DAVID Q. McDOWELL
As we become more dependent on digital imaging and color management, the interrelationship of the standards behind tools such as densitometers, colorimeters, and viewing booths becomes more critical (see Fig. 1). The standards that define colorimetric and densitometric measurements, viewing conditions, and color-management profile building need to be consistent with each other and consistent with current industry practice. It goes without saying that these standards must also be technically sound.
For a variety of reasons (some of them historical) the responsibilities for key standards are spread across several ISO (International Organization for Standardization) groups. Viewing conditions (ISO 3664) and densitometry (ISO 5) are the responsibility of ISO TC (technical committee) 42, Photography. Colorimetric measurements are defined in ISO 13655, which is the responsibility of ISO TC 130, Graphic technology. Color management profiles are defined in ICC.1 and ISO 15076 which are the responsibility of the International Color Consortium (ICC) and TC 130 (see "The official names of the standards" below). All of these documents are currently in review or revision with the goal of achieving consistency among these documents as well as with industry practice. However, as we begin the process we find that achieving these goals is easier said than done.
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The official names of the standards
ISO 5-1:1984, Photography--Density measurements--Part 1: Terms, symbols and notations
ISO 5-2:2001, Photography--Density measurements--Part 2: Geometric conditions for transmission density
ISO 5-3:1995, Photography--Density measurements--Part 3: Spectral conditions
ISO 5-4:1995, Photography--Density measurements--Part 4: Geometric conditions for reflection density
ISO 3664:2000, Viewing conditions--Graphic technology and photography
ISO 13655:1996, Graphic technology--Spectral measurement and colorimetric computation for graphic arts images
ISO 15076-1:2005, Image technology colour management--Architecture, profile format and data structure--Part 1: Based on ICC.1:2004-10
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The issues
The standards for colorimetry, viewing conditions, and profile building represent the core of color-management and color-data exchange. Although densitometry is a separate issue related to process control, there is a strong desire to maintain consistency between colorimetry and densitometry because users increasingly want to compute both density and colorimetry from the same measurement of spectral reflectance.
The real riddle is how to reconcile the standards with each other and with industry practice, while keeping them technically sound. When the colorimetric measurement standard and the current version of the viewing standard were created in the mid-to-late 1990s, the use of colorimetric measurements was in its infancy in the graphic arts. In addition, the viewing-condition standard had been the exclusive prerogative of the photographic standards community and had not been updated in 25 years. Industry practice was inconsistent. Getting workable documents out was the first priority.
Today, digital proofing and color management are widely used and we are learning some of the issues related to their use. Some of these issues are directly related to how we make measurements and how we view the printed/proofed products that are produced. The issue of proof-to-print comparison and the desire for metrology-based certification of proofing systems also poses a significant part of the puzzle, whose pieces might be described in terms of several primary issues.
Backing
The question of which backing to use when making spectral reflectance measurements to compute densitometry and colorimetry has many answers depending on the application and perspective of those using the data. Densitometry standards all call for a black backing to minimize the impact of back-printing, to minimize variability due to translucency effects, and to avoid problems associated with local variations in opacity and backing uniformity. On the other hand, the color-management world finds more consistent results between visual comparisons of proof and print when using profiles based on white or self-backing. On top of that, everyone would prefer to be able to compute density and colorimetry from the same set of spectral data without having to measure everything twice.
The existing viewing standard specifies viewing over black to achieve consistency with the measuring standard and minimize any effects of printing on the back side of the material being examined. However, a black backing is rarely used in practice for evaluating proofs and printed sheets.
The most significant effect of black backing, particularly on a slightly transparent substrate, is to reduce the color gamut (including the dynamic range) compared to that achieved with white backing. This is seen in both -viewing and colorimetric measurements. Although it can be argued that this shouldn't affect the use of a color management profile, in practice, the experience of many ICC members is that it is better to use a white backing for measurements of data used to create color-management profiles.
An additional complicating factor is that proofs are often made on a substrate that is less transparent than the media used for the actual printing. If a proof and a print are being compared, the judgments made can differ, depending on the backing used and the difference in transparency of the two materials--this is not a good situation.
The bottom line issue is that the differences seen or measured on typical printed material between black and white backing is greater than the acceptable tolerances for proof to print acceptability--we must pick one or the other.
