights...i End user license...ii Introduction... 1 Organization of this guide...1 Hardware and software requirements...1 User experience requirements...2 Isis Imaging workflow test forms suite components...2 Test forms...2 Graphic Arts Technical Foundation option...2 Analytic test form...3 Grayscale comparison test form...3 Nature scene photograph...3 People photographs...4 Industry standards...4 Test forms overview... 5 What are test forms...5 When to use test forms...5 When not to use test forms...5 When to use CMYK and grayscale test forms...5 Benefits of using test forms...6 Test form criteria...6 Advantages of the GATF standardized test form option...7 Advantages of the Isis Imaging test forms...7 Color management system... 9 Incorporating the workflow test forms suite in your CMS...9 Workflow...11 Workflow order...11 Workflow color-space configurations...11 CMYK reprographics...12 RGB to CMYK at prepress...13 RGB to CMYK at the RIP...13 Mixed RGB and CMYK to CMYK at the RIP...14 Workflow complexity...15 Simple RGB to CMYK workflow...15 Complex workflows...16 Mixed color-space workflow...16 Isis Workflow Test Forms Suite User Guide Table of contents iii

6 Image quality testing overview...17 Test images must fail to reproduce perfectly Image quality evaluation The human visual system Measuring color using your HVS Properties of the HVS Measuring color by visual comparison Color matching objectives Color preferences Measuring output device characteristics using HVS and instruments Measuring grayscale appearance on color devices HVS failures Lighting conditions and metamerism Lighting conditions and surround Lighting and adaptation Workflow testing procedures...27 Recommended viewing conditions Test image preparation Basic testing procedure Test image evaluation considerations Common HVS observation failures Test image characteristics should not be altered Sweeps Workflow testing strategy Color transformation points Grayscale testing Using the test forms Graphic Arts Technical Foundation test form option Analytic test form Grayscale test form Nature scene photograph People photographs Recording your workflow image quality measurements Sample image quality history document Communicating with your printer service representative...39 Appendix A. References...41 Appendix B. Monotone to color migration...42 Appendix C. Analytic test form s RGB saturation points...43 Appendix D. Commonly tested image properties...44 Appendix E. Subjective analytical evaluation...45 Appendix F. Image quality history form...46 Appendix G. Test form color spaces and gray balance...47 Appendix H. Preparing for an Isis image quality report...48 Appendix I. Glossary...49 iiii Table of contents Isis Workflow Test Forms Suite User Guide

7 Introduction This guide introduces you to the practice of visually measuring output results at image processing points in your workflow to achieve assurance of the accuracy of your color workflow. Using the supplied test forms and images, this guide explains how to test your workflow for image quality problems and describes several correction methods. The higher the quality throughout every step of production, the higher your printing quality. This guide can be used with any workflow and color management system. This user guide does not advise you on how best to manufacture your printed product, or calibrate and characterize your output devices. Please refer to the manuals supplied with your color management components. Organization of this guide This guide is organized as follows: Introduction describes the minimum hardware and software required to use the test forms, the expertise required by the person using the test forms, and brief descriptions of the supplied test forms. Test forms overview describes what test forms are, how they are used, and criteria for using them. It also lists the advantages of using Isis Imaging s test forms. Color management system describes how to incorporate the Isis Imaging Workflow Test Forms Suite in your color management system. Workflow describes common workflow configuration classifications. Image quality testing overview discusses the practice of evaluating the image quality of your workflow using your eye as a measuring instrument. Workflow testing procedures describes basic workflow testing procedures, evaluation considerations and the importance of recording your test results. Communicating with your printer service representative describes how to use the Isis Imaging Workflow Test Forms Suite to communicate image quality problems with your technical service provider. Hardware and software requirements The Isis workflow test forms are compatible with Apple Macintosh and PC computers supporting an ICC color management system and PDF software such as Adobe Acrobat and Photoshop. Isis Workflow Test Forms Suite User Guide Introduction 1

8 User experience requirements A trained Isis Imaging workflow test forms user will have a solid background in color reproduction, color management procedures, prepress capabilities, and RIP and digital press operation. This individual should also be an expert at using the software applications and hardware components that comprise the color workflow. See Appendix B for a basic requirements list. Isis Imaging workflow test forms suite components Test forms The Isis workflow test forms suite consists of: three test photographs, two test forms, the Workflow Test Forms User Guide (this document), and, with the GATF option: two standardized GATF test forms and the GATF/ColorSync Test Form User Guide. Each form and image tests for specific image quality properties. For detailed information on how to use each form refer to Using the test forms in Workflow testing procedures. Graphic Arts Technical Foundation option Use the Graphic Arts Technical Foundation (GATF) standardized test form to test for: consistency in time and between printers gray balance tonal accuracy halftone problems Figure 1, GATF test form 2 Introduction Isis Workflow Test Forms Suite User Guide

