1. Lecture 16 Scanner Characterization and Calibration - Sanjyot Gindi M.S.E.C.E, Purdue University July 18th 2008

2. Masters Thesis Presentation, July 18th, 2008Sanjyot Gindi Slide 2 of 43 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.

3. Masters Thesis Presentation, July 18th, 2008Sanjyot Gindi Slide 3 of 43 Basic working of a flatbed scanner Lamp Glass-top Paper to be scanned ‘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.

4. There are also Sheetfed Scanners text Masters Thesis Presentation, July 18th, 2008Sanjyot Gindi Slide 4 of 43 HP LaserJet Pro 400 Color MFP Visioneer

5. Masters Thesis Presentation, July 18th, 2008Sanjyot Gindi Slide 5 of 43 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]

6. Masters Thesis Presentation, July 18th, 2008Sanjyot Gindi Slide 6 of 43 Methods of characterization 1. Regression based method:  Considers the scanner system as a ‘black box’  Mapping from linear RGB to CIE XYZ 2. Model based method:  Uses known spectral functions of the components of a scanner system.

7. Masters Thesis Presentation, July 18th, 2008Sanjyot Gindi Slide 7 of 43 1. Regression based method Transformation Matrix T NL R NL G NL B RR GG BB RlRl GlGl BlBl X Y Z R , G , B  are the values obtained from the scanner with gamma on, shading on. R l, G l, B l 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 R , G , B  values and R l, G l, B l values

8. Masters Thesis Presentation, July 18th, 2008Sanjyot Gindi Slide 8 of 43 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 R l = Y.

9. Kodak Q60 target Masters Thesis Presentation, July 18th, 2008Sanjyot Gindi Slide 9 of 43

10. Masters Thesis Presentation, July 18th, 2008Sanjyot Gindi Slide 10 of 43 Gray Balance curves X axis: R,G,B values from Scanner (0-255) Y axis: (Y) Luminance values from X-Rite (0-100) a = 79.63 b = 2.272 c = 0.5905 a = 81.58 b = 2.033 c = 1.344 a = 84.14 b = 2.198 c = 1.093

11. Spectra-radiometers used for this study text Masters Thesis Presentation, July 18th, 2008Sanjyot Gindi Slide 11 of 43 X-Rite DTP 70Photo Research PR-705

12. Masters Thesis Presentation, July 18th, 2008Sanjyot Gindi Slide 12 of 43 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

13. Masters Thesis Presentation, July 18th, 2008Sanjyot Gindi Slide 13 of 43 2. Model Based Method R channel G channel B channel Sensor [F] Target Kodak Q60 reflectance [R] Lamp [L] Spectral Model of a Scanner

14. Masters Thesis Presentation, July 18th, 2008Sanjyot Gindi Slide 14 of 43 Experiments: 1. The Lamp spectrum [L], is obtained using a spectroradiometer and a white diffusion target. 2. The Sensor spectral response [F], is obtained using the Monochromator (400nm to 700nm) 3. The Spectral Reflectance of the Patches [R], were obtained using the X-Rite spectrophotometer.

15. Masters Thesis Presentation, July 18th, 2008Sanjyot Gindi Slide 15 of 43 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.

16. Masters Thesis Presentation, July 18th, 2008Sanjyot Gindi Slide 16 of 43 Schematic of the experimental assembly for lamp response measurement Spectroradiometer (PR-705) Lamp Direction of scan head motion White diffusion standard Samsung Scanner Light reflected from the diffuser Scan-head clamp

17. Masters Thesis Presentation, July 18th, 2008Sanjyot Gindi Slide 17 of 43 Wavelength in nm Lamp Spectrum - Measured by Spectroradiometer. Lamp spectrum matrix = [ L ], 31x31 diagonal matrix

18. Masters Thesis Presentation, July 18th, 2008Sanjyot Gindi Slide 18 of 43 1. The Monochromator SP-150 was used to input light with wavelengths in the range 400-700nm in steps of 10nm. 2. During the measurement, the lamp of the scanner was off. 3. Scanner setting: Gamma ‘on’ and shading ‘off’, 150dpi. 4. The Assembly used for measurement was as shown in the following figure: Expt. 2: Sensor Response.

19. Masters Thesis Presentation, July 18th, 2008Sanjyot Gindi Slide 19 of 43 Cross-section view of scan head and measurement assembly Lamp (turned ‘off’ during measurement) To CCD sensor glass Incident Light Mirror Lens Fiber optic cable of the Monochromator Clamp to hold the optic cable Scanner head

20. Masters Thesis Presentation, July 18th, 2008Sanjyot Gindi Slide 20 of 43 5. The sample output images obtained are as shown below: 1.2.3. 700nm 460nm 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.

21. Masters Thesis Presentation, July 18th, 2008Sanjyot Gindi Slide 21 of 43 Responses of the three channels were taken separately with different intensity settings of Monochromator- 25 for red, 50 for green, 60 for blue. Wavelength in nm

22. Masters Thesis Presentation, July 18th, 2008Sanjyot Gindi Slide 22 of 43 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.

