Transmitted Infrared Photography (IRT)

Transmitted Infrared photography (IRT) is part of the Technical Photography documentation and allows to detect underdrawing and pentimenti. It is a very effective imaging method since pigments become even more transparent than in the usual IR photography method. This method can be realized with our Technical Photography kit.

This method is useful only for art on translucent supports, such as paintings on canvas, drawings on paper and historical documents and manuscripts. The lamp providing IR radiation should face the back of the painting while the camera focus on the front. The lamp should be shielded so that only the radiation through the canvas can reach the camera. Any other source of radiation in the examination room should be turned off to avoid diffused light (actually diffused infrared). In some cases, could be worth testing the method
changing the actual configuration and having the camera facing the back of the painting. In general, the first setup is preferred because
the drawing lines will appear sharper since the infrared will not be diffused by the canvas.

IRT often provides better images compared to IR for detecting underdrawing, underpainting, pentimenti, or just the actual build-up technique of the painter to shape the figures. IRT is so powerful in particular for white pigments, such as lead white and titanium white, the most common in art, the most used, respectively, before and after about 1920′. These pigments reflect a lot of the incoming infrared and, consequently, their hiding power is barely affected by infrared coming from the front. They will just reflect most of the IR and they will not produce contrast between the ground and the underdrawing. When the infrared radiation comes from the back (transmission), the infrared can penetrate the paint and the underdrawing becomes apparent in the resulted IRT image.

Applications in Art examination

Transmitted infrared photography. Pigments become more transparent with transmitted infrared. — This Pigments Checker was made on canvas specifically to test IRT imaging. Pigments become more transparent with transmitted infrared. This is particularly important for the white pigments that are in general very opaque in IR images but also most of the other pigments show much better transparency in IRT, like in this example, Naples yellow. This figure shows also that the IRT image taken with the camera facing the actual paint shows a sharper underdrawing than the IRT image taken with the camera facing the back.

Transmitted Technical Photography using SALVO

SALVO is our UV-VIS-IR Imaging Panel for Transmitted Technical Photography. This video shows how to make Infrared Transmitted (IRT) photography on Pigments Checker. Pigments become much more transparent than in standard IR photography.

Salvo – UV-VIS-IR Imaging Panel

Transmitted infrared photography. An example application on a mock-up painting containing historical pigments. The mock-up had an extensive underdrawing, also with a number of changes. The drawing was eventually painted with historical and modern pigments. Its IR and IRT images show that IRT provides much more contrasted (rich in details) image of the drawing. Notice on the upper right side that the sky painted with azurite (very opaque in IR photography) become transparent in IRT. The dark border all over the painting in the IRT image is due to the frame.

Experimental Setup

Transmitted infrared photography. Setup. The camera faces the front of the painting while the lamp its back. The camera is set for infrared photography. As sources of infrared radiation, halogen lamps can be used but it is recommended a LED lamp to avoid any heating of the canvas or even flashes.

References

Publications on Transmitted Infrared photography (IRT)
A. Cosentino "Infrared Technical Photography for Art Examination” e-Preservation Science, 13, 1-6, 2016.