9 Analytic test form Use the analytic form to test for: color mixing accuracy sweep or gradient accuracy Figure 2, Analytic test form Grayscale comparison test form Use this form to determine: accuracy of grayscale levels directional dot gain dot gain gamma Figure 3, Grayscale comparison test form Nature scene photograph Use this photograph to test for: dot gain saturation detail gray balance global color accuracy Figure 4, Nature scene test image Isis Workflow Test Forms Suite User Guide Introduction 3

10 People photographs Use these photographs to test for: high-key sweeps and highlight color detail low-key sweeps and shadow color detail color accuracy in flesh tones detail reproduction Figure 5, People test photographs Industry standards All images in the Isis workflow test forms suite meet the following industry standards: PDF/X-3 compliant ICC profile format specification SWOP specifications for web offset printing 4 Introduction Isis Workflow Test Forms Suite User Guide

11 Test forms overview This chapter describes background information about test forms what they are, when to use them and the criteria for evaluating the usefulness of a particular test form. What are test forms Test forms are visual diagnostic and measurement tools used to verify the level of quality achieved at every image processing point and workflow component. They provide the basis for controlling and measuring systemwide image quality at the creation, prepress, and printer stages of the workflow. The test images provided in the Isis workflow test forms suite are device-independent, capable of testing various types of workflows. A test form contains images that test the results of calibration, characterization, consistency and color conversion. Test forms provide the basis for consistent testing, communication and description of image quality characteristics. When to use test forms The Isis workflow test forms suite is used after you have calibrated and characterized your color management system (CMS). Use the workflow tests for assurance that the state of your CMS is of high quality and consistently performs within your required range. When not to use test forms Do not use the Isis workflow test forms to calibrate your monitor, production printer or proofer. Use the test images provided by the manufacturer to calibrate, characterize and check the consistency of your monitor or printer. While test images display the accuracy of the calibration, they are not intended to guide calibration. When to use CMYK and grayscale test forms A CMYK version and a grayscale version of the memory and people photographs are supplied on the CD-ROM. Use these versions only if your workflow does not incorporate ICC color management and you require CMYK or grayscale test forms. Otherwise, use the RGB versions of the test forms and the ICC profiles that are part of your color-managed workflow. Isis Workflow Test Forms Suite User Guide Test forms overview 5

12 Benefits of using test forms Test form criteria For the creative professional: The Isis workflow test forms suite offers the ability to confirm that reproduction of images are representative of their origin. The test forms provide an indication that an image looks wrong, and image-dependent color correction is needed or CMS adjustment is required. You can test for the appearance of color without having to learn anything new or making any changes to the way you work. The software you currently use is all that is required. There is no need to purchase any additional products. It is an effective test when color is critical. For the prepress professional: The Isis workflow test forms suite provides assurance of image quality of your separation, OPI, trapping and imposition software. It provides general communication of problems and correction to others. For the print professional: The Isis workflow test forms suite delivers assurance of high quality color at little additional cost. The test suite helps you track down problem points in your workflow. Caution: Print jobs do not provide an analysis of the system s state of quality Isis workflow test forms are needed for assurance of the quality and consistency of your system. Each test form meets the following criteria: The form exceeds the output device s capability. This means that the test form s failure indicates the print engine s parameters. The form provides a comparison to a test up-stream in the workflow or to a prior testing session. The form is clear in its purpose. No single test form tests all color reproduction aspects. The form tests appropriate device limitations. For example, a test for offset press slur is an inappropriate monitor test. There are no optical illusions. The colors are observed objectively. The form is without poor image quality or noise that may be confused with a device characteristic. For example, if the test photograph has pixel noise, then the test may indicate a halftone problem that does not really exist. 6 Test forms overview Isis Workflow Test Forms Suite User Guide