23. Masters Thesis Presentation, July 18th, 2008Sanjyot Gindi Slide 23 of 43 Output variation is from 4.3e-5 for 400nm to 1.2e-3 for 700nm (in units of Radiance)~ approximately 27 times

24. Masters Thesis Presentation, July 18th, 2008Sanjyot Gindi Slide 24 of 43 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

25. Masters Thesis Presentation, July 18th, 2008Sanjyot Gindi Slide 25 of 43 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

26. Kodak Q-60 target Masters Thesis Presentation, July 18th, 2008Sanjyot Gindi Slide 26 of 43

27. Masters Thesis Presentation, July 18th, 2008Sanjyot Gindi Slide 27 of 43 Measurement of Kodak Q60 patches by X-Rite and Spectroradiometer X-Rite, 2 deg Observer, D65 illuminant Spectroradiometer, ½ deg observer, Illuminant: ‘Daylight’ (Sample # 1) Wavelength in nm

28. Masters Thesis Presentation, July 18th, 2008Sanjyot Gindi Slide 28 of 43 X-RiteSpectroradiometer Sample # 2 Wavelength in nm

29. Masters Thesis Presentation, July 18th, 2008Sanjyot Gindi Slide 29 of 43 X-RiteSpectroradiometer Wavelength in nm Sample # 3

30. Masters Thesis Presentation, July 18th, 2008Sanjyot Gindi Slide 30 of 43 Spectral Model of a Scanner : R channel G channel B channel Sensor [F] Target kodak Q60 reflectance [R] Lamp [L]

31. Masters Thesis Presentation, July 18th, 2008Sanjyot Gindi Slide 31 of 43 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] Based on the above model: TT

32. Masters Thesis Presentation, July 18th, 2008Sanjyot Gindi Slide 32 of 43 Results: Regression based method:  Mean Delta E = 3.82  Max Delta E = 18.20  histogram of number of patches with Delta E values.

33. Masters Thesis Presentation, July 18th, 2008Sanjyot Gindi Slide 33 of 43 Model Based Method  Mean delta E = 4.205  Max delta E =19.332  Histogram: 115 patches with delta E < 3

34. Masters Thesis Presentation, July 18th, 2008Sanjyot Gindi Slide 34 of 43 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

35. Masters Thesis Presentation, July 18th, 2008Sanjyot Gindi Slide 35 of 43 3D Gamut plots Samsung Scanner L* b* a* HP scanner

36. Masters Thesis Presentation, July 18th, 2008Sanjyot Gindi Slide 36 of 43 3D gamut plot Epson Scanner

37. Masters Thesis Presentation, July 18th, 2008Sanjyot Gindi Slide 37 of 43 L* Slices in Gamut Samsung HP Epson L* =20 L* = 30 Horizontal axis: b* values Vertical axis: a* values

38. Masters Thesis Presentation, July 18th, 2008Sanjyot Gindi Slide 38 of 43 Samsung HP Epson L*=40, L*=50 Horizontal axis: b* values Vertical axis: a* values

39. Masters Thesis Presentation, July 18th, 2008Sanjyot Gindi Slide 39 of 43 L*=60, L*=70 Samsung HP Epson Horizontal axis: b* values Vertical axis: a* values

40. Masters Thesis Presentation, July 18th, 2008Sanjyot Gindi Slide 40 of 43 Samsung HP Epson L* = 80, L*= 90 Horizontal axis: b* values Vertical axis: a* values

41. Masters Thesis Presentation, July 18th, 2008Sanjyot Gindi Slide 41 of 43 Chromaticity diagram Samsung HP Epson Adobe RGB 0.0 0.1 0.50.9 0.5 0.9 x chromaticity y chromaticity How can sensor chromaticities lie outside the spectral locus?

42. Masters Thesis Presentation, July 18th, 2008Sanjyot Gindi Slide 42 of 43 Chromaticity diagram Samsung HP Epson Adobe RGB 0.0 0.1 0.50.9 0.5 0.9 x chromaticity y chromaticity 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.

43. Masters Thesis Presentation, July 18th, 2008Sanjyot Gindi Slide 43 of 43 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  E units given by: Where P1, P2, P3 and P0 represent the vertices of a tetrahedron and ‘| |’ denotes the determinant. 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*) 1/6 * Volume =

44. Masters Thesis Presentation, July 18th, 2008Sanjyot Gindi Slide 44 of 43 Gamut Volumes (in cubic  E 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

45. Masters Thesis Presentation, July 18th, 2008Sanjyot Gindi Slide 45 of 43 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.

46. Masters Thesis Presentation, July 18th, 2008Sanjyot Gindi Slide 46 of 43 Delta EMeanMaxMeanMaxMeanMax Samsung2.8312.814.3517.333.1512.06 HP1.649.662.5812.281.969.21 Epson2.2411.023.3014.411.727.65 NameR channelG channelB channel Quantization Error in delta E units.

47. Masters Thesis Presentation, July 18th, 2008Sanjyot Gindi Slide 47 of 43 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.

48. Masters Thesis Presentation, July 18th, 2008Sanjyot Gindi Slide 48 of 43 References: 1. “Digital Color Imaging Handbook”, Gaurav Sharma. 2. “Color Science Concepts and Methods Quantitative Data and Formulae”, by Wyszecki and Stiles 3. ”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 4. ECE638, ”Principles of digital color Imaging systems”, notes by Prof. J. Allebach, Purdue University. 5. ECE 637, “Image Processing”, notes by Prof. C. Bouman, Purdue University. 6. "A Review of Linear Color Descriptor Spaces and Their Applications," M. Wolski 7. "Imaging Colorimetry Using a Digital Camera," W. Wu et al 8. Non-Contact Imaging Colorimeter for Human Tooth Color Assessment Using A Digital Camera," D. Ng et al

49. Additional slides…

50. Masters Thesis Presentation, July 18th, 2008Sanjyot Gindi Slide 50 of 43 Red: transformed values Black: actual values

51. Masters Thesis Presentation, July 18th, 2008Sanjyot Gindi Slide 51 of 43 Red: transformed values Black: actual values

52. Masters Thesis Presentation, July 18th, 2008Sanjyot Gindi Slide 52 of 43 Variation in Spectral Radiance over a time period of 400 seconds Wavelength in nm