44th Annual Meeting, May 16, Research and Technical Studies, Visible-Induced Luminescence Imaging: Past, Current and Future Applications in Conservation Research, Dawn Kriss and Anna Serotta

I was excited to see the most recent update on VIL imaging as it is an accessible imaging technique that can be used to localize pigments with specific characteristics. It is useful for anyone interested in painted surfaces, and can be used in conjunction with other multispectral imaging, or as a standalone technique.
The basic idea is that you need a light source to produce visible light, a camera with its infrared filter removed, and a bandpass filter to limit the type of light that gets to the camera sensor, along with some standards to help process the images. The pigment particles on the object are excited in the visible range, and emit infrared radiation which is detected by the modified camera. This technique can be used to detect trace remains of pigments that are all but undetectable to the naked eye. The technique was developed by scientists from the British Museum and the Courtauld Institute (see Verri et al., 2009) [1].
In the case studies shown in Dawn and Anna’s presentation the focus was on Egyptian blue, which produces luminescence in the infrared (~910nm) when exposed to visible light. Optimizing the capture and processing protocols will mean better results and hopefully, a means of standardizing and sharing information between conservators working in different labs. While VIL is gaining popularity as more museums add it to their workflow (for example. as part of the APPEAR project spearheaded by the Getty), the technique is still being developed, with much more progress on the horizon. Dawn and Anna reported on results of a survey of VIL users to show where progress has been made and where we can still expect some improvements in the technique.

gif showing VIL and normal illumination images of Brooklyn Museum's portrait, Noblewoman, ca. 150 C.E. Encaustic on wood. Brooklyn Museum, Gift of the Ernest Erickson Foundation, Inc., 86.226.1, image courtesy Brooklyn Museum — Click for larger image and to view transition! A gif showing VIL and normal illumination images of Brooklyn Museum’s portrait, Noblewoman, ca. 150 C.E. Encaustic on wood. Brooklyn Museum, Gift of the Ernest Erickson Foundation, Inc., 86.226.1, image courtesy Brooklyn Museum

Capture: varying light sources

There are many options for lights used for excitation, so choosing a light source that is targeted to your research question is critical. As an example, the authors described work by my classmate Brian Castriota showing that red LEDs with an output centered at 630 nm caused greater luminescence of Egyptian blue compared to white LEDs. More research on the luminescence characteristics of other pigments will help users optimize their light sources to target specific pigments.

Processing: calibration, standards, and protocols

While many conservators using VIL use the CHARISMA protocols (developed by the British Museum), others are using Photoshop to process the images. Egyptian blue VIL images are usually shown in monochrome, but as the technique is expanded different overlays or crossfades will help communicate the results by registering the images with other photographs, as Dawn and Anna did for the images shown in their presentation. This is one of the greatest advantages of VIL: it’s very easy to understand the images that are generated and easy to communicate the results to the public. However, capturing good metadata and using appropriate standards are critical for the intercomparability of these data in the future. It will be crucial to develop a luminescence scale or target in order to compare images from institutions who may not be using the exact same capture or processing parameters.

What do we have to look forward to?

While its initial development as a tool for identifying Egyptian blue has led to its popularity among archaeological conservators, it seems like the technique is ripe for more widespread adoption for research into modern pigments, some of which also have unique luminescence properties.
Conservators can use a variety of wavelengths using targeted or tunable light sources (e.g. the CrimeScope, adapted from the forensics field) to survey visible-induced luminescent pigments (other examples of which include dragon’s blood, Indian yellow, Han blue, cadmium red and yellow). Dawn and Anna showed an example of imaging surveying cadmium pigments used in Stuart Davis’s Mellow Pad carried out by their Brooklyn colleague Jessica Ford. For more on the work from the team at the Brooklyn Museum, see their recent blog post here.

References.

[1] Verri, Giovanni, et al. “Assyrian Colours: Pigments on a Neo-Assyrian Relief of a Parade Horse.” The British Museum Technical Research Bulletin 3 (2009): 57–62.