The IR method
consists in shining on the artwork a near
IR radiation source, generally halogen
lamps, and capturing the radiation reflected by
it with a suitable device.
In order to avoid the reflected visible radiation
to sum to the IR radiation, a visible light
blocking filter is generally used before the
sensor.
The IR radiation is able to penetrate the paint
layer and is generally reflected by the ground
layer underneath it, while the underdrawing
is generally realized with a carbon-based media,
which, on the contrary, absorbs IR radiation.
The visibility of the underdrawing depends on
three conditions, which are typical of each
investigated object:
1. The difference between the reflectance of
the material used for the preparatory layer
and the one used for the underdrawing
2. The paint layer transparency to IR radiation
3. The equipment used to collect the reflected
IR radiation
The first two conditions depend on the art object.
Art-Test can help you a great deal with the
third one.
Some history
The first form of infrared analysis to be applied
to paintings was IR photography, using a standard
photo camera and special IR films. In the fifties
it became a standard diagnostic technique, at
least on Flemish paintings, where it could be
effective, since these paintings generally present
relatively thin paint layers, and the IR film
sensitivity extend till only 0.9 µm.
The technique evolved when in 1968 the Dutch
scientist Van Asperen De Boer, employed Infrared
Vidicon Television Systems, which are
cameras sensible to IR radiations till 2.2 µm,
and allow the possibility to penetrate dark
and thicker layers as well. With the introduction
of digital CCD cameras, the
possible penetration into the paint layer is
diminished; however, the image resolution can
be dramatically improved, as well as the tonal
dynamics, which could be 8 to 16 bit, depending
on the systems used (for a example a commercial
camera or a scientific cooled camera.
The most advanced equipment for IR reflectography today
is the IR scanner
with InGaAs sensor, whose spectral sensitivity
extends till 1,7 µm.
|
