Spectral tools - ISO 3664 (Illumination analysis)
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ISO 3664 (Viewing conditions - Graphic Technology and Photography) is mostly a collection of standards regrouped to define the various viewing conditions required to judge the quality of color images. The P2 condition shown above, as defined in ISO 3664, is dedicated to the evaluation of prints under ambient light, or light booth illumination, at an illuminance level around 500 lux. A similar interface is presented for the P1 and T1 conditions, respectively defined for Critical appraisal of printed color (at a high illuminance level of 2000 lux) and for the direct viewing of transparencies with backlit illuminators. Any viewing condition can be measured at up to nine positions, as selected in the "Brightness uniformity" section. In the screenshot above, the center position is selected and the corresponding "Brightness", "Chromaticity", "Color Rendering Index (CRI)", and "MI" data is shown for this position. The "MI", or Metamerism Index", measured here is defined in CIE Standard 12 (CIE S 012) and presented as a Quality Grade for daylight simulators, from "A" to "E", with "A" being the best score. This is not the same MI as the one computed in the "MI tools". Combining the CRI data with the quality grade has been recognized as a very effective way of characterizing illumination. You may note that the MI test specifically called for in ISO 3664 is the CIE 51 procedure; however, CIE S 012 is a recent standard, published in the last quarter of 2004, which addresses the same concerns as CIE 51, is computed in the same way, has the same grade criteria, and effectively replaces it. These tools are called ISO 3664 with a "+" to indicate that alternate reference illuminants can be selected by the user for each test. In the screenshot above, we have measured the light falling through a window on a work table. Prior to our measurements, we had identified, on our table, measurement locations on a 3x3 grid over a 1m by 1m area, corresponding to the nine positions. Because the ambient light was near 6000 K, we selected D60 as our target chromaticity. We also selected "Auto" for the reference CRI Illuminant, and D65 for the MI reference (D60 is not available for the MI). We see that the brightness in the center position, the position selected in the "Brightness uniformity" control group, is 551 lux, meeting the P2 goal, and we thus see a "PASS" result. The "PASS" result is shown with a green colored font to indicate it corresponds to an ISO 3664 requirement. The chromaticity, CRI and MI results for this position also pass the selected requirements, but since these requirements are not exactly those of ISO 3664, they are shown with a yellow colored font. We see that the brightness uniformity test fails because of a fall-off in the lower-left corner, shown with a red light. We also note that the illumination is higher for the three top positions, which are closer to the window from which the light comes from. Of course, better results would be expected for a light booth, but this example shows that we can evaluate "real life" conditions as well. A screenshot of the the ISO3664+ tool when used for characterizing color monitors is shown next. |
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