Desktop Scanners

Robert G. Gann received his Ph.D. in Electrical Engineering in 1986 from Colorado State University. Since 1989, he has worked as a design engineer at Hewlett-Packard where he has been responsible for image quality specification and evaluation of HP ScanJet scanners. He has traveled worldwide, giving classes in testing scanners to audiences ranging from novice users to image scientists. Previous editions of his highly-successful Reviewing and Testing Desktop Scanners provide a strong basis for this new guidebook.

List of Figures.


List of Tables.


1. What's Desktop Scanners: Image Quality Evaluation All About?


Answers to Your Questions. For the first-time scanner buyer and user:. For the web user:. For the experienced user:. For technical reviewers or those responsible for recommending scanners:. For engineers and scientists. How to Use the Book. What's New in Desktop Scanners: Image Quality Evaluation. A Word from the Author. About the Author.



2. A Guide to the Book.


For the Scanner Buyer. For the Reviewer. For the User. For the Engineer or Scientist. A Guide to Each Chapter.



3. Buying a Scanner.


What You'll See When You Walk into the Store. What you will find in the store:. The two most common specifications-ppi and “bits”. Ignore ppi and Bit Depth Claims. A big hint. Technology Trade-Offs, You Can't Have It All. Light versus everything. A case study. Bit Depth Claims. Questions You Should Ask Yourself. Why am I buying a scanner? Aren't scanners too hard to use? How much should I expect to spend for a scanner? What about speed? Don't I need a fast scanner? Should I buy a digital camera or a scanner? I'm scanning for the web. What should I worry about? You keep saying I don't need high resolution, but does it hurt? Do I need a new printer for my scanner? If I'm going to buy a printer and a scanner, how about an all-in-one instead? How much memory and disk space do I need in my computer? Which Interface should I use to connect my computer to my scanner? What Accessories Do I Need? Claims to Watch Out For. 1200-ppi (or more) scanner. 36 bit (or more) scanner. Pretending that the ppi and Bit Depth Specifications Mean Something. ppi Used as a Measure of Resolution. What ppi would you need-assuming the claim were true? The reality of ppi claims. Reading ppi specifications. Bit Depth, Loosely Related to the Number of Colors a Scanner Can Recognize. What bit depth do you need-assuming it were true? It looks like 24 or 30 bits are enough! Reading Bit Depth Specifications.



4. For the Technical Reviewer.


The Challenge of Reviewing Scanners Today. Changes in Reviewing Scanners. Scanner users of today. Sophisticated controls may be unnecessary. Sophisticated results are required-automatically. Sophisticated controls can be useful. Steps to a Successful Review. Identify the type of review you are doing. Other factors to consider. An example test plan. Some Guidelines to Consider. Testing and Reviewing an All-in-One Scanner. Testing Digital Cameras.



5. A Look at Scanners.


What is a Scanner? Like a camera. Like a fax. Like a copier. Types of Scanners. Handheld scanners. Sheetfed scanners. Flatbed scanners. Film and photo scanners. All-in-one peripherals. Digital cameras. What Scanners Can Do. How Scanners Work. Bit depth and density claims. An Intro To Image Quality. A different view of image quality. Scanners and Printers. Using the right sampling rate. Dye-sub printers. Photo-quality inkjet printers. Halftoning printers.



6. Scanner Trends.


How Scanners Are Changing. Scanner Trends. The Perversion of Specifications. Scanning for the Internet. sRGB standard. Graphic file formats. Resolution and color.



7. Scanning Software.


Scanning Software. Changes in Scanning Software. Evaluating Scanning Hardware and Software. Proprietary vs. third-party scanning software. A Scanning Solution or Pieces of a Puzzle? A New Scanning Interface-Task Automation Software. A comparison to printer drivers. Ease of Use. Image Transforms. Page analysis. Image auto-type. Image auto-exposure and gamma compensation. Spot color. Raster-to-vector conversion. OCR. Challenges and limitations of OCR.



8. Scanner Gear.


Hardware Interfaces. SCSI. Parallel interface. USB (Universal Serial Bus). 1394 or Firewire. Accessories-ADFs and XPAs. Automatic document feeders. Transparency adapters. Requirements for overhead slides (foils). Comparing transparency adapters to dedicated slide and film scanners. Software in the box. Bundled Software. Service and Support. Warranty period and repair or exchange. 1-800 Or regular long-distance numbers. On-line support. Faxback services. OCR/Text Scanning. Documentation. Out-of-box experience. Attractive and efficient. On-line or paper or both?



9. Setting the Controls.


Overview. Overuse of Controls. Sampling Rate. Color. Automatic White Point. Exposure Controls. Tone maps. Gamma correction (compensation). Histograms. Highlight and shadow. Automatic exposure. Brightness and contrast. Emphasis. Other Controls. Sharpening. Preview scans. Trend in Controls.



