Multispectral imaging with the Phase One camera system

The Phase One Rainbow process at Tripont

As the Eastern European centre of Phase One, Tripont has been developing its cultural heritage digitisation lab and training centre since 2018. In 2021 we purchased the latest multispectral equipment (Phase One iXH 100MP WS) in order to put it at the service of Hungarian scientific life. At the end of March the camera system was commissioned and calibrated, with the participation of the international Phase One team. Those present at the occasion included Gerald Schnittger, the Danish company’s regional manager for Germany, Poul Husum, area sales manager, and Annette T. Keller, an expert in machine vision and multispectral imaging, who with 20 years of experience has contributed to the development of the camera system with the Phase One team since 2017. With her help Tripont analysed the first results.

Phase One Rainbow (source: https://www.fotointern.ch/)

The new “Phase One Rainbow” process makes it possible to use several multispectral imaging technologies in a single camera system. Its essence is that, by using light sources emitting in different light ranges and suitable filters, information invisible to the human eye can be displayed on digital images. For example, with the device, signs of human fluids, fingerprints or the use of weapons can be detected during a crime-scene investigation. The technology can also be used for numerous other purposes, for analysis or authentication in the fields of art history and criminal investigation. Use of the device makes it possible to make texts in poor condition legible, and to analyse polychrome surfaces (paintings, stone, other materials) or any other textile (for example, to determine the material and age in the case of historical research).

During imaging, the filter wheels that are part of the camera system and the various light sources are switched on and off automatically, controlled by the equipment’s own software called Rainbow MSI. In the process, which takes a few minutes, 8 images are produced, described below. The system is able to adjust the focus automatically as a function of wavelength, so imaging proceeds without interruption. With multispectral technology we examine the analysed object by a non-invasive method, that is, without any intervention. The 8 images need to be taken every time, because the human eye cannot decide in which light spectrum the information relevant to the research or authentication process can be found.

Representation of the wavelength of light in nanometres

The first examination

As a test, we placed a painting under the camera system in order to examine the images exposed under different illumination and recorded through light filters. The order of the images is as follows:

• VIS (visible-reflected): imaging under visible-light illumination.
• IRR (infrared-reflected): imaging under infrared illumination.
• IRRFC (infrared-reflected false color): infrared illumination, shown in false colour. (False-colour display means that we represent bandwidths other than the red, green and blue components that make up the image on the display and are visible, so that we can visualise wavelengths invisible to the human eye)
• UVRFC (ultraviolet-reflected false color): ultraviolet illumination, shown in false colour.
• UVL (ultraviolet-induced visible luminescence): imaging of the visible luminescence induced by ultraviolet.
• UVR (ultraviolet-reflected): imaging under ultraviolet illumination.
• UVIL (UV light with IR pass filter): imaging of ultraviolet illumination through an infrared filter.
• VIL (visible-induced infrared luminescence): infrared luminescence induced by visible light

Luminescence is one form of the generation of light. Light emission of non-thermal origin is called by this name. (Wikipedia)

The first image (VIS) is the one containing the visible information. It is a high-resolution reference that helps in analysing and interpreting the results obtained, and it is also of primary importance for creating the false-colour images.

On the image exposed under infrared (IRR) illumination we can observe traces of retouching done in different periods. This phenomenon is explained by qualitative differences in the materials used over time. IRR illumination also makes visible the artist’s preliminary sketches beneath the surface paint layers.

This is followed by the image of the infrared false-colour display (IRRFC). This method can be used to identify overpainting, technical details of restorations carried out in different periods, and the pigments used. IRRFC is used, for example, to distinguish azurite from ultramarine, and to map cadmium pigment (which is still used today for red, yellow and green paints).

Azurite blue pigment (left), ultramarine blue pigment (right)

UVRFC can be used to map, for example, whether the artist used zinc white or titanium white. In the attached image you can see that the white hair takes on a yellowish tint.

The next technology that Phase One’s camera system is capable of is called UVI

L, which records the image under ultraviolet illumination through an infrared filter. In the attached image it is clearly visible that this imaging mode highlights even more clearly the repairs made with materials different from the original.

The image made with the UVL technique records the visible light range while the object is illuminated with ultraviolet light. We illuminate our painting with UV in order to be able to identify organic materials.

UVR displays and images UV light under ultraviolet illumination. This imaging makes visible the varnish or adhesive applied during restoration and added to the picture.

When using VIL (visible-induced infrared luminescence imaging) technology, we record the information in the infrared light range while the object is illuminated by visible light. This method has been used for decades to identify Egyptian blue; in our case it highlights a few of the details already mentioned.

The multispectral imaging system of the Tripont laboratory is ready for use. In the coming period our team of experts will concentrate on perfecting and effectively applying the use of the equipment. We are open to, and excitedly await, our readers’ ideas for development, and we also welcome those interested who would like to think together with us in a joint project.

Author: Sára Szabó

Sources used:

1. Keller-Lenz-Artesani-Mosca-Comelli-Nevin: Exploring the Ultraviolet Induced Infrared Luminescence of Titanium White Pigments. In: UV-Vis Luminescence Imaging Techniques. Ed.: Marcello Picollo, Maartje Stols-Witlox, Laura Fuster-López. Valencia, 2019. 341 pages.

Dyer-Verri-Cupitt: Multispectral Imaging in Reflectance and Photo-induced Luminescence modes: A User Manual. The British Museum, London, 2013. 184 pages.

Vyshnavi Anandan: Roman Egyptian Mummy Portraits. Week 5–Multispectral Imaging. Online. https://archaeologicalmuseum.jhu.edu/the-collection/object-stories/roman-egyptian-mummy-portraits/week-5-multispectral-imaging/ Accessed 7 April 2021.

Joanne Dyer et al.: A multispectral imaging approach integrated into the study of Late Antique textiles from Egypt. Online. 4 October 2018. https://doi.org/10.1371/journal.pone.0204699 Accessed 7 April 2021.

Lumineszcencia [Luminescence]. Wikipedia, the free encyclopedia. Online. https://hu.wikipedia.org/wiki/Lumineszcencia. Accessed: 7 April 2021.

Image sources:

Phase One Rainbow: Multispectral Imaging Solution. Online. https://www.phaseone.com/en/Cultural-Heritage/Rainbow-Multispectral-Imaging-Solution Accessed: 7 April 2021.

Dyer-Verri-Cupitt: Multispectral Imaging in Reflectance and Photo-induced Luminescence modes: A User Manual. The British Museum, London, 2013. 184 pages.

Azurite. Wikipedia, the free encyclopedia. https://en.wikipedia.org/wiki/Azurite#Pigments Accessed: 7 April 2021.

Ultramarine. Wikipedia, the free encyclopedia. https://en.wikipedia.org/wiki/Ultramarine Accessed: 7 April 2021.

External links:

Phase One Rainbow: Multispectral Imaging Solution. Online. https://www.phaseone.com/en/Cultural-Heritage/Rainbow-Multispectral-Imaging-Solution Accessed: 7 April 2021.

Annette T. Keller: ArtImaging: Art Documentation & Multispectral Imaging. Online. https://artimaging.de/ Accessed: 7 April 2021.