Infrared Photography (IR)

Infrared photography (IR) is used to detect underdrawing. IR is part of the Technical Photography documentation and it is used for the examination of many kinds of artifacts.

Photographic set up

Infrared photography can detect underdrawing that absorbs infrared radiation such as carbon black paint.

Check out the Infrared Photography Lenses Database

infrared-photography-source — Halogen lamps are the best source of infrared radiation for documentation of large artworks, such as wall paintings.

infrared-photography-source-manuscripts — LEDs and flashes are suitable infrared sources for heat-sensitive materials such as manuscripts.

Applications in Art examination

The Pigments Checker is used to check which pigments become transparent in infrared photography.

IR versus IRR

Infrared Reflectography (IRR) is imaging infrared over 1100 nm and up to 1700 nm or 2500 nm (depending on the imaging detector type). A full spectrum digital camera can record light until about 1100 nm while an InGaAs camera until about 1700 nm.

Pigments that are MORE transparent with an InGaAs camera

Some 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 (IRR).

Increase in transparency using an InGaAs camera it’s shown also by red ochre, Prussian blue, bitumen, burnt umber, Van Dyke brown, titanium white, phtahlo green, verdigris and yellow ochre.

azurite1 — Azurite is one of those pigments that benefit from the inspection with IRR rather than IR. Transmittance is already at about 50% in the range of a full spectrum digital camera. The InGaAs camera provides another 50% transmittance increase.

prussian1 — Prussian blue seen through a digital full spectrum camera (IR) and an InGaAs camera (IRR). The latter increases the transparency of the pigment (underdrawing crossed lines become visible).

Pigments that have the SAME transmittance in digital full spectrum camera (IR) and InGaAs camera (IRR)

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 other pigments such as red lead, cadmium red, indigo, and phthalo blue.

vermilionswatch1 — Vermilion seen 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 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 IRR (InGaAs) than in IR.

cobaltblue1 — Transmittance curve for cobalt blue . Transmittance is higher in the IR range (digital full spectrum camera) than in the IRR range.

cobalt-blue-multispectral-imaging1 — Cobalt blue seen though a digital full spectrum camera (IR) and an InGaAs camera (IRR). The transparency is less in the InGaAs image.

References

Publications on Infrared Photography (IR)
A. Cosentino "Infrared Technical Photography for Art Examination” e-Preservation Science, 13, 1-6, 2016.
A. Cosentino “Identification of pigments by multispectral imaging a flowchart method” Heritage Science, 2:8, 2014.
A. Cosentino "Effects of Different Binders on Technical Photography and Infrared Reflectography of 54 Historical Pigments” International Journal of Conservation Science, 6 (3), 287-298, 2015.