Technical Report Artist Paint Target (APT): A Tool for Verifying Camera Performance Roy S. Berns June 2014 1 Executive Summary Building camera profiles is most commonly accomplished using color targets such as the Xrite ColorChecker Classic or ColorChecker Digital SG. These targets may not well represent artist materials in a given work of art. As a consequence, color errors can result that would not present themselves during the profiling (calibration) stage. A new color target was developed, the Artist Paint Target, or APT, which was produced using artist paints. This target is an excellent tool to verify camera profiles. It can also be used to evaluate color rendering of light sources used in the display of artwork. Introduction The accurate imaging and of artist paints presents interesting challenges. Artists have hundreds of paints to choose from leading to considerable spectral variability and a large color gamut. The applied materials can have a range of gloss, texture, and opacity. Some painting styles eschew the use of black pigments and as a result neutral and low-­‐chroma passages have considerable spectral selectivity, increasing their sensitivity to camera optical deficiencies in spectral response. Although there are a number of commercial targets that can be used to both calibrate and evaluate color accuracy, these targets do not have the range of spectral properties found in artist paints. This is of particular concern for blue pigments that may be azurite, natural ultramarine, synthetic ultramarine, cobalt blue, Prussian blue, indanthrone blue, and copper phthalocyanine blue. Targets such as the Xrite ColorChecker Digital SG and those printed with CMYK inks only contain phthalocyanine blue. The ColorChecker Classic has one bluish sample not made using phthalocyanine blue: Purplish Blue (row 2, column 2). Given that common camera calibration targets are either the ColorChecker Digital SG or printed, there is a need for a color target that has spectral variability encompassing artist paints. Such a target has been under development within the artwork spectral imaging and archiving of cultural heritage research program (www.art.si.org).1 These experiences and those gained using current commercial targets have led to the new Artist Paint Target, APT. Target Design The Artist Paint Target was modeled after the ColorChecker Classic,2 particularly because it contains only 24 colors, has high-­‐chroma colors sampling the hue circle (CMYRGB), and a 1 M. Mohammadi, M. Nezamabadi, R. S. Berns, L. A. Taplin, “Spectral imaging target development based on hierarchical cluster analysis,” Proc. IS&T/SID Twelfth Color Imaging Conference, 59-­‐64 (2004); M. Mohammadi, M. Nezamabadi, R. S. Berns, L. A. Taplin, “A prototype calibration target for spectral imaging,” Proc. 10th Congress of the International Colour Association, Granada, 387-­‐390 (2005); R. S. Berns and M. I. Haddock, “A color target for museum applications,” IS&T/SID Eighteenth Color Imaging Conference 27-­‐32 (2010); R. S. Berns and M. I. Haddock, “A Color Target for Lighting and Camera Evaluation,” Proc. 16th Triennial Meeting Lisbon, ICOM Committee for Conservation, 1601-­‐1-­‐1601-­‐9 (2011). 