3 ImageQualityforScanningand Digital Imaging Systems Donald R. Lehmbeck Xerox Corporation, Webster, New York, USA (retired) CollegeofImagingArtsandSciences,RochesterInstituteof Technology, Rochester, New York, USA (Adjunct Faculty) ImagingQualityTechnologyConsulting,Penfield,NewYork,USA TorreyPinesResearch,Fairport,NewYork,USA John C. Urbach Portola Valley, California, USA (deceased) CONTENTS 3.1 Introduction ........................................................................................................................ 135 3.1.1 Imaging Science for Scanned Imaging Systems ................................................ 135 3.1.1.1 Scope ......................................................................................................... 135 3.1.1.2 The Literature .......................................................................................... 136 3.1.1.3 Types of Scanners.................................................................................... 136 3.1.2 The Context for Scanned Image Quality Evaluation ........................................137 3.2 Basic Concepts and Effects ............................................................................................... 140 3.2.1 Fundamental Principles of Digital Imaging ...................................................... 140 3.2.1.1 Structure of Digital Images ................................................................... 140 3.2.1.2 The Sampling Theorem and Spatial Relationships ............................145 3.2.1.3 Gray Level Quantization: Some Limiting Effects .............................. 148 3.2.2 Basic System Effects ............................................................................................... 152 3.2.2.1 Blur ............................................................................................................ 152 3.2.2.2 System Response ..................................................................................... 153 3.2.2.3 Halftone System Response .................................................................... 155 3.2.2.4 Noise ......................................................................................................... 159 3.2.2.5 Color Imaging .......................................................................................... 160 3.3 Practical Considerations ................................................................................................. .. 166 3.3.1 Scan Frequency Effects ......................................................................................... 166 3.3.2 Placement Errors or Motion Defects ................................................................... 169 3.3.3 Other Nonuniformities ......................................................................................... 173 3.3.3.1 Perception of Periodic Nonuniformities in Color Separation Images ....................................................................................................... 173 3.4 Characterization of Input Scanners that Generate Multilevel Gray Signals (Including Digital Cameras)............................................................................................. 174 3.4.1 Tone Reproduction and Large Area Systems Response................................... 175 3.4.2 MTF and Related Blur Metrics ............................................................................. 181 3.4.2.1 MTF Approaches..................................................................................... 183 133 134 Handbook of Optical and Laser Scanning 3.4.2.2 The Human Visual System’s Spatial Frequency Response ............... 189 3.4.2.3 Electronic Enhancement of MTFs: Sharpness Improvement ............ 189 3.4.3 Noise Metrics .......................................................................................................... 190 3.5 Evaluating binary, thresholded, scanned imaging systems........................................ 193 3.5.1 Importance of Evaluating Binary Scanning....................................................... 193 3.5.1.1 Angled Lines and Line Arrays .............................................................. 193 3.5.2 General Principles of Threshold Imaging Tone Reproduction and Use of Gray Wedges....................................................................................... 194 3.5.2.1 Underlying Characteristic Curve and Noise ....................................... 194 3.5.3 Binary Imaging Metrics Relating to MTF and Blur.......................................... 195 3.5.3.1 Resolving Power (A Measure for Discrimination of Fine Detail) ....195 3.5.3.2 Line Imaging Interactions ...................................................................... 198 3.5.4 Binary Metrics Relating to Noise Characteristics ............................................. 198 3.5.4.1 Gray Wedge Noise .................................................................................. 198 3.5.4.2 Line Edge Noise Range Metric.............................................................. 199 3.5.4.3 Noise in Halftoned or Screened Digital Images.................................200 3.6 Summary Measures of Imaging Performance............................................................... 202 3.6.1 Basic Signal-to-Noise Ratio................................................................................... 202 3.6.2 Detective Quantum Efficiency and Noise Equivalent Quanta .......................204 3.6.3 Application-Specific Context ................................................................................204 3.6.4 Modulation Requirement Measures ...................................................................204 3.6.5 Area under the MTF Cure (MTFA) and Square Root Integral (SQRI) ...........205 3.6.6 Measures of Subjective Quality ........................................................................... 206 3.6.7 Information Content and Information Capacity ...............................................209 3.7 Specialized Image Processing.......................................................................................... 214 3.7.1 Lossy Compression ................................................................................................ 214 3.7.2 Nonlinear Enhancement and Restoration of Digital Images .......................... 216 3.7.3 Color Management ................................................................................................ 218 3.8 Psychometric Measurement Methods Used to Evaluate Image Quality ...................219 3.8.1 Relationships between Psychophysics, Customer Research, and Psychometric Scaling............................................................................................. 219 3.8.2 Psychometric Methods .......................................................................................... 220 3.8.3 Scaling Techniques ................................................................................................ 221 3.8.3.1 Identification (Nominal) .........................................................................222 3.8.3.2 Rank Order (Ordinal) .............................................................................222 3.8.3.3 Category (Nominal, Ordinal, Interval)................................................222 3.8.3.4 Graphical Rating (Interval)....................................................................222 3.8.3.5 Paired Comparison (Ordinal, Interval, Ratio).....................................222 3.8.3.6 Partition Scaling (Interval).....................................................................223 3.8.3.7 Magnitude Estimation (Interval, Ratio)...............................................223 3.8.3.8 Ratio Estimation (Ratio) .........................................................................223 3.8.3.9 Semantic Differential (Ordinal, Interval) ............................................223 3.8.3.10 Likert Method (Ordinal) ........................................................................223 3.8.3.11 Hybrids (Ordinal, Interval, Ratio) ........................................................ 224 3.8.4 Practical Experimental Matters Including Statistics......................................... 224 3.9 Reference Data and Charts............................................................................................... 226 Acknowledgments ...................................................................................................................... 237 References .................................................................................................................................... 238 Image Quality for Scanning and Digital Imaging Systems 135 3.1 INTRODUCTION 3.1.1 Imaging Science for Scanned Imaging Systems This chapter presents some of the basic concepts of image quality and their application to scanned imaging systems. In this revised edition, we have added more on tonal rendition including system plots and halftones, new approximations to MTF’s and revised pointers to the current industry standards in image quality, as well as more reference data and charts while reducing content on binary imaging and overall quality. New references and other technical details have been added throughout. The emphasis in this chapter will be on the input scanner. Output scanners will be dealt with mainly by inference since many input scanner considerations and metrics are directly applicable to the rest of a complete electronic scanned imaging system. Expanded discus­ sions of halftone methods, tone reproduction, and nonuniformity