1 / 52

Lecture 17 Scanner Characterization and Calibration

Learn about the spectral-based characterization and calibration of different scanners, such as Samsung, HP Photosmart, and Epson Photo RX700. Explore color gamuts and compare scanner performance.

louisee
Download Presentation

Lecture 17 Scanner Characterization and Calibration

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Lecture 17Scanner Characterizationand Calibration - Sanjyot Gindi M.S.E.C.E, Purdue University July 18th 2008 Papers/theses covering these materials can be found in the references under “Capture”

  2. Objectives: • Spectral model based characterization of Samsung SCX5530 scanner. • Empirical or regression based characterization of Samsung, HP Photosmart and Epson Photo RX700 scanners. • Plotting of color gamuts and comparison of the 3 scanners based on them. Sanjyot Gindi

  3. Basic working of a flatbed scanner • ‘Target’ is placed on the glass top • A movable scanner-head consists of a lamp and sensors • Light from the lamp incident on the target is reflected back to the scan head. • This light passes through an optical assembly and is received by sensors. • RGB values of the color of target – ‘device-dependent’ color space. Paper to be scanned Glass-top Lamp Sanjyot Gindi

  4. There are also Sheetfed Scanners • text Visioneer HP LaserJet Pro 400 Color MFP Sanjyot Gindi

  5. Modeling of a scanner system • Understanding the color characteristics and consistently predicting the color values. • Two steps : • Calibration: Linearization or gray balancing • Characterization: Transformation from device dependent RGB values to co-ordinates in the device independent color space like CIE XYZ, L*a*b*. [1] Sanjyot Gindi

  6. Methods of characterization • Regression based method: • Considers the scanner system as a ‘black box’ • Mapping from linear RGB to CIE XYZ • Model based method: • Uses known spectral functions of the components of a scanner system. Sanjyot Gindi

  7. Rg Rl X NLR Gg Gl Transformation Matrix T Y NLG Bg Bl Z NLB 1. Regression based method • Rg,Gg , Bgare the values obtained from the scanner with gamma on, shading on. • Rl, Gl, Bl are the linearized values • X,Y,Z values are the CIE XYZ values obtained using the X-Rite Spectrophotometer with D65 illuminant. • NL represents the non-linear relationship between output Rg,Gg , Bg values and Rl, Gl, Bl values Sanjyot Gindi

  8. Calibration procedure: • ‘Gray balancing’ using Y (Luminance) values of the neutral gray patches on the Kodak Q60 target (see following slide). • Linear R,G,B values obtained using a power-law curve fit given by: , where Rl = Y. Sanjyot Gindi

  9. Kodak Q60 target Sanjyot Gindi

  10. Gray Balance curves a = 79.63 b = 2.272 c = 0.5905 X axis: R,G,B values from Scanner (0-255) Y axis: (Y) Luminance values from X-Rite (0-100) a = 84.14b = 2.198 c = 1.093 a = 81.58 b = 2.033 c = 1.344 Sanjyot Gindi

  11. Spectra-radiometers used for this study • text X-Rite DTP 70 Photo Research PR-705 Sanjyot Gindi

  12. Characterization: • Find scanner RGB values for Kodak Q60 target : 240 color patches • CIE XYZ values of color patches using X-Rite spectrophotometer for D65 illuminant condition. • Determine the 3x3 transformation matrix T given by: [ B ] = [ A ] [ T ] is found by least squares approximation where: 240x3 240x3 Sanjyot Gindi

  13. Lamp [L] R channel G channel B channel Target Kodak Q60 reflectance [R] Sensor [F] 2. Model Based Method Spectral Model of a Scanner Sanjyot Gindi

  14. Experiments: • The Lamp spectrum [L], is obtained using a spectroradiometer and a white diffusion target. • The Sensor spectral response [F], is obtained using the Monochromator (400nm to 700nm) • The Spectral Reflectance of the Patches [R], were obtained using the X-Rite spectrophotometer. Sanjyot Gindi

  15. Expt 1: Lamp output spectrum measurement. • The 99% diffuse reflectance target (Labsphere, N. Sutton, NH) was used to reflect the light from the lamp of the scanner ( Setting: ‘Document Feed’ mode) • Spectroradiometer PR-705 (Photo Research Inc.)-was used to measure the spectrum of the lamp by focusing the aperture (1/2 degree) on the white diffusion standard. The spectrum is obtained in the range of 380nm to 780nm. • The outputs of the PR-705: • Spectral Radiance (W/sr/sq.m) • X,Y (Luminance in cd/sq.m), Z ; L*,a*,b*,L,u,v, and chromaticity x,y. Sanjyot Gindi