2 Breneman, E. J., A color chart for use in evaluating quality of color reproduction, Journal of Photographic Science and Engineering 1, 74-­‐78, 1957; McCamy, C. S., Marcus, H., and Davidson, J. G., A color-­‐rendition chart,, Journal of Applied Photographic Engineering 2, 95-­‐99, 1976. 2 neutral scale with nearly flat spectra. It also has specialty colors, designed to evaluate 1970 era color-­‐imaging systems. Accordingly, the design goal was to create a 24-­‐sample color target, include a range of blue pigments, sample the hue circle at high chroma, include several specialty colors, and have a neutral scale with nearly flat spectra. A final consideration was gloss. There were several criteria. The new target samples are currently produced by hand mixing and applying to a paper substrate using a drawdown bar. This procedure can produce air bubbles trapped in the paint. Reducing gloss seems to reduce surface tension enabling the air to escape the paint film and produce a smooth surface. The second consideration was uniformity in gloss. All the samples should have identical gloss. The third criterion was color gamut, particularly for dark colors. Matte paints do not achieve a sufficiently low lightness for black. The decision was made to use light absorbing coated foil. Golden Artist Colors Matte Fluid acrylic-­‐dispersion paints (www.goldenpaints.com) were used to produce the samples except for the black. The specific paint names and their color index (CI) numbers are listed in Table I. The specific paints for each sample are shown in Table II. White was added to every sample except D3, E3, E4, F3 and F4. Titan Buff was added to E3. An image of the target is shown in Figure 1. The black sample was Acktar Metal Velvet. Table I. Golden Artist Colors Matte Fluid paints used to produce color target. Golden Artist Color CI Number Golden Artist Color CI Number Ultramarine Blue PB 29 Pyrrole Red PR 254 Cobalt Blue PB 28 Quinacridone Magenta PR 122 Phthalo Blue (Green Shade) PB 15:4 Quinacridone Crimson PR 206, PR 202 Phthalo Green (Blue Shade) PG 36 Dioxazine Purple PV 23 Permanent Green Light PG 7, PY3 Titan Buff PW 6 Hansa Yellow Opaque PY 74 Titanium White PW 6 Yellow Ochre PY 43 Bone Black PBk 9 Pyrrole Orange PO 73 Table II. Materials used to produce color target. A B C D E F 1 Ultramarine Blue Cobalt Blue Phthalo Blue Phthalo Green Permanent Green Light 2 Permanent Green Light Hansa Yellow Permanent Green Light Hansa Yellow Hansa Yellow Phthalo Blue Phthalo Green Pyrrole Orange Hansa Yellow Pyrrole Orange Hansa Yellow Pyrrole Red 3 Quinacridone Magenta Quinacridone Magenta Dioxazine Purple Dioxazine Purple Titan Bluff 4 Titanium White Bone Black Raw Umber Bone Black Raw Umber Bone Black Raw Umber Ultramarine Blue Yellow Ochre Quinacridone Crimson Bone Black Ultramarine Blue Yellow Ochre Quinacridone Crimson Acktar Metal Velvet 3 Figure 1. sRGB image of the color target. The sample holder was manufactured by Image Science Associates. The first two and one half rows are high-­‐chroma colors with a range of hues. Samples A1, B1, and C1 are different blue paints: ultramarine, cobalt, and