ISO 3664 Update: Why Standard Viewing Conditions Are Essential for Print Color Assessment

Color and density measurement are widely used to control reproduction quality in printing, yet for an individual image they cannot replace the human eye’s final assessment of color reproduction. In print reproduction, the reproduced image is usually compared with various originals to determine how faithfully color has been reproduced. For example, when evaluating the color quality of a reproduced color image, a common approach is to compare it side by side with the original and then judge the degree of fidelity.

Ideally, the viewing conditions used to control print reproduction should match the environment in which the finished product will ultimately be seen. If that final environment is clearly defined, printing staff can monitor quality under the same conditions. In practice, however, this is rarely possible; more importantly, the final viewing conditions often do not allow direct comparison of the original, photograph, or contract proof with the final printed piece. Under different light sources and viewing conditions, each material renders color in a way that directly affects the final image, and their appearance often differs. To avoid misinterpreting color reproduction, stable viewing conditions during the reproduction process are essential.

Surrounding objects and surfaces in a viewing environment—their color and brightness—also strongly influence how an observer perceives the color of the target. This is why standard viewing booths are used, but an ordinary viewing station is often merely part of a room, making control of the viewing environment important. To ensure uniform viewing conditions across all reproduction steps, a unified viewing standard is imperative. For color prints and photographs, the recommended standard is ISO 3664, and a new version is to be issued. Two earlier standards already existed: ISO 3664, Viewing Conditions for the Illumination of Color Transparencies and Reproductions, and PH2.30-1989, Viewing Conditions for Printed Matter, Photographs, Color Prints and Projected Laser Imaging Reproductions. Their recommendations are similar but not identical. Revision of the new ISO 3664 began at the end of the year. According to the association, the new edition includes all required recommendations. In print reproduction, stable and consistent viewing conditions depend on key factors: the spectral energy distribution of the light source; its luminous intensity and uniformity; the viewing environment (covering both viewing and illumination conditions); and the stability of the illumination environment. The new version also recommends conditions for viewing images on color displays and for exhibitions and appreciation of images, and applies to both the printing and photographic industries.

The updated ISO 3664 addresses key items such as reference illumination, color difference, CIE 13.2 metamerism index, CIE 51, reflectance of ambient illumination, and viewing conditions for print appraisal and comparison. It recommends CIE D50 as the basic viewing illuminant, specifying illumination as daylight spectral distribution with blue, green, and red energy distribution close to equal energy. Lamp makers have tried various phosphors and filters to produce D50-like sources, but no lamp matches D50 exactly; the standard therefore provides parameters to compare real viewing sources with the theoretical source. In older standards, matching was specified mainly by color rendering index, which reflects how similarly a set of standard color patches appears under a theoretical source and an actual viewing source. For colors with spectral emission curves—provided gloss effects do not interfere—the older standard could still apply. However, color imaging and proofing technologies such as dye sublimation, thermal transfer, inkjet, and electrostatic imaging are increasing color contrast. The primary colorants they use often show abrupt changes in reflectance curves when matching photographic or ink-printed colors, creating metameric matches that are highly sensitive to lighting changes. Color rendering index alone is therefore no longer sufficient. The new standard retains CRI while adding two parameters—visibility and color-change index—drawn from light-source quality evaluation methods dating from 1981, to distinguish actual sources more strictly from the ideal CIE D50 illuminant. Viewing equipment compliant with the new standard should provide more stable conditions, reducing problems caused by new materials or by the sensitivity of paper and ink to fluorescent lamps. Preliminary tests indicate that most viewing devices considered good under the old standard also meet the new one; early devices may have issues and need updating, so they cannot yet show which recommended criteria are workable. The GATF/RHEM light source indicator can warn when viewing conditions are incorrect; installing it directly in the working environment makes it convenient to alert staff, though it does not confirm that viewing conditions are correct. The new standard recommends two illuminance levels: high illuminance (P1) of 2000–500 lx for evaluating and comparing images, and low illuminance (P2) of 500–25 lx for tone viewing that simulates conditions closer to final observation. The low-illuminance recommendation has stirred debate, with concerns that too many illuminance recommendations may cause confusion; a note was therefore added after the standard: Note: 'emphasizing comparison of image detail effects'... (original was too long; excerpt here).