  16. White diffusion standard clamp Light reflected from the diffuser Direction of scan head motion Spectroradiometer (PR-705) Scan-head Lamp Samsung Scanner Schematic of the experimental assembly for lamp response measurement Sanjyot Gindi

  17. Lamp Spectrum - Measured by Spectroradiometer. Wavelength in nm Lamp spectrum matrix = [ L ] , 31x31 diagonal matrix Sanjyot Gindi

  18. Expt. 2: Sensor Response. • The Monochromator SP-150 was used to input light with wavelengths in the range 400-700nm in steps of 10nm. • During the measurement, the lamp of the scanner was off. • Scanner setting: Gamma ‘on’ and shading ‘off’, 150dpi. • The Assembly used for measurement was as shown in the following figure: Sanjyot Gindi

  19. Fiber optic cable of the Monochromator Clamp to hold the optic cable glass Scanner head To CCD sensor Lamp (turned ‘off’ during measurement) Incident Light Mirror Lens Cross-section view of scan head and measurement assembly Sanjyot Gindi

  20. 1. 2. 3. • 5. The sample output images obtained are as shown below: 460nm 700nm 540nm 6. For the sensor response, the R,G,B values were averaged over the length of the page and 3-4 pixels across the breadth of the page. Sanjyot Gindi

  21. Wavelength in nm Responses of the three channels were taken separately with different intensity settings of Monochromator- 25 for red, 50 for green, 60 for blue. Sanjyot Gindi

  22. Intensity calibration of Monochromator: • The White diffusion standard was used to reflect the monochromator output and the spectroradiometer measured this reflectance. • The monochromator output measured by the spectroradiometer for all wavelengths from 400-700nm is plotted. • Based on the maximum output (found to be obtained at 690nm) the scaling factor is obtained for all wavelengths • The scaling factor at l= (Intensity at 690nm)/( intensity at l) • The output of each wavelength is multiplied with this scale factor to obtain the calibrated response. Sanjyot Gindi

  23. Output variation is from 4.3e-5 for 400nm to 1.2e-3 for 700nm (in units of Radiance)~ approximately 27 times Sanjyot Gindi

  24. Sensor Response curves • The RGB values are linearized • For each channel, the response < 1% of the maximum value is considered = 0. • The monochromator calibration is applied to each channel Sanjyot Gindi

  25. Expt 3: Measurement of Reflectance spectra of KodakQ60 color patches. • Using X-Rite DTP70 Spectrophotometer, 2 degree observer, D65 illuminant. • Using Spectroradiometer setup, 1/2 degree observer to measure the Kodak Q60 target illuminated by daylight setting using a Macbeth SpectraLight II viewing booth Sanjyot Gindi

  26. Kodak Q-60 target Sanjyot Gindi

  27. Measurement of Kodak Q60 patches by X-Rite and Spectroradiometer (Sample # 1) Wavelength in nm Wavelength in nm X-Rite, 2 deg Observer, D65 illuminant Spectroradiometer, ½ deg observer, Illuminant: ‘Daylight’ Sanjyot Gindi

  28. Wavelength in nm Wavelength in nm Sample # 2 X-Rite Spectroradiometer Sanjyot Gindi

  29. Sample # 3 Wavelength in nm Wavelength in nm X-Rite Spectroradiometer Sanjyot Gindi

  30. Lamp [L] R channel G channel B channel Target kodak Q60 reflectance [R] Sensor [F] Spectral Model of a Scanner : Sanjyot Gindi

  31. Based on the above model: Let [S] = [R] * [L] * [F], then [X] = [M] * [S] L = 31 x 31 diagonal lamp spectrum matrix. R = 240 x 31 matrix- reflectance spectrum of patches on Q60 Target. F = 31 x 3 sensor sensitivity function matrix. S = 3 x 240 matrix of scanner output data obtained from the model. M = 3 x 3 calibration matrix X = 3 x 240 matrix of CIE XYZ values of the same patches • Matrix M was obtained by simple least squares approximation as [M] = ([S] * [S] ) * [S] * [X] T -1 T Sanjyot Gindi

  32. Results: • Regression based method: • Mean Delta E = 3.82 • Max Delta E = 18.20 • histogram of number of patches with Delta E values. Sanjyot Gindi

  33. Model Based Method • Mean delta E = 4.205 • Max delta E =19.332 • Histogram: 115 patches with delta E < 3 Sanjyot Gindi

  34. Color Gamuts: • The regression method was used to characterize HP, Epson along with the Samsung scanner. • Transformation matrices used to plot 3D gamuts in L*a*b* spaces. • Plots of L* Slices of the 3D gamut • Chromaticity diagram • Metrics for comparison: • Gamut Volume • Quantization error for each channel This is based on inputting uniformly spaced samples from the RGB color cube (0 – 255)x(0 – 255)x(0 – 255) that comprises the output space of the scanner to the inverse model for the scanner. Sanjyot Gindi