phthalocyanine, respectively. Sample D3 was made using titan buff and approximates aged lead white paint. Samples E3 and F3 are skin tones with their spectra similar to the Buckley and Grum data,3 used as aim spectra for the ColorChecker Classic. The last row contains neutrals. The CIELAB coordinates (CIE Illuminant D50, 1931 standard observer) for the target are plotted in Figure 2. As a comparison, the CIELAB coordinates for the Xrite ColorChecker Classic are plotted in Figure 3. The target has a larger color gamut, both for chroma and lightness. Figure 2. Artist Paint Target plotted in CIELAB. 3 W. R. Buckley and F. Grum, “The measurement of skin color, spectrophotometric technique,” J. Society Cosmetic Chemistry 15, 79-­‐85 (1964). 4 Figure 3. Xrite ColorChecker Classic plotted in CIELAB. The spectra of the Artist Paint Target are plotted in Figure 4. There are the long-­‐
wavelength “tails,” characteristic of ultramarine and cobalt blues. The addition of an orange pigment produced a series of sharp-­‐cutting spectra for yellow through red. Quinacridone magenta and dioxazine purple further extend the sharp-­‐cutting spectra. Titan buff is similar to aged lead white that has yellowed. The skin spectra have the absorption characteristics of hemoglobin. Finally, the neutral scale is quite flat spectrally and ranges from 0.01 – 0.93 reflectance factor. The colorimetric and spectral data are listed in Tables III and IV, respectively. Image Science Associates is manufacturing and selling the Artist Paint Target, in a variety of sizes and configurations. See www.imagescienceassociates.com. 5 Figure 5. Spectral reflectance factor of Artist Paint Target, measured with bi-­‐directional geometry. 6 Table III. Nominal colorimetric L*a*b* data for the Artist Paint Target: CIE illuminant D50 and the 1931 standard observer. Sample L* a* b* C*ab hab A1 42.07 14.07 -­‐57.19 58.90 283.82 A2 61.37 -­‐47.75 39.35 61.87 140.51 A3 46.37 54.01 -­‐5.07 54.25 354.63 A4 97.67 -­‐0.56 1.33 1.44 113.05 B1 45.59 4.29 -­‐52.39 52.57 274.68 B2 71.36 -­‐32.81 71.21 78.41 114.74 B3 46.85 31.79 -­‐25.75 40.91 320.99 B4 77.52 -­‐0.03 0.45 0.45 93.32 C1 45.93 -­‐15.15 -­‐44.75 47.25 251.30 C2 87.93 6.14 97.92 98.12 86.41 C3 46.81 23.42 -­‐37.83 44.49 301.76 C4 57.52 -­‐0.16 0.15 0.22 137.55 D1 50.85 -­‐36.91 -­‐21.19 42.56 209.87 D2 75.27 36.03 80.97 88.62 66.01 D3 84.78 2.31 15.52 15.69 81.55 D4 37.20 0.16 0.52 0.55 72.71 E1 53.54 -­‐48.53 -­‐2.65 48.60 183.13 E2 65.54 55.35 66.57 86.58 50.26 E3 62.64 20.55 21.17 29.51 45.86 E4 21.88 -­‐0.05 -­‐0.23 0.23 256.46 F1 59.42 -­‐57.33 18.70 60.30 161.93 F2 48.44 62.31 30.74 69.48 26.26 F3 39.40 15.60 14.69 21.43 43.28 F4 10.41 0.45 1.18 1.27 69.09 7 Table IV. Nominal spectral reflectance factor data of the Artist Paint Target, based on the average of three measurements using an Xrite i1 bi-­‐directional spectrophotometer. Wavelength A1 A2 A3 A4 B1 B2 B3 B4 380 0.1006 0.0621 0.0932 0.1218 0.1173 0.0444 0.1050 0.1146 390 0.1765 0.0743 0.1385 0.2108 0.1956 0.0462 0.1726 0.1936 400 0.3148 0.0829 0.2021 0.4138 0.3263 0.0472 0.2789 0.3424 410 0.4382 0.0857 0.2359 0.6806 0.4428 0.0480 0.3512 0.4710 420 