10. More on Image Quality.


Scanning with Default Settings. Why default exposure settings are used when testing scanners. Why scanning with defaults can result in misleading tests. Why users sometimes scan with default settings. Why scanning with defaults is not optimal for users. Default or auto-exposure from a testing standpoint. Irreversible Transformations. Image Quality Requirements for Scanning Transparent Originals. Resolution requirements for 35-mm slides. Resolution requirements for larger transparent originals. Tonal requirements for transparent originals. Condition of the Object Being Scanned. What Do You Get for Your 10 (Or 12) Bits? Advantages. Tonal transformations in a scanner. Tone maps for 10-bit scanners. PRNU and DSNU. Dark signal non-uniformity (DSNU). Compensation for DSNU. PRNU. Digital compensation. Analog compensation. Misleading bit depth claims in digital compensation scanners. Advanced Image Processing. Sharpening or filtering. Gamma compensation and tone mapping. Interpolation or scaling. 3 x 3 Matrixing. Dual image processing. Calibration. Types of calibration. SRGB. Screen calibration. System calibration. Closed-loop calibration. Incident-Light Angle Changes and Multiple-Exposure Scans.



11. Some Scanner Technologies.


Optical Reduction or Contact Image Sensor Scanners. Optical reduction scanner. Contact image sensor scanner. Passes and Exposures. Three Primary Color Separation Technologies. Transmissive filter scanners. Light source separation scanners. Beam-splitting scanners. Transmissive Filter Scanners. Manual filter placement scanners. Automatic filter placement scanners. Three-exposure scanners with on-chip color filters. Light Source Separation Scanners. Three-pass, colored-light-source scanners. One-pass scanners with colored light sources. LED-illuminated scanners. Beam-splitting scanners. HP Trichromatic beam splitter scanner. Flying-Spot Drum Scanners. Slide or Film Scanners. Photo Scanners. All-in-one Products. Digital Cameras. Color separation in a digital camera. The light source for a digital camera. Color accuracy in a digital camera. Resolution in digital cameras. Confusing claims about the number of pixels captured in digital cameras.



12. Scanning Speed.


Task Speed Versus Scan Speed. Ease of use can be more important than raw scanning speed. Typical Tests. Recommended Test. Detailed test procedure.



13. Resolution.


What is Resolution? Resolution is not optical sampling rate. Resolution and Sampling Rate. You can't reliably evaluate the resolution of a scanner by looking at images. X- and y-direction optical sampling rates. Nyquist Frequency, Aliasing and Moir Patterns. Limitations of typical tests. Recommended Test. Detailed Step-by-Step Procedure. Removing the effect of gamma correction. What to do if clipping occurs. Signs of errors. Comparison to Another Common MTF Measurement. Simplified Step-By-Step Procedure.



14. Scaling and Interpolation.


Overview of Scaling and Interpolation. Jaggies. X-Direction Scaling. Y-Direction Scaling. Magnification or Scaling Accuracy. Positional or Stop-Start Errors. Limitations of Typical Tests. Recommended Tests. Detailed test procedure.



15. Tonal Resolution, Density Range and Bit Depth.


Tonal Resolution. Limits and Needs. Tonal Transformations. Limitations of the Typical Test. Recommended Tests. Test one: tonal resolution test. Test two: tonal transformation test (quantization). Test three: signal-to-noise ratio test.



16. Image Noise.


Impact of Noise on Bit Depth and Density Range. Correlated Noise. Non-Correlated Noise. Comparison of Correlated and Non-correlated Noise. Typical Tests. Recommended Test.



17. Uniform Illumination.


Types of Lamps. Intensity and Temperature. Blowout and Clipping. Design Trade-off. Light Sources. Xenon gas discharge. Hot cathode fluorescent. Cold cathode fluorescent. Limitations of Typical Test. Recommended Test. Detailed test procedure for uniform lighting.



18. Color Registration.


Overview. Typical Test. Recommended Test. Test 1: measurement of misregistration due to optical misalignment. Test 2: y-direction misalignment due to inconsistent carriage motion.



19. Color Fidelity.


What Determines the Color of an Object. The Object. The Detector. Light Source. CIELAB Color Model. Color Errors: Recoverable and Non-recoverable. Gray Balance. Limitations of Typical Tests. Recommended Tests. Test one: calibrated color target. Test two: gray balance.



20. Scanning for Black and White Output.


Scans of Black and White Photos. Colored Objects in Grayscale Mode. Grayscale Objects in Color Mode. Limitations of Typical Tests. Recommended Tests. Detailed Test Procedures. L* comparison test. NTSC Conversion. Photoshop comparison test. Appendix A: CIELAB to RGB Calculations and NTSC Equations. RGB-to-LAB Conversion Overview. Assumptions. RGB-to-LAB Conversion: Detailed Steps and Equations. Calculating color differences (CIELAB). Perception of color or lightness difference. Inverse calculations (LAB to RGB). NTSC Equations. Applied Image Test Target.



Glossary.


Index.