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Infrared & Ultraviolet Imaging


Reflected Infrared Digital Photography

Infrared imaging is a non-destructive technique used by conservators to examine paintings and artworks and detect hidden details under the upper layers such as added paint, underdrawings, and hidden signatures or watermarks. The technique has been used to discover details of artists' creative processes, alterations and reworking. Infrared examination also can be used as a tool to differentiate between certain groups of pigments and inks.

Infrared imaging is the recording of the variable absorption and reflectance of infrared light by an object. The infrared wavelengths may penetrate an object below the surface, unlike visible light. The way that a material interacts with infrared radiation, reflection, absorption or transmission can aid in the characterization and differentiation of materials. In some cases infrared imaging can enhance the contrast between materials and/or features. In the case of paint or paper with charcoal or carbon-based inks, the contrast can be dramatic.

A modified DSLR is used for reflected infrared digital photography. Most DSLRs have IR-cut filters on the sensors that increase the quality of the visible light image by blocking any infrared radiation from the sensor. The modification of this camera included the removal of the IR-cut filter and the color filter array (CFA) allowing the camera to have a maximum sensitive of around 330nm to 1200nm and to acquire monochrome images.¹ The spatial resolution of a full-frame DSLR allows for an entire object to be captured at a fairly high resolution in a single image. Tungsten halogen lamps are used for illumination.

Kodak Wratten IR filters are used on the camera. These filters included 88A, 87, 87C, 87B and 87A cutting on at different wavelengths within the near infrared.

¹Information from http://maxmax.com/IRCameraConversions.htm.



Infrared Reflectography


Comparison of Infrared and Visible

Infrared Reflectography reveals underdrawing (as seen in the first image) that was not seen in the visible light (right image).
[Right Image by Don Hurlbert, CSIP, Smithsonian Museum of Natural History]

Infrared reflectography is also the recording of the variable absorption and reflectance of infrared light by an object. In the case of the systems at MCI, infrared reflectography has a higher spectral resolution than reflectance infrared digital photography. Due to the low spatial resolution of the camera for IRR (320x256 pixels), we either can only acquire details of an image or we take enough overlapping images to stich a mosaic of the area of interest.

MCI is equipped with a system that has an uncooled InGaAs (indium gallium arsenide) detector with a spectral range of 900- 1700nm (near infrared).

MCI received a grant from the Smithsonian Women's Committee in 2009 to acquire a digital infrared imaging system. It was the first system of its kind in the Smithsonian Institution, allowing advanced non-destructive documentation of latent images.



Ultraviolet Imaging


Copybook in Visible Light

The left images are of a copybook illuminated and photographed using visible light and with Ultraviolet Radiation. The right images are looking closer at the text of the copybooks with both visible and ultraviolet light.


Ultraviolet radiation has been widely used in art conservation as a non-destructive examination technique. UV-induced fluorescence is used for the identification, characterization, condition assessment, and treatment of objects. It is particularly useful in the cases of organic materials such as plastics, coatings, and adhesives. The technique often shows alterations of objects over time, as in the case of varnishes on paintings.

UV-induced fluorescence technique can be documented using a digital camera. We have used the technique to record changes in objects, and to make documents legible (as seen above).



IR & UV Resources

Elizabeth Walmsley, Catherine Metzger, John K. Delaney and Colin Fletcher, "Improved Visualization of Underdrawings with Solid-State Detectors Operating in the Infrared," Studies in Conservation, 39, 1994, 217-231.

John K. Delaney, Elizabeth Walmsley, Barbara H. Barrie, and Colin F. Fletcher, "Multispectral Imaging of Paintings in the Infrared to Detect and Map Blue Pigments," Scientific Examination of Art / Modern Techniques in Conservation and Analysis, 120-136.

Warda, Jeffrey, Franziska Frey, Dawn Heller, Dan Kushel, Timothy Vitale, and Gawain Weaver. The AIC Guide to Digital Photography and Conservation Documentation. 2nd ed. Washington: American Institute for Conservation of Historic and Artistic Works, 2011. Print.