13 Advantages of the GATF standardized test form option The GATF standardized test form has all the advantages of industry standards. Standard test forms are consensus agreements containing precise criteria, consistent rules and definitions. They provide consistent results, and provide you with assurance that Isis Imaging products and services meet a recognized level of quality. If an image quality problem lies with prepress and image preparation, the test form will test the point in the workflow where the problem resides. Frequently the image quality problem occurs before the image reaches the printer output. Other advantages of the GATF standardized test form are: It provides consistent comparison in time and across distance. It allows standard error correction and image quality improvement procedures. It provides a common understanding and facilitation among all users. It is currently the only method to check digital printer systems. Advantages of the Isis Imaging test forms The advantages of the four Isis Imaging test forms (analytic, grayscale comparison, nature scene photograph, people photographs) are: They assure accurate calibration. They check the use of proper characterization. They monitor consistency over time. They display consistency between components and printers. They confirm the accuracy of color-space conversion. They test the entire workflow from digital input to print. Isis Workflow Test Forms Suite User Guide Test forms overview 7

14 Isis Workflow Test Forms Suite User Guide

15 Color management system Your color management system (CMS) is your collection of hardware and software tools designed to control the color capabilities of your scanners, monitors, proofers and digital printers. Your software tools include software programs like Adobe InDesign, Adobe Photoshop, Icefields and profile making software. Your CMS may be a simple CMYK throughput or include complex device-linked ICC profile color transformations. The basic premise of testing is that color is always dependent on a specific device s input or output. Your CMS prepares the image s color for the requirements of each device. You cannot test an image defined in a device independent space since it is not characterized for any particular device. The Isis workflow test forms provide you with assurance that your CMS is at its optimal state. Incorporating the Isis Imaging workflow test forms suite in your CMS Here is an example of the usefulness of the Isis workflow test forms suite. You have calibrated and characterized every device in your workflow scanner, document creation monitor, prepress monitor and digital printer. You are ready to publish your next print job. Now it is time for a final test before you print the job. You view the nature scene photograph on the monitor. It looks good. You print a hard proof and compare it to the monitor. The hard proof compares favorably to the monitor. Lastly, you print the test on your digital press, and compare that print to the monitor and the hard proof. You notice that there is a slight color-cast difference between the printed sample and your hard proof. You check your RIP s ICC profile and find the wrong profile is selected. A quick change in profile selection, another test print to verify the correction, and you are confident that your customer will accept the printed job. That s the value of the Isis workflow test forms suite achieving assurance that a print job s color reproduction is acceptable to your customer. Isis Workflow Test Forms Suite User Guide Color management system 9

16 Isis Workflow Test Forms Suite User Guide

17 Workflow Workflow order This chapter provides an overview of workflow configuration classifications. You probably use one of the configurations described. The points in the workflow where you test will vary from configuration to configuration. The order you process images determines your testing points and also influences the quality of your workflow. Like an assembly line, print jobs are assembled in a certain order. You can use the Isis workflow test forms in any order testing from origination to output or from prepress to origination. You can also use the tests to evaluate and determine the order of job construction and printing. Step-by-step procedures are often described as a pipeline through which your images flow. Some image-pipeline configurations result in higher quality than others. For example, Photoshop s rotation, sharpening, blurring, and scaling features can enlarge a CMYK gamut beyond the capabilities of the output device, therefore you should use Photoshop in RGB color space before the image is transformed to the CMYK device-space. Keep the number of color transformations to a minimum and your color gamuts boundary shapes and sizes as close as possible. Always decrease the size of the gamut as the image moves through your pipeline. Never back up through the pipeline (see figure 6). Keep your pipeline as simple as possible. Don t change to a new device space until absolutely necessary. The fewer the number of image transformations and the closer the various color space gamut boundaries are in size and shape, the higher the image quality in your pipeline. The image s gamut should be largest at the beginning of the pipeline and smallest at the end. Fortunately, your scanner produces images with a larger gamut than your monitor s gamut, which is larger than your printer s gamut. Figure 6, Backing up through the workflow does not reverse the color gamut compression Workflow color-space configurations This section describes four basic configuration categories. The simplest configuration does not use ICC profiles while highly complex configurations use profiles to manage color transformations at every point in the print job assembly line scanner, monitor, repository, OPI, imposition, proofing and printing. Isis Workflow Test Forms Suite User Guide Workflow 11

