Infrared Reflectography (IRR) is a non-invasive method of studying a painting by looking beneath the visible layers of paint. This allows you to examine the underdrawing along with any changes or pentimenti present in the work.
Infrared imaging has been used within the fields of conservation and art history for over 90 years, initially through the use of wet photographic emulsions and extended IR film. In the late 1960's Dutch physicist J. R. J. van Asperen De Boer developed Infrared Reflectography using vidicon tubes to extend much further into the infrared than photography had done before.
Modern sensor technology, such as the InGaAs sensor used in the Osiris, allows us to build on these techniques with higher resolution and improved sensitivity to reveal a painting's secrets like never before.
The most common and well known application of IRR is for studying a painting’s underdrawing, as you read about in the case study The Portrait of Alexander Mornauer. This can reveal compositional techniques of the artist, along with changes conducted during both composition and painting. It’s not uncommon to find changes to the positioning of people or the nature of backgrounds, or even whole figures that were drawn but never painted, as in The Death of Anacreon attributed to Pierre-Claude Desmarets.
It can also aid research into the provenance of a painting. For example, some studios produced many copies of a single painting and it can be virtually impossible to discern which is the original. However, original paintings tend to display some form of change, however minor, while copies don’t have such features.
It’s also common for an artist to reuse a canvas, and infrared reflectography is capable of revealing the original composition hidden beneath the later painting, as in Liu Wei’s Revolutionary Family Series.
See our Case Studies for more examples of how infrared reflectography is used.
How It Works
The ‘reflect’ element of reflectography refers to the reflection of a light source from the ground beneath a painting.
It is particularly effective when a painting features a white, or light-coloured, ground with an underdrawing or preliminary sketch executed in a carbon-rich material such as charcoal, or bone black. Infrared light is absorbed by this carbon-rich material, while it is reflected from a light-coloured ground, and it is this contrast that IRR records.
The obvious problem is that both underdrawing and ground are covered with layers of paint. Paint pigments are generally designed to be opaque to light in the visible part of the spectrum, but they are often increasingly transparent to light at longer wavelengths.
When visible light is used to illuminate a painting, the absorption and scattering by the paint layers is generally so high that the paint is all we see. Very little light is reaching the ground, and that which does is subject to further scattering and absorption on its way back.
By shining an infrared light source such as a Tungsten Halogen bulb at a painting and recording the light reflected back, we can see through the paint and study otherwise invisible elements of the painting and its composition.
Theory of Reflectance
A simple model of reflectance proposed by Kubelka-Munk helps explain the processes involved. In this model, light illuminates a painting and light reflected back is recorded. To image the underdrawing there must be sufficient difference or contrast between areas where light is reflected from the substrate and those where light is absorbed by the underdrawing.
Infrared Photography using Near Infrared (NIR)
The scattering and absorption co-efficients in the NIR region are a little less than in visible light. This allows a certain amount of detail beneath the paint to be seen by a detector sensitive to NIR. For this part of the spectrum we can use standard silicon sensors, which offer a cost-effective way to record information from paintings with thin paint layers.
However, this will still typically contain a large amount of light scattered from the paint layers so there may be relatively little contrast between areas with and without interesting features.
Infrared Reflectography using Short-wave Infrared (SWIR)
When illuminated and imaged in SWIR using infrared reflectography, the paint becomes much more transmissive and the proportion of light reflected from the ground is greatly improved. This boosts the contrast in the image and makes the underdrawing and other features much more visible for study.
To record light of this wavelength, it’s necessary to use a specialised sensor such as the InGaAs (or indium gallium arsenide) sensor used in the Osiris. This is sensitive to light in the 1-1.7um part of the spectrum and is optimised to produce high contrast images.
So if Longer’s Better…
The logic would follow that using even longer wavelengths would make the paint even more transmissive and we could study more details.
However, once we reach the MIR part of the spectrum we encounter a problem with thermal radiation. All materials radiate light with a wavelength that’s related to their temperature and it just so happens that room temperature objects radiate light in the MIR region. This effectively precludes us from using MIR for infrared reflectography.
Features of the Painting
The results achieved for any given painting are naturally also affected by the qualities of the painting itself. There are two features that can contribute significantly to the results of an IR reflectogram.
The thickness of the paint layers is a very important factor when using IRR. Paintings with thinner paint layers are generally easier to image, while those using particularly thick paint layers can be impossible to penetrate regardless of wavelength used.
The composition of the paints themselves will also have a significant impact on its absorption and scattering qualities. Generally, older paints such as those used in the sixteenth century lend themselves better to IRR than modern oil paints. This is due to the finer size of the pigment particles in modern paints that are more effective at scattering light. The medium used to suspend the pigment is also more absorptive in modern paints.
However, it is still entirely possible to use IRR on more modern artworks to reveal interesting features, and artists such as Picasso, Van Gogh and Kandinsky have all been successfully studied using infrared reflectography.
Multispectral Imaging (MI) has become something of a catch-all term to describe a large number of differing imaging systems.
All multispectral imaging systems share the concept of recording light from a number of defined wavelength ranges, but what these wavelength ranges are can vary greatly.
While infrared light is excellent at revealing things such as underdrawings and pentimenti, different wavelengths can be used to reveal different features of a painting. Ultra-violet (UV) light can be particularly useful for studying the varnish on top of a painting, while x-radiography (X-rays) can be used to ‘look through’ a canvas. Defined bands within the visible spectrum can also be useful for exploring the types of pigments used in a painting.
Multispectral imaging techniques can be applied within the infrared spectrum itself. Rather than studying light reflected from the painting’s ground, this is often looking for light scattered from within the pigment layers. These images can be used in combination with images from visible parts of the spectrum to provide further information about pigment composition, as well as revealing under painting or areas of change or repair.
In addition to reflectography and multispectral SWIR imaging, the Osiris can be used for transmitted infrared, or infrared transmittography (IRT). Rather than reflecting light off the painting, this technique shines light through the painting to reveal features that are behind the white ground, such as a canvas maker’s mark, or the canvas lining.