SPIE DCS Digital Forum: Tri-modal imaging spectroscopy of artworks

30 Apr 2020

Illuminating plenary describes material classification of classical paintings using reflectance and fluorescence imaging spectroscopic techniques.

The identification of materials used in the creation of paintings and illuminated manuscripts can provide conservators with knowledge on how to better preserve objects, and supply scholars with valuable information about the history and origins of the objects. Significant headway has been made with non-invasive imaging techniques in the past decade.

“The title of my talk is Trimodal Imaging Spectroscopy of Paintings," said John K. Delaney, Senior Imaging Scientist at the US National Gallery of Art, Washington, DC,and a plenary speaker at this week’s SPIE Defense + Commercial Sensing Digital Forum and exhibition. He describes recent highlights of the work his group, also featuring Tania Kleynhans and David W. Messinger.

“I will start off by reviewing the three imaging modalities: diffuse reflectance; molecular fluorescence, or that is fluorescence it is emitted by pigments; elemental fluorescence, and using that technique to get at actually the atoms that are present.

“I also explain how we use these different imaging modalities to address questions that have been posed by conservators and art historians, such as what pigments were used in painting this composition? What were the working methods of a given artist? And probably the most fun part is to look for hidden changes in the paintings. That is, did the artist paint over a prior composition?”

'Materials, methods, and hidden changes'

The presentation reviews three imaging modalities employed in the Delaney’s group’s artwork assessment processes: diffuse reflectance imaging spectroscopy (RIS); molecular fluorescence imaging spectroscopy (FIS); and elemental fluorescence (X-ray fluorescence) imaging spectroscopy (XRF-IS). The key applications are in determining the types of artists’ material used and their locations in a work; visualizing artists’methods; and finding hidden changes.

The main optical sensing systems used by the group at the US National gallery of Art in Washington, DC, are a Macro XRF Scanner (specified at 0-26keV, 0.0137keV), which provides the elemental information about material content; and VNIR and SWIR hyperspectral cameras (for RIS purposes), which operate across the range 400-2500nm and provide the molecular information about the artworks.

An arrangement of painting support and movable camera system enables the painting to be scanned (the scientist need to be very careful at this stage because only a small temperature rise in the artwork of a few degrees Celsius is permitted), after which the data points are processed and mapped to yield the composition and reveal potential underpainting and other interesting changes.

What makes Delaney’s presentation come to life is its frequent presentation of some of the National Gallery’s famous works, and how they are inspected. These include: Cosmè Tura’s The Virgin Annunciate (1470), which is pigment-mapped using RIS; and van Gogh’s The Olive Orchard (1889), which is searched for a particular paint “Eosin Lake”, which the artist favored at that time.

Delaney’s piece de resistance is his report of the group's work on The Feast of The Gods, by Bellini and Titian (1514 / 1529), described by the National Gallery as “one of the greatest Renaissance paintings in the United States by two fathers of Venetian art.”

The process of SWIR RIS analysis reveals “Feast” to have undergone a significant background transformation from woodland to dramatic hillside when the Bellini original, intermediately modified by Dossi, was later reworked by Titian.

Other presentations from this week’s SPIE Defense + Commercial Sensing Digital Forum can be watched on the Digital Library, where there are also many thousands more photonics presentations archived.