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Home > News & Events > Fiber Optic Reflectance Spectroscopy for Pigment Identification

Fiber Optic Reflectance Spectroscopy for Pigment Identification

Optical sensing techniques are useful for researchers, archaeologists and other art investigators seeking to learn more about the heritage of artwork and artifacts. For example, researchers can use reflectance or fluorescence spectroscopy to measure pigments used in paintings, which help to identify the era in which a painting originated. Similar techniques can be applied to the testing of media used in aged documents, parchment and textiles.

Physicist and cultural heritage scientist Antonino Cosentino is a proponent of innovative and affordable instrument solutions for technical examination and documentation of art. Dr. Cosentino provides art documentation, training and consultation for both professionals and institutions through his Cultural Heritage Science Open Source blog.

In this first-person account, Cosentino describes his work using Ocean Optics spectrometers and accessories for reflectance measurements of historical pigments. Some of his comments have been edited for length and clarity.

Figure 1: Fiber optic reflectance spectroscopy (FORS) is a useful technique in identifying pigments found in works of art.

Figure 1: Fiber optic reflectance spectroscopy (FORS) is a useful technique in identifying pigments found in works of art.

FORS (Fiber Optics Reflectance Spectroscopy) has been used for art examination and art conservation for at least two decades for identification of pigments [1, 2, 3, 4] and dyestuffs [5]. The strength of this method is the relative simplicity of the equipment: a source of light, a spectrometer and two fiber optics — one to deliver the light on the object and the other to collect the light reflected. A reflectance spectrum shows for each wavelength the ratio between the intensity of the reflected light and the intensity of the incident light.

I had been following the development of a series of miniaturized spectrometers by Ocean Optics, a leader in innovative optical systems. For my daily work in on-site art examination I’m always looking for new low-weight solutions, and these instruments looked exactly as what I was searching [for].

I wrote to Ocean Optics and they put me in contact with their Italian representative, GHT Photonics, based in Padua, Italy, the town where Giotto [a late 13th-early 14th century Florentine painter and architect] decorated the Scrovegni Chapel, one of his most famous frescoes. GHT was helpful in arranging for me to have a look at their low-cost FORS system at the Cultural Heritage Science Open Source lab in Sicily. I was so delighted by this system. Ocean Optics has made a kind of revolution in spectroscopy: miniaturized and low-cost spectrometers [that are] really, really affordable.

[Miniature spectrometers] open possibilities to researchers and private professionals that can now introduce optical spectroscopy into their workflow. Indeed, with just one spectrometer it is possible to run a different number of spectroscopic methods, just [by] changing lamps and probes. The same spectrometer can be used for reflectance spectroscopy and transmittance spectroscopy as well as fluorescence spectroscopy. So, just purchasing one spectrometer and using some of our geekiness with optical tools we can run a number of art examinations. Should I use one word to describe this system, I would say FUN!

Editor’s Note: Dr. Cosentino has generously shared photos and video of the FORS system he’s using, as well as the results of testing he’s conducted in Sicily on his collection of historical pigments and on a number of art objects. Click on each image to enlarge its view.

Figure 2: A system comprising a miniature spectrometer, tungsten halogen light source, integrating sphere and optical fibers was used to measure reflection of pigments mixed with various binders.

Figure 2: A system comprising a miniature spectrometer, tungsten halogen light source, integrating sphere and optical fibers was used to measure reflection of pigments mixed with various binders.

Figure 3: Burnt umber pigment, a natural brown color, has its highest reflectivity in the shortwave NIR.

Figure 3: Burnt umber pigment, a natural brown color, has its highest reflectivity in the shortwave NIR.

Figure 4: Red ochre pigment is another earth color that was tested.

Figure 4: Red ochre pigment is another earth color that was tested.

Figure 5: Yellow lake pigments are derived from organic dyestuffs.

Figure 5: Yellow lake pigments are derived from organic dyestuffs.

Figure 6: Chrome green is an inorganic pigment with olive-green color.

Figure 6: Chrome green is an inorganic pigment with olive-green color.

Figure 7: As anticipated, blue bice pigment has its strongest spectral response in the blue spectral region.

Figure 7: As anticipated, blue bice pigment has its strongest spectral response in the blue spectral region.

Figure 8: Ivory black pigment has a spectral response that is relatively “flat” from 450-950 nm.

Figure 8: Ivory black pigment has a spectral response that is relatively “flat” from 450-950 nm.

Figure 9: The spectrum for gypsum pigment varies dramatically depending on binder mixture.

Figure 9: The spectrum for gypsum pigment varies dramatically depending on binder mixture.

References

[1] T. Cavaleri, A. Giovagnoli, M. Nervo “Pigments and Mixtures Identification by Visible Reflectance Spectroscopy,” Procedia Chemistry 8 ( 2013 ) 45 –54.

[2] E. Cheilakou, M. Troullinos, M. Koui “Identification of pigments on Byzantine wall paintings from Crete (14th century AD) using non-invasive Fiber Optics Diffuse Reflectance Spectroscopy (FORS),” Journal of Archaeological Science 41 (2014) 541–555.

[3] M. Picollo, M. Bacci, A. Casini, F. Lotti, S. Porcinai, B. Radicati, L. Stefani “Fiber Optics Reflectance Spectroscopy: A Non-destructive Technique for the Analysis of Works of Art,” Optical Sensors and Microsystems: New Concepts, Materials, Technologies, edited by Martellucci et al., Kluwer Academic/Plenum Publishers, New York, 2000.

[4] Scientific examination for the investigation of paintings, AA.VV. Edited by D. Pinna, M. Galeotti, R. Mazzeo, 2009.

[5] M. Gulmini, A. Idone, E. Diana, D. Gastaldi, D. Vaudanc, M. Aceto “Identification of dyestuffs in historical textiles: Strong and weak points of a non-invasive approach,” Dyes and Pigments 98 (2013) 136–145.