6 MAINTAINING LONG-TERM ACCURACY OF PRINT MONITORING SYSTEMS

THE MOST DIFFICULT aspect of a monitoring program is maintaining long-term system calibration; the ultimate usefulness of a monitoring program will be determined by the degree of accuracy of initial and future instrument calibrations. In fine art applications, one must be able to measure accurately very small changes in the visual characteristics of color prints.

The calibration system must not be affected by changes in color densitometers or in other measuring equipment. It is obvious that present equipment and filters will become obsolete and will be replaced many times during the next several hundred years with new optical density measurement instruments. As two different color densitometers may give significantly different readings from the same print samples (Table III), the data obtained with one piece of equipment will have to be translated accurately to permit comparison with readings taken with the other instruments.

TABLE III

A specific densitometer should be set aside for use in the monitoring program, and it should not be used for any other application. The densitometer should be used in an environment with constant temperature and constant relative humidity and should be allowed to warm up until calibration readings stabilize (depending on the type of densitometer, an adequate warm-up period may range from 30 minutes to about 12 hours). Before density readings are made of each print (or each fading monitor), the densitometer should be carefully calibrated using the fired porcelain enamel plaque supplied by the manufacturer of the instrument. In addition, before each measurement session, or at least once each day the densitometer is used, the accuracy of the densitometer should be checked with a Kodak Reflection Densitometer Check Plaque;14 this plaque will indicate changes in spectral response of the densitometer that result from fading of the filters or from other causes. The Kodak plaque should be permanently assigned to the densitometer and should be stored carefully between uses.

In addition, measured gray scales and color scales made of each type of color photographic material in the collection should be kept in cold storage at − 18�C (0�F) or lower and at 30% relative humidity; very low-temperature storage will reduce changes in these photographic calibration standards to a negligible amount during the next 1,000 or more years, according to current estimates based on accelerated test data.15

A Macbeth ColorChecker16 is recommended as a suitable original gray scale and color patch image for use in preparing the photographic material calibration standards; the standards should be printed in a size of about 7.8 � 12.7 cm (3 � 5 inches), including a border of at least 1 cm (⅜ inch) to protect the image area. All of the color patches, including those with low-saturation colors, should be read and the data recorded. While the porcelain calibration plaques will generally be adequate for the continued calibration of a specific densitometer, their use will not necessarily produce accurate readings after an instrument has been repaired or if the filters have faded or have been replaced; instrument response may also change if the light sensor has been replaced. Each dye of each type of color photographic print has a distinct set of spectral absorption characteristics; for this reason, any change in the spectral response of a densitometer will produce different readings from a given photographic sample even if the instrument has been calibrated with a porcelain plaque.

When the color photographic calibration standards are needed, they should be removed from cold storage and the packages containing the standards should be allowed to reach room temperature before they are opened. The densitometer should be calibrated with the porcelain plaque, and readings should then be taken, with the color photographic calibration standards and numerical conversions determined for each gray scale density and color patch of each material; use of these conversions will allow continued accuracy of the overall system during the lifetimes of the color prints. In future years, most institutions will have computer-based cataloging and data storage systems; the densitometer can be interfaced directly with the computer, and a program can be devised for automatic conversion of current data to a form that can be compared directly with the initial and subsequent densitometric readings for an original print or fading monitor (Fig. 5).

Fig. 5. Density measurements of color print being read with color densitometer (left). Data is entered into microcomputer (center) and recorded on a magnetic “floppy” disc using computer disc drive (right). The computer can be programed to analyze data in terms of the criterion specified for the densitometric limits of change for color photographs.

While other long-term calibration procedures may be devised in the future, the author believes that at present, long-term system accuracy can be maintained only by the use of preserved photographic calibration standards for each type of material in a collection. It is presumed that in the near future most institutions with significant color photographic collections will install humidity-controlled cold-storage facilities for the preservation of their color collections;17 the calibration standards can be kept in these cold-storage areas. If humidity-controlled cold storage is not available, the photographic calibration standards should be sealed in suitable vapor-proof packages18 and should be placed in a normal − 18�C (0�F) household-type freezer.