18 The following symbols are used in figures 8 through 13. Scanner Soft proof Color transform Printed output Comparison Result of test Saved document CMYK to CMYK RGB to CMYK CMYK to RGB RGB to RGB CMYK reprographics Figure 7, Workflow diagram symbols (red circled symbols indicate major transformation points.) Images in CMYK device-space are generated at the scanner or at the prepress station. The RIP is not used to prepare images for the digital press. No ICC profiles are used (see figure 8). Prepress Figure 8, Typical CMYK reprographics workflow (See figure 7 for diagram symbols.) 12 Workflow Isis Workflow Test Forms Suite User Guide

19 Prepress RGB to CMYK at prepress Images originate in RGB color space and are transformed into CMYK separations using Photoshop or an Adobe Acrobat plug-in at the prepress station with color look-up tables or ICC profiles (see figure 9). Figure 9, Typical RGB to CMYK at prepress workflow (See figure 7 for diagram symbols.) RGB to CMYK at the RIP Images originate in RGB colorspace and are changed into CMYK separations at the RIP using ICC profiles or PostScript Color Rendering Dictionaries (CRDs) (see figure 10). Prepress Figure 10, Typical RGB to CMYK at the RIP workflow (See figure 7 for diagram symbols.) Isis Workflow Test Forms Suite User Guide Workflow 13

20 Mixed RGB and CMYK to CMYK at the RIP Source RGB images and CMYK separated images are mixed in the document. This is a common situation. Logos, line art and fonts may be in CMYK device color space and photos in RGB device color-space. Usually the final transformation to device CMYK color space is performed for images defined in both RGB and CMYK color spaces (see figure 11). Prepress Figure 11, Mixed RGB and CMYK to CMYK at the RIP workflow (See figure 7 for diagram symbols.) 14 Workflow Isis Workflow Test Forms Suite User Guide

21 Workflow complexity Workflows vary in color management complexity. Simple RGB to CMYK workflow A simple reprographics workflow requires color space transformations occurring at each device (see figure 12). It consists of three hardware components: scanner monitor (where color correction, page layout and prepress are performed) digital printer A more complex workflow adds a hard proofer to the configuration. Prepress Figure 12, Typical simple RGB to CMYK workflow (See figure 7 for diagram symbols.) Isis Workflow Test Forms Suite User Guide Workflow 15

22 Complex workflows Very complex workflows include many input and output devices. Complex workflows benefit most by the Isis Imaging workflow test forms suite. The biggest challenge met by test forms is evaluation of consistent color from device to device. Mixed color-space workflow A mixed color-space workflow may include RGB, CIE L*a*b* and CMYK images (see figure 13). At the printer s controller, or RIP, the image is transformed to CMYK device space. Color transformations are complex, and you should test each transformation. The color space transformations are RGB and CMYK to CIE L*a*b* to production printer CMYK and then to proofer CMYK. The original CMYK image will not match the same image defined by the CIE L*a*b* to CMYK transformation. This color pipeline is complex because the output profile is used at the controller and an identical profile is used at the proofer. If you change the output profile, then you must also select the same new profile at the proofer. The test form s color-cast will warn you if you have used different profiles. If the original CMYK image is to match the printed image, then that image must undergo the same color space transformations CMYK source space to CMYK production printer color space to CMYK proofer space. Prepress Figure 13, Typical mixed color space workflow (See figure 7 for diagram symbols.) 16 Workflow Isis Workflow Test Forms Suite User Guidee

23 Image quality testing overview This chapter introduces you to the practice of determining image quality using your eye s capability of comparing various image properties. It describes how you can evaluate the image quality of your workflow and maintain that consistent level of quality. Testing for image quality reproduction problems is similar to finding out why your electrical outlet does not work. An electrician checks the outlet, the circuit, other outlets on the circuit and the circuit-breaker box to find out where the problem occurs in the electrical flow. The problem may not be at the outlet. Likewise an image quality problem can occur throughout the image workflow pipeline, not just at your digital printer. In the future there may be a software agent that can test every point in your workflow pipeline, evaluate the standard test image, and automatically correct any problem. However, the large number of configurations and hardware components makes this option very difficult to develop. Currently, only you can be the testing agent, and only you can provide the detailed and consistent testing that your workflow requires. The four C s of workflow quality calibration, characterization, consistency and color conversion alone do not ensure image quality. A well-controlled CMS can still produce unexpected color reproduction problems. The Isis workflow test forms suite used consistently at every image processing point provides you with assurance that your printed products are of high quality. You do not require formulas or measurements of spectral and tristimulus data to use the test forms and images in the suite. Test images must fail to reproduce perfectly Good test images always fail at reproducing perfectly. If they did not fail, they would miss exposing an important image quality problem. If the test form s gamut is smaller than the digital printer s gamut then the test would not engage the full potential of your printer. Test images test the limits of a printer s reproduction capability. They are not intended to compare the quality of various printing presses unless those digital presses are calibrated, characterized and color managed. Image quality evaluation Color image encoding uses a relatively small range of numbers. (A monitor requires three columns of 256 numbers [8-bit] per pixel. Each pixel is composed of three primary colors and each color is 8-bits in depth. See figure 14.) The data is displayed as light-emitting tristimulus pixels on a monitor or light-reflecting four-color rosettes on paper. The numbers are displayed as wavelengths, and the human eye interprets the wavelengths as colors. Isis Workflow Test Forms Suite User Guide Image quality testing overview 17

