Printing high quality color is a challenging and complicated process that demands accuracy and consistency. Often, color communication problems arise due to incorrect or inconsistent lighting and viewing conditions that are used during the color reproduction process. There are many factors that influence the way a given color is perceived. As humans, we all perceive color differently due to the complex nature of the human eye. When this fundamental factor is combined with the significant impact physical viewing conditions have on color appearance, any variance in lighting and viewing environment can produce undesirable results. Requiring all participants in the color communication chain to view color in controlled viewing conditions (designed to meet the industry standard) is the best way to guarantee optimum color reproduction. When this goal has been achieved, viewing environments should be regularly maintained to make certain they are in compliance with the standard. If components in the system are not operating a full potential, accurate color viewing cannot be achieved.

　　ISO 3664:2000 is the International standard for graphic technology and photography that was established to combat color communication problems. Frustrating and costly errors can result when deviation from the standard occurs. The standard requires viewing conditions to meet stringent specifications including color quality, light intensity, illumination evenness, viewing/illumination geometry, and surrounding environmental conditions.  All can affect the appearance of color.

　　Illuminant Definition

　　The viewing illuminant specified in ISO 3664:2000 is CIE Illuminant D50 which defines the spectral energy distribution of daylight with a color temperature of 5000 Kelvin. D50 was originally picked because it is the closest illuminant to having equal red, green, and blue spectral energy. Unfortunately, there are no lamps that can exactly match CIE D50.

　　In the current standard, the basic definition of “goodness of the match" to D50, is a CIE procedure called Color Rendering Index or CRI. The CRI is a unit of measure that defines how well colors are rendered in a given illuminant as compared to the colors that are rendered using a theoretically perfect D50. The CRI is rated on a scale of 0-100. A rating of 100 indicates that all color samples illuminated by the light source in question, would appear no different as those same samples illuminated by a reference illuminant.

　　Intensity Levels

　　When "viewing of photomechanical reproductions, photographic prints, and other color reflection materials", the current ANSI standard specifies 2200 ± 470 lux. The ISO standard calls for an illumination level of 2000 ± 500 lux for "comparison viewing" of reflection prints. The new standard proposes a level of 2000 ± 500 lux "for the critical comparison between two or more copies of an image." It goes on to say that these comparisons may be between an original and a reproduction, a proof and a print, or multiple prints. ANSI is a U.S. organization which has agreed to assist in revising the ISO standard in hopes that the revised ISO standard will be adopted as the new ANSI standard

　　The ANSI standard also makes provision for a level of 800 lux for the "display, judging, or routine inspection of photographic color prints." The new standard recognizes that for practical appraisal of the appearance of an image all by itself, 2000 lux is too high of an illumination level. Most office and home environments are somewhere between 300 and 800 lux. For practical appraisal, the new standard is suggesting a level of 500 ± 125 lux with exactly the same conformance to D50 that is specified for the comparison of images.

　　Monitor Viewing Conditions

　　Up until now, the industry has not had any recommended conditions for viewing images on a monitor. This was not a problem as long as people were not trying to make final judgments based on a soft display. Certainly, a first step in attempting to achieve some degree of consistency is to define a recommended viewing condition.

　　The current draft of ISO 3664 contains specifications for the viewing of a monitor independently of any form of hard copy. Thus, these specifications can be seen as being primarily relevant where successive viewing of hard copy and soft copy takes place. ISO 12646, Graphic Technology - Color proofing using a Color Monitor, currently being developed by TC130, will hopefully provide a more detailed recommendation where direct comparison of prints is required.

　　The chromaticity of the white displayed on the monitor should approximate a white point of D65. The luminance level should be greater than 100 cd/m2 and shall be greater than 75 cd/m2 (When a standard says “should”, it’s a strong recommendation; “shall” means the requirement must be met). Unfortunately, these are brightness levels that many monitors cannot achieve while also achieving a white point of D65. Monitors have a hard time producing enough red output. To keep the red, green, and blue in balance for D65, they must lower the overall brightness. For most monitors, brighter output produces a bluer output (higher color temperature of the white point).

　　The standard specifies the ambient illuminate should be less than 32 lux and shall be less than 64 lux. The standard also specifies both surround and environmental conditions. A dark gray surround on the monitor is specified which should be approximately the same chromaticity as the white point of the monitor.

　　The environmental conditions are: "The monitor shall be situated so there are no strongly colored areas (including clothing) directly in the field of view or which may cause reflections in the monitor screen. Ideally, all walls, floors, and furniture in the field of view should be gray and free of any posters, notices, pictures, wording, or any other object that may affect the vision of the viewer."

　　Many of the issues included here are rarely monitored today, yet our industry continues to fight color reproduction on a daily basis. Having clearly defined goals which conform to established standards should allow anyone to move ahead in the communication of the appearance of color images and their reproduction in the final print.

[时间：2007-10-19  作者：Johnny Shell  来源：信息中心]