This post compares an InGaAs camera – used for Infrared Reflectography (IRR) with a Digital camera modified for Infrared photography (IR). An InGaAs camera is your choice when it comes to imaging underdrawings. Though, the advantages of the InGaAs camera, even if real, are limited. So, we should be aware of them before we decide to spend a huge sum of money to buy one. This post collect my thoughts and a bit of experiments on this topic.
A digital camera can record light until about 1100 nm while an InGaAs camera until about 1700 nm. A first issue with the InGaAs camera, other than their cost is their low pixels count. With detectors in the order of 640×520 pixels, they deliver a resolution of 0,3MP. Compare it with the digital camera Nikon D800 that has 36MP. This translates into a lot of time-consuming stitching. That said, let’s see what are the actual advantages in terms of pigments’ transparency in the infrared.
Pigments that are MORE transparent with an InGaAs camera
I posted already a set of Multispectral images of historical pigments here. Pigments become considerably more transparent at longer infrared wavelengths (i.e. using an InGaAs camera). This statement is true for some historical pigments, not for all of them. And in many cases the increase in transmittance is negligible.
Azurite, the affordable blue used in pre-industrial age European art, is an example of those pigments whose transmittance increases at longer infrared wavelength Though, the transmittance of the pigment in the modified IR digital camera spectral range is already about 50%
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Pigments that have the SAME transmittance in both IR modified Digital cameras and InGaAs cameras
Vermilion is among the most used red historical pigments. It is common in oil and tempera paintings. The InGaAs camera does not increase its transparency, at all .
The same can be said for red lead, cadmium red, indigo, phtalo blue, Egyptian blue, ultramarine, cobalt violet, ivory black, vine black, gesso, lead white, zinc white, green earth, malachite, cadmium green, viridian, chrome green, cobalt yellow, realgar, orpiment, Naples yellow, massicot, lead tin yellow and cadmium yellow.
Pigments that are LESS transparent with the InGaAs camera
Cobalt blue was loved by Vincent van Gogh who said to his brother Teo, ‘Cobalt blue is a divine colour and there is nothing so beautiful for putting atmosphere around things…”. Even if it seems counter intuitive, cobalt blue, cobalt green and smalt, are less transparent in the far infrared (InGaAs) than in the closest infrared (Digital camera) . So, bottom line, a digital camera can better see through them!
Bottom line, the two techniques are complementary.
References M. Gargano, N. Ludwig, G. Poldi “A new methodology for comparing IR reﬂectographic systems” Infrared Physics & Technology 49 (2007) 249–253.  Lawrence Berkeley National Laboratory Pigment Database http://coolcolors.lbl.gov/assets/docs/PAC-2003-03-11/Pigment-Charts-2003-03-09.pdf.  J. R. J. van Asperen de Boer “Reflectography of Paintings Using an Infrared Vidicon Television System” Studies in Conservation, Vol. 14, No. 3 (Aug., 1969), pp. 96-118. [ws_table id=”4″]