0.4992 0.0852 0.2415 0.8539 0.5103 0.0478 0.3725 0.5131 430 0.5354 0.0847 0.2338 0.9136 0.5379 0.0479 0.3720 0.5201 440 0.5583 0.0840 0.2165 0.9261 0.5428 0.0475 0.3572 0.5208 450 0.5608 0.0836 0.1945 0.9298 0.5316 0.0467 0.3321 0.5210 460 0.5344 0.0850 0.1731 0.9345 0.5192 0.0471 0.3035 0.5222 470 0.4755 0.0905 0.1527 0.9361 0.4864 0.0484 0.2724 0.5218 480 0.3982 0.1011 0.1315 0.9372 0.4496 0.0505 0.2366 0.5212 490 0.3197 0.1241 0.1129 0.9393 0.4623 0.0615 0.2005 0.5214 500 0.2517 0.2000 0.0999 0.9399 0.4199 0.1138 0.1720 0.5211 510 0.1984 0.3484 0.0881 0.9405 0.3193 0.2476 0.1474 0.5210 520 0.1607 0.4747 0.0753 0.9429 0.2260 0.4294 0.1230 0.5222 530 0.1336 0.5057 0.0665 0.9415 0.1604 0.5765 0.1066 0.5217 540 0.1152 0.4700 0.0652 0.9420 0.1211 0.6398 0.1037 0.5226 550 0.1025 0.4091 0.0671 0.9423 0.1058 0.6268 0.1058 0.5238 560 0.0934 0.3441 0.0671 0.9400 0.0974 0.5747 0.1047 0.5240 570 0.0876 0.2851 0.0728 0.9427 0.0872 0.5126 0.1110 0.5265 580 0.0834 0.2295 0.0970 0.9406 0.0806 0.4419 0.1373 0.5260 590 0.0808 0.1798 0.1521 0.9410 0.0792 0.3695 0.1841 0.5264 600 0.0795 0.1411 0.2392 0.9407 0.0803 0.3042 0.2309 0.5260 610 0.0791 0.1187 0.3499 0.9403 0.0824 0.2606 0.2511 0.5252 620 0.0801 0.1086 0.4657 0.9403 0.0818 0.2383 0.2456 0.5241 630 0.0822 0.1041 0.5652 0.9394 0.0833 0.2265 0.2496 0.5224 640 0.0860 0.1015 0.6405 0.9405 0.0950 0.2186 0.2858 0.5217 650 0.0917 0.1005 0.6916 0.9417 0.1273 0.2142 0.3499 0.5207 660 0.1005 0.1037 0.7267 0.9440 0.1932 0.2187 0.4320 0.5203 670 0.1128 0.1111 0.7484 0.9435 0.3007 0.2316 0.5152 0.5185 680 0.1306 0.1208 0.7638 0.9433 0.4467 0.2484 0.5923 0.5167 690 0.1548 0.1299 0.7746 0.9422 0.6012 0.2634 0.6590 0.5144 700 0.1865 0.1362 0.7848 0.9424 0.7138 0.2733 0.7155 0.5134 710 0.2246 0.1366 0.7940 0.9431 0.7692 0.2730 0.7590 0.5123 720 0.2661 0.1319 0.7991 0.9412 0.7907 0.2632 0.7884 0.5099 730 0.3102 0.1313 0.8058 0.9423 0.8023 0.2608 0.8121 0.5089 8 Table IV. continued. Wavelength C1 C2 C3 C4 D1 D2 D3 D4 380 0.0807 0.0481 0.1057 0.0932 0.0751 0.0451 0.1027 0.0596 390 0.1269 0.0498 0.1789 0.1427 0.1095 0.0466 0.1606 0.0743 400 0.2058 0.0512 0.3015 0.2071 0.1507 0.0472 0.2503 0.0873 410 0.2760 0.0514 0.3976 0.2435 0.1790 0.0476 0.3273 0.0929 420 0.3101 0.0512 0.4362 0.2514 0.1956 0.0472 0.3764 0.0940 430 0.3490 0.0511 0.4525 0.2531 0.2143 0.0471 0.4174 0.0943 440 0.4153 0.0505 0.4538 0.2535 0.2399 0.0465 0.4538 0.0944 450 0.4715 0.0495 0.4382 0.2538 0.2745 0.0456 0.4859 0.0946 460 0.4902 0.0497 0.4096 0.2545 0.3261 0.0457 0.5144 0.0949 470 0.4884 0.0510 0.3697 0.2543 0.3929 0.0466 0.5358 0.0950 480 0.4697 0.0527 0.3184 0.2539 0.4498 0.0482 0.5513 0.0948 490 0.4371 0.0643 0.2596 0.2539 0.4750 0.0584 0.5641 0.0950 500 0.3914 0.1200 0.2102 0.2538 0.4576 0.1032 0.5766 0.0950 510 0.3341 0.2614 0.1732 0.2537 0.4117 0.1897 0.5909 0.0951 520 0.2730 0.4562 0.1441 0.2541 0.3539 0.2533 0.6070 0.0954 530 0.2149 0.6348 0.1273 0.2536 0.2923 0.2650 0.6200 0.0954 540 0.1663 0.7606 0.1236 0.2540 0.2352 0.2615 0.6340 0.0957 550 0.1269 0.8273 0.1235 0.2545 0.1841 0.2891 0.6479 0.0961 560 0.0971 0.8533 0.1206 0.2545 0.1419 0.3988 0.6600 0.0963 570 0.0793 0.8672 0.1227 0.2556 0.1135 0.5756 0.6747 0.0971 580 