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Imatest Documentation Using Print Test Measure print quality factors: color response, tonal response, and Dmax Introduction Imatest Print Test measures several of the key factors that contribute to photographic print quality, Color response, the relationship between pixels in the image file and colors in the print, Color gamut, the range of saturated colors a print can reproduce, i.e., the limits of color response, Tonal response, the relation between pixels and print density, and Dmax, the deepest printable black tone: a very important print quality factor, using a print of a test pattern scanned on a flatbed scanner. No specialized equipment is required. You can observe response irregularities by viewing the test print, but you need to run Print Test to obtain detailed measurements of the print's color and tonal response. The essential steps in running Print Test are Assign a profile to the test pattern, if needed. Print the test pattern, noting (as applicable) the printer, paper, ink, working color space, ICC profile, rendering intent, color engine, and miscellaneous software settings. Scan it on a flatbed scanner, preferably one that has been profiled. Best results are obtained by scanning it next to a step chart such as the Koda Q-13. Run Imatest Print test. Color quality is a function of gamut— the range of saturated colors a print can reproduce, as well as overall color response— the relationship between file pixels and print color. Tonal quality is strongly correlated with Dmax— the deepest attainable black tone, as well as the tonal response curve. Prints usually appear weak in the absence of truly deep black tones. The test pattern, shown (reduced) on the right, was generated using the HSL color representation. H = Hue varies from 0 to 1 for the color range R→ Y→ G→ C→ B→ M→ R (horizontally across the image on the right). (0 to 6 is used in the Figures below for clarity.) S = Saturation = max(R,G,B)/min(R,G,B). S = 0 for gray; 1 for the most saturated colors for a given Lightness value. LHSL = Lightness = (max(R,G,B)+min(R,G,B))/2. (The HSL subscript is used to distinguish it from CIELAB L.) L = 0 is pure black; 1 is pure white. The most visually saturated colors occur where LHSL = 0.5. The key zones are, 1. 2. 3. S=1. A square region consisting of all possible hues (0 ≤ H ≤ 1) and lightnesses (0 ≤ LHSL ≤ 1), where all colors are fully saturated (S = 1), i.e., as saturated as they can be for the Lightness value. L=0.5. A rectangular region consisting of all possible hues (0 ≤ H ≤ 1) and saturation levels (0 ≤ S ≤ 1) for middle Lightness (LHSL = 0.5), where colors are most visually saturated. Two identical monochrome tone scales, where pixel levels vary linearly, 0 ≤ {R=G=B} ≤ 1. The zones labelled K, Gry, and W are uniform black (pixel level = 0), gray (pixel level = 127), and white (pixel level = 255), respectively. 402 of 451
Imatest Documentation Detailed instructions The printer test patterns are located in the images subfolder of the Imatest installation folder. They can also be downloaded from the following links: Print_test_target.png (no embedded profile), Print_test_target_sRGB.tif , Print_test_target_Adobe.tif, Print_test_target_WGRGB.tif (embedded profiles as indicated in the file name). These patterns all contain the same data; only the profile tag differs. [I was unable to save embedded profiles in PNG files with any of my editors, so I used LZW-compressed TIFF. (I found a small error when using maximum quality JPEGs.)] Assigning a profile: To explore the effect of working color space on print quality you may want to change the ICC profile of the test image without changing the image data. In Photoshop: Click Image, Mode, Assign Profile... In Picture Window Pro: Click Transformation, Color, Change Color Profile... Set Change: (the bottom setting) to Profile Setting Only. Open a test pattern in an image editor, preferably an ICC-aware image editor such as Photoshop or Picture Window Pro with color management enabled. For most printer testing I recommend Print_test_target_Adobe.tif, which has an embedded Adobe RGB (1998) profile, but patterns with other color spaces are also of interest. Print test lets you see the effect of printing with different color spaces. Print the pattern, approximately 6.5x10 inches (16x25 cm), from your image editor. Carefully record the paper, ink, working color space, ICC profile, rendering intent, and printer software settings. I recommend running a nozzle check before making the print— just make sure everything is OK. Scan the print on a flatbed scanner at around 100 to 150 dpi. The scanned image should be no larger than about 1500 pixels high. Higher resolution is wasted; it merely slows the calculations. The pattern must be aligned precisely (not tilted). The scanner's auto exposure should be turned off and ICC Color Management should be used if possible. On the Epson 3200 flatbed scanner, I click on the Color tab in the Configuration dialog box, then select ICM. Source (Scanner): is either be EPSON Standard or a custom scanner profile. Target: should be one of the three profiles recognized by Print test: sRGB (small gamut; comparable to CRTs), Adobe RGB (1998) (medium gamut; comparable to high quality printers), or Wide Gamut RGB (larger gamut than any physical device). Since the Epson standard profile, which represents the scanner's intrinsic response, has a large gamut (between Adobe and Wide Gamut RGB, according to ICC Profile Inspector), and since the measured printer gamut is bounded the gamut of the working color space, a moderate to large gamut color space should be chosen for best accuracy. I recommend Adobe RGB. The profile is not embedded in the output file; you'll need to remember it: it can be part of the file name. Save the scan with a descriptive name. TIFF, PNG, or maximum quality JPEG are the preferred formats. A reduced scan of a print made on the Epson 2200 printer with Epson Enhanced Matte paper and the standard Epson ICC profile is shown above. Colors are somewhat more subdued than glossy, semigloss, or luster papers. Accuracy is improved if you scan a Q-13 or equivalent reflective step chart next to the print (shown on the right of the above image), then run Stepchart. When the run is complete, check the box labelled "Save Print test calibration data" in the Save Q-13 Stepchart Results window. This saves the tonal response and Dmax calibration data, even if you click No to Save Q-13 Stepchart results? This only needs to be done once for a set of scans made under identical conditions. This optional step improves the accuracy of the tonal response and Dmax. Scanner settings and performance verification For the Epson 3200 flatbed scanner, the Configuration dialog box, shown on the right, is opened by clicking Configuration... in the main window. If you have a trustworthy custom profile for your scanner, you should enter it in the Source (Scanner): box. The Epson Standard (default) profile is shown. If the custom profile is not visible, go to Control Panel, double-click on Scanners and Cameras, right-click on your flatbed scanner, click Properties, then click on the Color 403 of 451
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