Analytical Techniques: Additional Techniques

Back to the Paintings Page

Back to Main Catalogs Page

The information presented on the Paintings Conservation Wiki is the opinion of the contributors and does not imply endorsement or approval, or recommendation of any treatments, methods, or techniques described.

Colorimetry

Author: Elizabeth Robson

Editors: Erin Stephenson, Kari Rayner

Colorimetry is a branch of color science used to numerically describe the color of a target area. The standards are defined by the CIE (the Commission Internationale de l’Eclairage, or the International Commission on Illumination), whose diagrams are based on normalized illuminants and an average observer (Johnston-Feller 2001). The most commonly used standard currently is the CIE 1976 L*a*b* colorimetric space (Figure 1). The “language” of colorimetry is used to unambiguously define the perceptual dimensions of color using three independent variables: hue/tint, saturation, and lightness (Johnston-Feller 2001). Practically, these variables are measured using a non-destructive analytical technique with a small, handheld device to obtain spectral reflectance curves (Figure 2). A standard sample, such as a tablet of barium sulphate or a disk of opal glass, should be used to calibrate the device (Lorusso et al. 2008). Templates can be created using mylar or paper and circular punches to ensure that the exact same surface area is measured each time. Once two or more measurements have been made of a surface area, the change in L* (lightness), a* (red-green shift), and b* (yellow-blue shift) values can be compared, and the ΔE (total color difference) can be calculated using various formulas (Johnston-Feller 2001, Lorusso et al. 2008). These colorimetric measurements can be used to monitor changes in color during treatment or display; quantify discoloration of media, including restorations, over time; and occasionally to characterize and identify pigments (Marchiafava et al. 2014, Johnston-Feller 2001).

• 

  Figure 1: Representation of the ΔE in the chromaticity diagram L*a*b* (Lorusso et al. 2008). In this diagram, the target color (A) refers to what would be considered the original measurement, while the “champion” (B) refers to a second measurement taken later in time.

• 

  Figure 2: Spectrophotometric colorimeter Minolta CM-2600 D model (Lorusso et al. 2008).

References

Johnston-Feller, R. 2001. Color Science in the Examination of Museum Objects: Nondestructive Procedures. Tools for Conservation. Los Angeles, Getty Conservation Institute.

Lorusso, S., A. Natali, and C. Matteucci. 2008. “Colorimetry applied to the field of cultural heritage: examples of study cases.” Conservation Science in Cultural Heritage 7 (1): 187-220.

Marchiafava, V., G. Bartolozzi, C. Cucci, M. De Vita, and M. Picollo. 2014. “Colour measurements for monitoring the conservation of contemporary artworks.” Journal of the International Colour Association 13: 36-42.