Advanced imaging
Infrared Reflectography
What is it
It is an optical imaging method that uses the infrared radiation. It is a totally non invasive technique.
What is it useful for?
This technique is able to provide IR images which allow us to penetrate the paint layer to reveal underdrawings, uncover hidden underpaintings, and to study changes made by the artist; all important aspects which can help establish attribution and authenticate works of art
It is also used in the field of documentation and diagnostics of paintings.
More details
Moreover, infrared reflectography is used by restorers before the restoration work and during it, and scholars 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 IR images all 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.
Selected
detail
Let’s take a detail, including light and dark tones.
IR Photography
Modified camera
IR photography does not show any underdrawing.
CCD Camera
Standard, not cooled
Cooled CCD Camera
filtered around 1100 nm
If instead we used a cooled CCD, filtering around 1000nm, we see some underdrawing and the squaring used by the painter to draw is appearing.
Infraredscanner Intravedo
till 1700 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.
Broadband vidicon camera
till 2200 nm
Even is we use a Vidicon camera the underdrawing is visible. However, the image is not geometrically correct and the spatial resolution is lower.
So we can see the preparatory drawing in its entirety only using an appropriate instrument. We risk otherwise to draw 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?
Painted surfaces are, generally speaking, multilayer structures.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 reflectography is an optical technique mainly used in the field of documentation and diagnostics of paintings.
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.