24 Figure 14 Three, 8-bit data planes describe the monitor s pixels It is the wavelength to color interpretation that makes assessing color quality difficult and makes the human visual system the perfect tool for assessing quality and the constancy of quality. Image quality is a comparison evaluation. An image is never judged in isolation. If an image is evaluated on its own, then color quality is based on a comparison of the observer s memory of the image or a similar image. The first time you view a test image, your only comparison is to your memory. (Three of the test photographs contain memory colors.) Color memory is not colorimetically accurate. Your color preference and your interest in the content of the test contributes to the perception of color accuracy. When you view synthetic test images in isolation, such as the IT8 7/3 target, (see figure 15), in the GATF test form, or the Isis grayscale test form, they are quality controlled internally. That is, you view each color patch in association with other patches. The arrangements of patches are as important as the spectral definitions of the patches themselves. Figure 15, IT8 target provided with the GATF test form All grayscale images are compared either to other renderings of the same grayscale image or internally. There are no naturally occurring grayscale source images. Memory colors will not serve as a comparison, and memory luminance levels do not exist. Internal evaluations of a grayscale image include a black point at the darkest area, a white point at the lightest area, and smooth gradient values between the darkest and the lightest points. A 50% gray is the gray level in the middle of a sweep. Other image quality determinates include high spatial-frequency detail and lack of noise in flat tonal areas. 18 Image quality testing overview Isis Workflow Test Forms Suite User Guide

25 The human visual system This section presents background information about how your eyes see printed images and how you can use the mental processing characteristics of your eyesight to assess color acceptability. Your eyesight is a critical measuring instrument. By observing, evaluating and recording, you use your human visual system (HVS) to quantify quality output. This is achieved by determining whether you see color differences between two samples, and by evaluating the significance of the perceptual differences. Your eyesight is an excellent color appearance evaluation tool. Measuring color using your HVS Properties of the HVS If you can measure color then you can control it is the guideline for testing the four C s of workflow quality calibration, characterization, consistency and color conversion. A spectrophotometer is the appropriate device for measuring wavelengths within the visual spectrum, which is between approximately 380 nm and 740 nm. Unfortunately each spectrophotometer varies in accuracy, and each output device displays colors with varying spectral power distributions. The Isis workflow test suite allows you to determine if your well-calibrated system provides excellent color appearance. Many halftone and rendering algorithms make implicit use of the properties of the HVS. Without the HVS we would never see an image, only an array of colored pixels or a pattern of colorant dots. We see images on a monitor and on paper because of the following properties of the HVS: Low-pass filter function. This function improves perception of images by blurring halftone dots and CRT pixels. The amount of blur is based on viewing distance, angle and resolution. HVS low-pass properties are non-uniform in all angle directions (see figure 16). HVS is least sensitive to a dot pattern angled in 45 diagonal directions and most sensitive to vertical and horizontal directions. (See figure 16.) Note that it is the filter function that blurs, not your lens optics. You still need good eyesight. Figure 16, HVS angular resolve. Darker tones indicate angles where lines are easy to see. Isis Workflow Test Forms Suite User Guide Image quality testing overview 19

