PMG Section 1.5.1 Monitoring Equipment
Photographic Materials Conservation Catalog
Exhibition Guidelines for Photographic Materials
Date: July 2004
Contributor to WIKI version: Your name could be here!
Compiler: Stephanie Watkins, 1993-2004
Initiator: Douglas Severson, 1992-1993
Contributors (Alphabetical): Catherine Ackerman, Nancy Ash, Sarah Bertalan, Jean-Louis Bigourdan, Barbara N. Brown, Ed Buffaloe, Carol Crawford, Corinne Dune, Thomas M. Edmondson, Debra Evans, Julia Fenn, Betty Fiske, Gwenola Furic, Judy Greenfield, Doris Hamburg, Marc Harnly, Pamela Hatchfield, Cathy Henderson, Nancy Heugh, Ana Hofmann, Emily Klayman Jacobson, Martin Jurgens, Nora Kennedy, Daria Keynan, Lyn Koehnline, Barbara Lemmen, Holly Maxson, Constance McCabe, John McElhone, Cecile Mear, Jennifer Jae Mentzer, Jesse Munn, Rachel Mustalish, Douglas Nishimura, Leslie Paisley, Sylvie Penichon, Hugh Phibbs, Dr. Boris Pretzel, Dr. Chandra Reedy, Nancy Reinhold, Andrew Robb, Grant Romer, Kimberly Schenck, Douglas Severson, Tracey Shields, Angela Thompson, Sarah Wagner, Clara von Waldthausen, Dr. Mike Ware, Stephanie Watkins, Dr. Paul Whitmore, Faith Zieske, Edward Zinn.
First edition copyright: 2004. The Photographic Materials Conservation Catalog is a publication of the Photographic Materials Group of the American Institute for Conservation of Historic and Artistic Works. The Photographic Materials Conservation Catalog is published as a convenience for the members of the Photographic Materials Group. Publication does not endorse nor recommend any treatments, methods, or techniques described herein.
Visual inspection of photographs may detect changes and damage but it is unreliable. Perceptions of color cannot be easily remembered and with increasing age, one's ability to see contrast and to distinguish between subtleties in color change diminishes. Also, perception of color varies with lighting conditions (color temperature and lumen level): the lower the light levels, the greater the change must be before it can be perceived. Changes to a photograph may be very slow to become visibly noticeable. Frequent checking of materials for changes might catch drastic fading or color shifts, but checking is time consuming. Relying on visual observation to detect changes is to wait until the change is significant.
The conservation community has not yet agreed upon one monitoring standard for photographic materials. It is confusing to compare data from various monitoring machines and methods. With all methods, it is necessary to make a template for each photograph in order to orient the machines to the same site, before, during, and after exhibition.
The reflection densitometer measures the amount of light reflected from the surface of an image and gives a numerical set related to the densitometry standard used in the photographic industry. Consult the most current ANSI/ISO standards, including:
- ANSI/ISO 5-2: 1991, ANSI/NAPM IT2.19-1994 (Photography --Density Measurements --Part 2: Geometric Conditions for Transmission Density);
- ANSI/ISO 5-3: 1995, ANSI/NAPM IT2.18-1995 (Photography --Density Measurements --Part 3: Spectral Conditions);
- ANSI/ISO 5-4: 1995, ANSI/NAPM IT2.17-1995 (Photography --Density Measurements --Part 4; Geometric Conditions for Reflection Density).
- ANSI/NAPM IT9.9-1996 (Imaging Materials-Stability of Color Photographic Images -Methods for Measuring). Corresponding ISO document is ISO 10977:1993.
The advantages of densitometers include ease of use, simple and reliable calibration, and that computer programs are not required for interpretation. Densitometers are the least expensive of current choices and are the standardized equipment found most commonly in museums and other institutions. Current stability standards are written in terms of density change and a body of readings has been developed. The machines are good at comparing between samples at anyone time and for monitoring changes in an image area, although less so in white areas. Densitometers provide quantitative proof of change that custodians can follow much more easily than other types of instrumental monitoring equipment. Densitometers are easy to transport, an important consideration when monitoring is a requirement for traveling exhibitions.