  35. L* b* a* 3D Gamut plots Samsung Scanner HP scanner Sanjyot Gindi

  36. 3D gamut plot Epson Scanner Sanjyot Gindi

  37. Samsung HP Epson L* Slices in Gamut Horizontal axis: b* values Vertical axis: a* values L* = 30 L* =20 Sanjyot Gindi

  38. Samsung HP Epson L*=40, L*=50 Horizontal axis: b* values Vertical axis: a* values Sanjyot Gindi

  39. Samsung HP Epson L*=60, L*=70 Horizontal axis: b* values Vertical axis: a* values Sanjyot Gindi

  40. Samsung HP Epson L* = 80, L*= 90 Horizontal axis: b* values Vertical axis: a* values Sanjyot Gindi

  41. 0.9 Samsung HP 0.5 Epson 0.1 Adobe RGB 0.0 0.1 0.5 0.9 Chromaticity diagram How can sensor chromaticities lie outside the spectral locus? y chromaticity x chromaticity Sanjyot Gindi

  42. 0.9 Samsung HP 0.5 Epson 0.1 Adobe RGB 0.0 0.1 0.5 0.9 Chromaticity diagram How can sensor chromaticities lie outside the spectral locus? – Scanner is projecting onto a different 3-D subspace than the human visual subspace. It doesn’t see color the same way as a human being. y chromaticity x chromaticity Sanjyot Gindi

  43. P1(L*) – P0(L*) P1(a*) – P0(a*) P1(b*) – P0(b*) P2(L*) – P0(L*) P2(a*) – P0(a*) P2(b*) – P0(b*) P3(L*) – P0(L*) P3(a*) – P0(a*) P3(b*) – P0(b*) Volume = 1/6 * Gamut Volume • Gamut volume metric from paper by Braun and Spaulding [3]. • Divide the RGB color Lattice into tetrahedrons—Tetrahedral Tesselation • Compute the L*a*b* values of the vertices • Calculate the volume of each tetrahedron in L*a*b* space in cubic DE units given by: • Where P1, P2, P3 and P0 represent the vertices of a tetrahedron and ‘| |’ denotes the determinant. Sanjyot Gindi

  44. Gamut Volumes (in cubic DE units): • Number of tetrahedrons used: 17,576 • The gamut volumes calculated are as follows: • Samsung: 2,012,700 units • HP: 819,700 units. • Epson Expression: 780,370 • sRGB color space: 811,180 • Adobe RGB color space: 1,186,315 Sanjyot Gindi

  45. Quantization Error [3] • For the set of L*a*b* values of patches on the KodakQ60 target, the number of points within the gamut of each scanner was determined. • For each of these in-gamut points, the R value was incremented by one and the corresponding L*a*b* value was determined. • The average delta E error between the original value and the incremented value is the quantization error for R channel • Similarly repeated for G and B channels. • Thus quantization errors for each channels is determined in delta E units. Sanjyot Gindi

  46. Quantization Error in delta E units. Sanjyot Gindi

  47. Summary of the discussion… • The model based method was used to characterize Samsung SCX6320 scanner • The regression based method was used to characterize the Samsung, HP and Epson scanners • 3D gamuts for all 3 scanners were plotted • Slices of the 3D gamut in L* were plotted • Gamut volume and Quantization error calculation for all 3 scanners. Sanjyot Gindi

  48. References: • “Digital Color Imaging Handbook”, Gaurav Sharma. • “Color Science Concepts and Methods Quantitative Data and Formulae”, byWyszecki and Stiles • ”Method for Evaluating the Color Gamut and Quantization Characteristics of Output-Referred Extended-Gamut Color Encodings”, Gustav Braun and Kevin Spaulding, Tenth Color Imaging Conference: Color Science and Engineering Systems, ISBN / ISSN: 0-89208-241-0 • ECE638, ”Principles of digital color Imaging systems”, notes by Prof. J. Allebach, Purdue University. • ECE 637, “Image Processing”, notes by Prof. C. Bouman, Purdue University. • "A Review of Linear Color Descriptor Spaces and Their Applications," M. Wolski • "Imaging Colorimetry Using a Digital Camera," W. Wu et al • Non-Contact Imaging Colorimeter for Human Tooth Color Assessment Using A Digital Camera," D. Ng et al Sanjyot Gindi

  49. Additional slides…

  50. Red: transformed values Black: actual values Sanjyot Gindi

More Related