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% [1, 3].
Increase in transparency using an InGaAs camera it’s shown by red ochre, Prussian blue, asphaltum, burnt umber, Van Dyke brown, titanium white, phthalo green, verdigris and yellow ochre.
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Transmittance curve for Azurite [1]. Transmittance is already at about 50% in the range of a modified digital camera. The InGaAs camera provides another 50% transmittance increase but at a much higher cost and much lower pixel count. Azurite observed through a digital IR modified camera (IR) and an InGaAs camera (IRR). The latter increases transparency of the pigment (a vertical underdrawing line becomes visible). Prussian blue observed through a digital IR modified camera (IR) and an InGaAs camera (IRR). The latter increases transparency of the pigment (underdrawing crossed lines become visible).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 [1].
The same can be said for red lead, cadmium red, indigo, phthalo 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.
Transmittance curve for Vermilion [1]. Transmittance is at about 70% in the range of a modified digital camera and does not increase with the InGaAs camera. Vermilion observed through a digital IR modified camera (IR) and an InGaAs camera (IRR). The transparency is the same (a faint vertical and horizontal underdrawing line become visible in both IR and IRR).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 color 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) [2]. So, bottom line, a digital camera can better see through them!
Transmittance curve for Cobalt blue [2]. Transmittance is higher in the range of a modified digital camera than in the InGaAs range.
Bottom line, the two techniques are complementary.
References
[1] M. Gargano, N. Ludwig, G. Poldi “A new methodology for comparing IR reflectographic systems” Infrared Physics & Technology 49 (2007) 249–253.
[2] Lawrence Berkeley National Laboratory Pigment Database http://coolcolors.lbl.gov/assets/docs/PAC-2003-03-11/Pigment-Charts-2003-03-09.pdf.
[3] 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.