Illumination
Another area that presents difficulties is the spectral power distribution of the illumination used for measurement and viewing. We usually identify illumination (and monitor white points) by its equivalent color temperature--D50, D65, and so on. However, this only takes into account one characteristic of the illumination. The spectral power distribution of the illumination specifies the amount of energy at each wavelength, including energy outside of the visible portion of the spectrum. Two illuminants with the same color temperature can have significantly different spectral power distributions. The dyes and pigments used in proofing systems often do not match the spectral reflectance of the printing inks. Therefore, differences in the spectral power distribution of viewing illumination (even when it meets the color temperature criteria) will change the match of proof and print.
Even more critical is the use of brightened papers and/or fluorescent inks. Their effect is directly proportional to the amount of UV light present, and can be dramatic. The current viewing standard defines the spectral power distribution of the reference D50 illuminant in both the visible and in the UV. It also has test criteria for how closely a viewing booth should match the specified spectral power distribution in both the visible and UV. Unfortunately, few systems match the UV criteria because it is difficult to produce illumination systems with the correct UV content.
The measurement standards state: "If the materials do not fluoresce, the spectral power distribution of the measurement source is not a concern and so no specification is given for the conformity of the spectral power distribution of the measurement source to the illuminant specified. . . ." They go on to say that if the materials do fluoresce then the spectral power distribution of the measurement source should match D50 for colorimetry and illuminant A for densitometry. The illumination source in most spectrophotometers is designed for efficiency, long life, and stability--not to match either D50 or illuminant A. However, some do include a UV cutoff filter and/or a filter that provides some enhancement of the UV but not to the extent that it matches D50.
Because it is very difficult, if not impossible, to properly match the specified UV content of a D50 illuminant in a spectrophotometer, some have suggested that to get better agreement between instruments, all colorimetric measurements should be made without any UV present at all. That would also work for densitometry because illuminant A has little UV.
If viewing and measurement are expected to match, however, then all UV should be blocked in the viewing booth. If we block UV in viewing, the effect of paper brighteners disappears--but the client is paying for brightened paper and the customer (at least in some viewing environments) is seeing the effect of brightened paper (or why else would the client be paying a premium for it?).
Others suggest that some UV, but not as much as D50 specifies, is a more manageable goal. But there are no real suggestions as to how much is enough. Bottom line is that if we are going to use brightened papers (and/or inks that fluoresce), the amount of UV present in both viewing equipment and measurement equipment must have some degree of consistency.
Another option offered is that for subjective customer evaluation of the finished product, illumination with UV present should be allowed to show the effect of paper brighteners. All other applications such as metrology and proof to print comparisons should use a UV cutoff filter.
The bottom line questions with respect to illumination are: Is it important for the illumination for viewing and measurement to match and is it important for viewing to show the effect of paper brighteners? It is not clear that we can have both.
Where are we headed?
Joint Working Groups (JWGs) have been formed by both TC130 and TC42 to revise the viewing, color measurement, and densitometry standards. These involve members of TC42, TC130, and the ICC, as well as other industry experts. These groups are meeting at the same time and location to facilitate the interchange of ideas and help ensure consistency between the standards.
The revised documents will likely place greater emphasis on practical implementation than on theoretical "correctness" and/or historical consistency. Significant weight will be given to input from the color-management community, which didn't exist when these standards were initially created. The color-management community, the printing and publishing community, and photographic community increasingly find that they must share common solutions to allow interoperability across the digital image space, which encompasses all three application areas (see Fig. 2).
The next few months will be a period of hard work and exciting opportunities for creative solutions to the conundrum of standards versus practical realities in this area. Those interested in participating in this adventure are invited to contact the author for information.
REFERENCES
Existing versions of ISO standards are available from any national standards organization and:
1. www.iso.org/iso/en/CatalogueListPage.CatalogueList
2. webstore.ansi.org/ansidocstore/default.asp
3. ISO 3664, 13655, and 15076 are also available from: www.npes.org/standards/orderform.html
DAVID Q. MCDOWELL is a standards volunteer for NPES--The Association for Suppliers of Printing, Publishing, and Converting Technologies--51 Parkwood Lane, Penfield, NY 14526; e-mail: [email protected].