0.0699 0.8708 0.1366 0.2553 0.0954 0.7299 0.6850 0.0972 590 0.0652 0.8745 0.1617 0.2553 0.0836 0.8178 0.6958 0.0973 600 0.0620 0.8766 0.1834 0.2548 0.0741 0.8555 0.7046 0.0973 610 0.0600 0.8783 0.1846 0.2542 0.0679 0.8703 0.7120 0.0971 620 0.0596 0.8802 0.1712 0.2534 0.0654 0.8763 0.7189 0.0968 630 0.0598 0.8813 0.1702 0.2521 0.0643 0.8786 0.7243 0.0964 640 0.0605 0.8844 0.1964 0.2514 0.0641 0.8825 0.7309 0.0964 650 0.0620 0.8872 0.2482 0.2506 0.0649 0.8862 0.7368 0.0961 660 0.0638 0.8911 0.3215 0.2500 0.0670 0.8907 0.7432 0.0960 670 0.0641 0.8919 0.4031 0.2488 0.0688 0.8921 0.7473 0.0957 680 0.0625 0.8922 0.4856 0.2475 0.0697 0.8928 0.7511 0.0953 690 0.0606 0.8920 0.5626 0.2461 0.0693 0.8926 0.7542 0.0948 700 0.0590 0.8931 0.6321 0.2451 0.0680 0.8936 0.7585 0.0945 710 0.0587 0.8942 0.6901 0.2442 0.0670 0.8948 0.7626 0.0943 720 0.0606 0.8928 0.7348 0.2426 0.0680 0.8936 0.7645 0.0938 730 0.0658 0.8942 0.7733 0.2419 0.0725 0.8949 0.7687 0.0935 9 Table IV. continued. Wavelength E1 E2 E3 E4 F1 F2 F3 F4 380 0.0673 0.0473 0.0768 0.0342 0.0619 0.0633 0.0454 0.0096 390 0.0853 0.0490 0.0983 0.0351 0.0782 0.0753 0.0471 0.0104 400 0.1035 0.0496 0.1213 0.0352 0.0925 0.0799 0.0489 0.0103 410 0.1152 0.0493 0.1360 0.0355 0.0995 0.0773 0.0507 0.0104 420 0.1237 0.0484 0.1454 0.0353 0.1019 0.0728 0.0529 0.0105 430 0.1345 0.0480 0.1577 0.0353 0.1041 0.0695 0.0568 0.0107 440 0.1486 0.0473 0.1735 0.0351 0.1074 0.0669 0.0623 0.0107 450 0.1713 0.0466 0.1869 0.0350 0.1117 0.0648 0.0662 0.0108 460 0.2137 0.0466 0.1940 0.0351 0.1219 0.0634 0.0677 0.0108 470 0.2818 0.0472 0.1974 0.0350 0.1602 0.0621 0.0682 0.0110 480 0.3628 0.0485 0.2006 0.0350 0.2701 0.0610 0.0692 0.0110 490 0.4313 0.0546 0.2069 0.0349 0.4167 0.0603 0.0716 0.0112 500 0.4517 0.0728 0.2167 0.0348 0.5045 0.0596 0.0755 0.0112 510 0.4309 0.0937 0.2274 0.0348 0.5198 0.0588 0.0797 0.0112 520 0.3916 0.0988 0.2376 0.0349 0.4925 0.0582 0.0835 0.0114 530 0.3426 0.0942 0.2486 0.0348 0.4444 0.0576 0.0872 0.0115 540 0.2901 0.0917 0.2623 0.0348 0.3877 0.0572 0.0913 0.0116 550 0.2372 0.1080 0.2675 0.0348 0.3265 0.0571 0.0921 0.0117 560 0.1899 0.1823 0.2600 0.0347 0.2676 0.0578 0.0885 0.0117 570 0.1516 0.3324 0.2600 0.0347 0.2170 0.0607 0.0877 0.0119 580 0.1195 0.5136 0.2914 0.0346 0.1713 0.0744 0.0983 0.0119 590 0.0938 0.6742 0.3704 0.0346 0.1324 0.1577 0.1275 0.0120 600 0.0759 0.7822 0.4524 0.0346 0.1034 0.3313 0.1594 0.0120 610 0.0666 0.8363 0.4949 0.0345 0.0872 0.5036 0.1754 0.0122 620 0.0631 0.8564 0.5123 0.0346 0.0803 0.6441 0.1817 0.0122 630 0.0617 0.8627 0.5205 0.0345 0.0775 0.7511 0.1857 0.0124 640 0.0612 0.8680 0.5281 0.0345 0.0759 0.8155 0.1909 0.0124 650 0.0616 0.8728 0.5366 0.0345 0.0754 0.8450 0.1982 0.0126 660 0.0633 0.8783 0.5481 0.0346 0.0780 0.8615 0.2093 0.0128 670 0.0663 0.8802 0.5606 0.0346 0.0837 0.8702 0.2241 0.0129 680 0.0699 0.8811 0.5758 0.0345 0.0910 0.8769 0.2439 0.0130 690 0.0731 0.8815 0.5931 0.0344 0.0981 0.8814 0.2695 0.0131 700 0.0756 0.8831 0.6139 0.0345 0.1031 0.8860 0.3023 0.0132 710 0.0757 0.8847 0.6353 0.0345 0.1036 0.8898 0.3399 0.0134 720 0.0739 0.8842 0.6535 0.0344 0.1003 0.8904 0.3783 0.0135 730 0.0742 0.8860 0.6722 0.0345 0.1001 0.8935 0.4181 0.0138 10