What is it?
Infrared reflectography is an optical technique mainly used in the field of documentation and diagnostics of paintings.
It is a non-invasive technique. This feature is fundamental in the analysis of works of art, for which the integrity of the artifact is paramount.
What is it useful for?
One of the key results obtained through infrared reflectography is the visualization of the underdrawing below the pictorial layer, which is essential for a historical and stylistic study of the painting, e.g. to understand what have been the stages to reach the artwork current appearance. In fact, this technique allows to investigate and document the methods employed by the artist for designing the composition, e.g.the degree of knowledge of the mathematical prospective rules, the techniques used for the drawing. The parts performed freehand and the portions instead made using transport techniques can be identified.
Moreover, infrared reflectography is used by restorers and art historians before the restoration work and during it, for documentation purposes.
Before the restoration intervention reflectography allows for the assessment of the state of conservation, by detecting the presence of layers of deposits, paints or repainting. During the course of the intervention itself, the reflectography can be profitably repeated as it would then be possible to go deeper and reveal other important details not initially perceptible.
It is also qualitatively possibly to identify hidden pictorial layer, by the artist himself in the process of creating the painting (pentimenti) or added in later periods, due to restoration. In this way, the history of the painting is reconstructed from the moment of its initial elaboration; this can also be useful in particular cases for authentication and attribution of the artifact or parts of it.
Reflectography can also be sometimes useful for the qualitative identification of pigments through visualization in false color.
Are all reflectographies the same?
No, they are not.
There are several devices and techniques that can be used to produce an image in the infrared range.
However, depending on what is used as instrument and how it is used, the results can be very different.
Let’s take this painting as an example. It is an oil on canvas, and the paint layer is not particularly thick.
(n.b. all the images have been downsized in order for them to be fast to load on a website. However, they have been all modified in the same way)
Let’s take a detail, including light and dark tones.
IR photography does not show any underdrawing.
IR digital photocamera modified to be sensible in the IR range.
Even if we acquire a reflectography with a CCD camera that is not cooled, we still do not see much of the underdrawing.
Standard, not cooled, CCD Camera
If instead we used a cooled CCD, filtering around 1000nm, we see some underdrawing the squaring used by the painter to draw is appearing.
Cooled CCD Camera, filtered around 1100 nm
We we use the IntraVedo infrared scanner we can see the whole of the underdrawing with great detail and contrast. The image is faitful and geometrically correct, with no distortion due to uneven illumination.
Infrared Scnner IntraVedo till 1700 nm, with standar resolution. We also have a 4X resolution available. The highest available worldwide.
Even is we use a Vidicon camera the underdrawing is visible. However, the image is not geometrically correct and the spatial resolution is lower.
Broadband Vidicon camera till 2200 nm
So we can see the preparatory drawing in its entirety only using an appropriate instrument. We risk otherwise drawing the wrong conclusions: no preparatory drawing, or free-hand drawing instead of a cross-reference drawing. Moreover, if we do not use a performing instrument, we will have to see poor visibility in the darker areas, difficulty in interpreting signs that are deformed by the optics etc ..
How do we get an IR image?
The method consists in illuminating the work to be examined with a radiation source in the near infrared, generally consisting of incandescent halogen lamps, and in registering the object’s reflected radiation with a detection system. To eliminate the visible backscattered radiation, a filter that eliminates the visible component of the radiation is generally placed in front of the objective of the detection system. The incident infrared radiation passes through the pictorial layer and it is possibly reflected by the underlying preparatory layer. Above this preparation draft, often a preparatory drawing, also known as underdrawing, was carried out using graphite or charcoal, materials that absorb infrared radiation and are therefore visible in the image acquired by our device.
Thanks to the reflectographic investigations we therefore obtain an image that gives us new information with respect to the visible data.
The underdrawing visibility depends on three parameters, of which two in relation to the material and two to the instrumentation used:
1. the difference between the reflectance of the materials used for the preparatory layer and of those used for the design (contrast factor)
2. the transparency of the pictorial material to the infrared radiation
3. the sensitivity of the sensor
4. the resolution capability of the detection system
A bit of history
Infrared photography became a routine analysis in the 1950s. It was mainly used on 15th century Flemish paintings, for which it obtains good results in reading the underdrawing, thanks above all to the small thickness of the pictorial layers. The spectral sensitivity of the photographic film is quite limited up to 0.9 µm and would not be able to reach the underdrawing on thicker paints.
In 1968 the Dutch physicist van Asperen de Boer developed a methodology using infrared sensitive cameras (Infrared Vidicon Television Systems, sensitive up to 2.2 µm).
With the introduction of digital systems such as the camera with CCD sensor (Charge-Coupled-Device), despite the pigment’s penetration capacity being reduced compared to Vidicon up to a maximum of 1.1 µm, a qualitative improvement of the image was reached, thanks to the greater spatial resolution and the greater tonal range (generally from 8 bits up to a maximum value of 16 bits per pixel) depending on the system used (for example a commercial Sony F828, or a cooled scientific camera for astrophysics).
The most recently developed instrument and better performing device is the IR Scanner equipped with InGaAs sensor, whose spectral sensitivity extends, compared to cameras with CCD sensors, up to a wavelength of 2.2 µm.