The disadvantages of densitometers include data limitations. They measure only the ratio of reflected light to incident light (a single quantity) over a broad spectral range. Densitometers do not relate changes to our perception as do colorimeters. Densitometers cannot detect changes in chromaticity and assume dye loss to be directly proportional to density loss. Regular maintenance (refurbishment and recalibration) and replacement of light source and filters is necessary to ensure quality of data. The reference tiles and filters need to be replaced every few years because they are not stable (Wilhelm 1993, 247). Densitometers have poor inter-instrument agreement; two densitometers may give different readings for the same sample.
Colorimeters measure reflected light using red, green, and blue electronic photoreceptors (filtered sensors) and convert these tristimulus values to the chromaticity components of color (e.g., CIE, L* a* b*).
The advantages of colorimeters begin with ease of use. One reading incorporates hue, lightness, and chromaticity. Samples can be compared and the machines are good at detecting color shifts of "white" areas. The data format makes it easy for custodians to perceive visible change and provides quantitative proof of change that custodians can follow. Recently, more software has become available and some newer machines come with internal programs, making them simpler to use. More recently designed machines are easy to transport, an important consideration for monitoring traveling exhibitions. Colorimeters are widely used in the color industry and already used by paper and paintings conservators making them potentially available in nearby labs.
The disadvantages of colorimeters begin with the complexity of the machine that can be intimidating or confusing to learn to use. The interpretation of data is more complex than with densitometers and spectrophotometers, especially for black-and-white prints. Calibration in older machines may be difficult whereas newer models are straightforward and no more difficult or unreliable than are densitometers. Regular maintenance (refurbishing and recalibration against a standard) and replacement of the light source are necessary to ensure quality of data. Reference tiles must be replaced every few years. "Reference readings" need to be established before and after any maintenance or repair to detect any large reading shifts. Colorimeters have poor inter-instrument agreement; two colorimeters may give different readings for the same sample. The lack of standard observer angle, geometry, or illuminant can lead to varying results among operators or readings. The cost for a colorimeter is less than a reflection densitometer and more than a spectrophotometer.
The spectrophotometer measures the ratio of reflected to incident light for one wavelength at a time, scanning through the desired wavelength range. The resulting "spectrum" represents the reflectivity of the measured sample at each wavelength. The advantages of spectrophotometers include reliability, maximum sensitivity, versatility and control. Calibration is not necessary with modern, so-called "doublebeam" instruments and data collected can be output in colorimetry units. The machines provide reflectance curves and numerical data as color coordinates. Newer models are easier to travel with than older models, an important consideration for monitoring traveling exhibitions. Spectrophotometers are the best choice for long-term monitoring of in-house collections and exhibits because readings are less prone to differences from differing operator technique or multiple operators.
The disadvantages of spectrophotometers include cost, as these are the most expensive machines of all the current choices. Calibration plates are easily breakable and expensive to replace. However, machines using aluminum oxide plates can be cheaper as rubbing the plates together can clean them. The top surface is removed a little, but can last a long time. Older machines are difficult to transport to multiple sites necessary for monitoring a traveling exhibition. Considerable time is needed to take measurements and generate the data. Taking reliable measurements requires some training and understanding of the science involved. The data provided by the machine is complex and copious. Nonscientific custodians will need interpretation of the data. Finally, few institutions or private conservators can afford to purchase or maintain spectrophotometry equipment.
Statistical analysis of data from monitoring
Error can be random, affecting the precision of the result, or systematic, affecting the accuracy of the result. Operator error, misalignment of the machine, poor instrument precision, and instrument drift contribute to statistical errors. Reduction of errors can be achieved with consistency in measuring, repeated readings of a single site, and realignment of the sample and measuring again. Standard deviation equations can be used to predict random error size. Some statistical calculations (linear regression) can be done on standard computer spreadsheet software. Confidence limits and t-tests are also available in computer statistics packages.
Visible and ultraviolet light meters
Visible and ultraviolet light meters are used to measure ambient and direct lighting levels. Light meters can measure footcandles (fc), lux (Ix), microwatts per lumens (μW/lm), and color temperature (K, degrees Kelvin). Dial and digital liquid crystal displays (LCD) are available. Camera meters can be used as an estimated general measure of lux in an area (CCI 1983).
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