26 High spatial-frequency band-pass function. The low-pass function does not flatten contrast. HVS is sensitive to contrast, edge detection and linear acuity. The higher the contrast and the greater the number of edges, the greater the required resolution (see figure 17). HVS is more sensitive to noise in the luminance channel than in the chromatic channels. (Noise here is defined as low spatial frequencies within the highcontrast sensitivity function.) Band-pass functionality allows the eye to view greater amounts of luminous levels in low bandwidths. Figure 17, HVS resolution resolve. Darker tones indicate where lines are easy to see. Contrast sensitivity function (CSF). CSF is a curve representing our sensitivity to spatial modulations in chroma and luminance contrast. The CSF sensitivity to chromatic spatial variations falls off faster as spatial frequencies increase than does the response to spatial variations in luminance. That is, our HVS is more sensitive to changes in gray levels than to changes in chroma levels (see figure 18). Our sensitivity to chroma varies across the spectrum. We are more sensitive to differences in blue hues than in yellow hues. Figure 18, HVS chroma resolve. Darker tones indicate where contrast is easy to see. Global nonlinear processing. The perception of an image depends not only on nearby luminance and color values, but on a global consideration of the image and its geometry. In the English language, words have various meanings and context is indicative of meaning. Within an image, the appearance of a colorant is dependent on its context or colorants surrounding that colorant. 20 Image quality testing overview Isis Workflow Test Forms Suite User Guide

27 Color vision is a two-stage process. The first stage is a point-wise sensing stage, and the second is a comparison of signals from the neighborhood to infer the individual color. Color perception is based on the distance between areas, the size of the areas, and the color of the surrounding area. Therefore the HVS uses both area and quanta information. Measuring color by visual comparison The eye measures color and grayscale by comparison. Unlike the ear s capability to recognize a musical note precisely, viewing colors and grays is contextual. You cannot select a 50% gray patch from a random arrangement of gray patches. However, if you place the gray patches in increasing density order, then you will select the middle gray patch as 50% gray even though the actual value may not be 50%. Our HVS adapts to various illuminations and to various gamuts. A densitometer may measure as gray a tone that appears to be black to the eye. Our HVS adapts the gray to appear black. This visual phenomenon of precise comparison, inexact accuracy of color in isolation, and color adaptation forms the basis for using the Isis Imaging workflow test forms. Seeing a black tone produced on an output device and comparing it to a black tone of lesser density on another device relies on two elements of the HVS: Constancy. Visual adaptation of the appearance of color remains constant when the level of illumination changes. The paintings on your wall look the same at night or in the daylight. Adaptation. Brightness adaptation of the gray range results in the lightest tone to appear white and the darkest tone to appear black regardless of the actual gray values at the image s black and white points. Chromatic adaptation helps you interpret colorants when the illuminant changes the actual colorimetric response. Adaptation is image dependent. Not all images have white or black points, and the eye does not force all images to appear to have white and black points. Adaptation is obvious at dusk when the low light levels no longer reflect saturated colors, yet we still see those colors. Adaptation is also obvious when you exit a dark house into the sunlight. The daylight levels jump in lightness, but your eyes quickly compensate to provide you with the same range of light levels. Adaptation is the HVS s capability to see colors from the darkest value to the lightest value in various ranges of illumination. We are sensitive to a wide range of light (1,000 to one) and yet can only process a narrow range (64 to 1). As a result, we compress the available light levels. The range of reflected ink densities from black to white (256 levels) is greater than our light processing range. We adapt to the printed range. Isis Workflow Test Forms Suite User Guide Image quality testing overview 21

28 Color matching objectives Color preferences The majority of media and device color matches are conditionally matched. The type of illumination, the surrounding colors and the optical illusions within the photograph affect the appearance of similar colors. Your test objective is to determine if there is an image match. If the outputs do not match, then determine if the appearance miss-match is of consequence and in need of correction. The appearance of an image reproduced by two devices may contain colors of various spectral differences yet their closeness of appearance will be acceptable. Observers sometimes prefer somewhat more saturated images, even when they clearly perceive them to be less natural probably the result of social conditioning (see figure 19). Complementary hues produce greater emotional impact and attract attention likely a genetic or hereditary response to coloration. When you observe a test image, compare the saturated areas. Sometimes it is beneficial if the areas do not match but the printed result is more saturated and of greater contrast. Figure 19, Nature scene in unsaturated, normal and saturated versions Measuring output device characteristics using HVS and instruments While there is no doubt that print quality metrics affect image reproduction, the results are not always significant. Print quality metrics may be global measurements and are image dependent. The global metrics of a printer with a small reproduction range can still reproduce high quality images if the density range of the printer is equal to the density range of the image (see figure 20). For example, if a photograph consists of only midtone pixels, then the range between the darkest and lightest pixels of that photograph lies within the range of the printer. In this case, mapping pixel densities to existing toner densities is a more appropriate image quality strategy. A spectrophotometer s measurement of the gamut boundary of an output device is not a predictor of the resulting quality of the reproduction of any particular image. 22 Image quality testing overview Isis Workflow Test Forms Suite User Guide

29 Figure 20, People photograph with short grayscale range and its histogram The number of available density levels within the gamut is not always an indicator of a particular picture s quality. A highly detailed image with contrast sweeping from white to black in 64 pixels or less requires only 64 levels of gray, not 256 levels (see figures 21 and 22). Therefore the higher the spatial frequency (amount of detail) of the photograph, the lower the number of density levels required to accurately reproduce the photograph. The Isis workflow test forms suite consists of images that demonstrate your printer can reproduce highly detailed images regardless of its gray range. Figure 21, People photograph with only 64 levels of gray and its histogram Resolution increases upward Detail decreases left to right Figure 22, The number of gray levels needed decreases with an increase in detail Measuring grayscale appearance on color devices The HVS uses two wavelength input systems: one for viewing luminosity or grays and another for viewing chroma. Your eye s luminosity reception system has greater sensitivity to contrast and high spatial-frequencies (detail) Isis Workflow Test Forms Suite User Guide Image quality testing overview 23

30 HVS failures and greater adaptability to various illuminations than your chroma reception system. Getting the grayscale right is basic to getting the color right. The Isis workflow test forms provide visual grayscale tests to assess the appearance of grayscale range accuracy. Test for accurate grayscale appearance before you test for chroma appearance. There may be times when you must communicate a spectral or colorimetric value of a colorant or the precise measurement of the difference between two colorants. Your eyesight compares two colors accurately, but it is difficult to communicate visual comparisons accurately. In such cases, use the spectrophotometer supplied with your printer or RIP. The HVS incorporates many aspects of visual processing, including light adaptation, luminance compression, chromatic channel bandwidth processing, spatial filtering, and contrast sensitivity. Use your trained HVS to evaluate the image quality of your workflow and predict the quality of your printed product. Experience and familiarity with the Isis workflow test forms will help you become an expert at solving color problems. Lighting conditions and metamerism While difficult to achieve, it is important to have consistent viewing conditions in order to achieve accurate and consistent evaluations. When two colorants match under fluorescent light, but not under D50 light, then the spectral power distribution of each source light differs and the effect is a metameric match. You will observe a close match of the appearance of two colors only in one lighting condition. Therefore, all your observations should occur under the same light. Two colors that are different can appear to be the visual equivalent under different lighting conditions. A printed color and a monitor color may not have a different spectral power distribution curve yet appear to match. This is a metameric match. A spectrophotometer will describe the colorants as different. Color appearance matching is better accomplished through your HVS with one constant source of illumination. Lighting conditions and surround The color that surrounds a test image can contribute to its global color-cast as demonstrated in figure 23. Always view a test image with a neutral gray border. Place the printed image on a large gray board to provide an adequate surround if you do not have a viewing booth. Figure 23, The surrounding blue color changes the blue square to gray 24 Image quality testing overview Isis Workflow Test Forms Suite User Guide

31 Lighting and adaptation Just looking at your monitor or printed page for a length of time can improve the accuracy of the image. Your HVS adapts its color interpretation of the wavelengths to agree more closely with the expected colors. Look away from the test image to a uniform area frequently. Lighting adaptation allows you to see colors the way they should look not the objective colors defined by their spectral frequencies. For example, at night under street lighting, car colors remain accurate to the pigmentation even though the spectral profile doesn t include the actual hues. Adaptation means that with training many colors can be compared accurately under many types of lights. Which colors cannot be viewed accurately? The colors for which you have no preconceived experience. Hence analytic test forms are accurately compared only under D50, other standard illuminations and cloudy daylight. Natural scenes may be accurately compared under many types of lights. Isis Workflow Test Forms Suite User Guide Image quality testing overview 25

32 Isis Workflow Test Forms Suite User Guide

33 Workflow testing procedures This chapter describes basic workflow testing procedures. It discusses evaluation considerations, how to use each test form in the Isis Imaging workflow test forms suite, and how to maintain an ongoing record of your test results. See Appendix D for a list of commonly tested image properties, and Appendix E for a list of subjective factors to test. Recommended viewing conditions The following illumination and surroundings are recommended to best achieve accurate and consistent results: Illumination: both D50 and typical lighting conditions Surround: 50% gray Test image preparation All test images have one of the following embedded profiles: Adobe 98 RGB SWOP CMYK 1.8 gamma grayscale Basic testing procedure The basic testing procedure consists of the following steps: 1. Print or display the test. 2. View and evaluate the test image. 3. Add your observations to the test history document. 4. Perform the correction. 5. Add the correction to the history document. Test image evaluation considerations This section describes how to observe basic image quality characteristics of your output devices. Common HVS observation failures Test forms provide accurate information, but assessing that information and correcting a problem is the result of experience. Isis Workflow Test Forms Suite User Guide Workflow testing procedures 27

34 It is not possible to list all probable causes of any specific observation failure. The most common observation failures include: Generation failure. Does today s test compare well with the last time you printed the test? You may need to compare the history of all the previous tests. Your memory will not provide adequate information. Test selection failure. Each portion of each test provides information about a certain reproduction characteristic. Some tests are not appropriate for the output device under examination. You should view only the pertinent test areas. Viewer vision and memory failure. Some observers have a rod or cone deficiency. Others have memories that present colors as brighter and more saturated than they are in reality. Color acuity and color memory vary with experience and from person to person. Environmental failure. The lighting in your viewing room may change during the day creating metameric viewing problems. Maintain consistent viewing surroundings. Temporal failure. Over time colorants may fade and change the ratio of primary colors. Frequently yellow is the first colorant to fade causing color changes. Do not trust comparisons to tests that are exposed to light and humidity. Test image characteristics should not be altered All test images have inherent characteristics that you must not change. These characteristics include: Contrast. Contrast is image dependent, and the amount of contrast is subjective. Do not change the contrast of the test images. Exposure or lightness. The correctness of the neutral values of an image is a subjective determination. Do not darken or lighten the test images. Skin tone. It is difficult to determine the accuracy of skin pigmentation without a source. This is an example of colors which are dependent on the image content. The eye has no frame of reference to determine the accuracy of brown and tan colors when it views a color patch in isolation. However when those tan colors determine the skin tones of a pictured person, then determination is accurate. (See figures 35 through 38.) Do not adjust the skin tonal values. Color to gray transformation. Transformation from a color space to a neutral axis is dependent on the accuracy of the look-up table or ICC profile. Do not further adjust the test image after completing a color to grayscale transformation. Caution: Altering the test image characteristics will degrade its usefulness. 28 Workflow testing procedures Isis Workflow Test Forms Suite User Guide

35 Sweeps Sweeps, or gradients, display the output device s density range and number of levels. Sweeps are important because they comprise the areas of a photograph without contrast and detail. They are the light to dark transitions on smooth objects. Number of density levels. The maximum number of density levels on most printers is 255. If some levels or bands of color seem thick, the device may not be capable of reproducing all 255 density levels of all primaries. Complete range from full saturation to white. The primary and secondary colors linearly change from white to maximum saturation. Workflow testing strategy Two strategies to achieve color consistency among your various tonerbased and offset printers are: 1. Lowest common color gamut. Select your printer with the smallest gamut. Aim your soft and hard proofs to that profile with CMYK-to-CMYK device linked profiles. Use the test forms to validate the closeness of the proofs. 2. Map from one gamut to another while maintaining the perception of a similar gamut. Calibrate and characterize each device for the largest gamut. Ensure that the gamut diminishes as your workflow assembly line moves images from creation to prepress to print. Your images should be described in an appropriate space for each point in the workflow. Find the point in the workflow where the image quality problem may occur. Print or view the test from that point in your workflow. If possible perform the same test again upstream. If you perform RGB to CMYK conversions in prepress, then convert and print the test image from the page assembly workstation. If you use ICC profiles, check to ensure the proper profile is selected. Begin testing at the printer stage and proceed upstream to your creative phase. All output devices have color quality limitations. More saturated shades of blue are available from a toner-based printer than an offset press. There are only two solutions for out-of-gamut matching problems: limit the larger gamut to the device with the smallest gamut, or color-manage the image so that the global color impression is similar. There are situations where you want to limit your printer s gamut to colors less saturated than its full capability. Most inkjet proofers have larger gamuts than offset presses. Use CMYK-to-CMYK linked profiles to compress the proofer s gamut to the offset gamut. Isis Workflow Test Forms Suite User Guide Workflow testing procedures 29