Infrared Reflectography (IRR)

Infrared Reflectography (IRR) allows us to identify underdrawing and pentimenti. It is performed with a scientific camera (InGaAs camera) that can image in the range of 1000- 1700 nm. Pigments such as azurite, prussian blue, and malachite become transparent only in this far infrared region.

IRR and IR are the methods used for detecting underdrawing and “pentimenti.”

Infrared Reflectography (IRR) can make pigments more transparent than IR photography. In this example, the green paint of the vest becomes transparent just in the IRR image, revealing the underdrawing.

In this example, only IRR allows us to clearly see the “squaring” below this painting. Painters copying another painting sometimes draw first a square grid to facilitate their work.

This illustration shows how IRR compares to Technical Photography methods. IRR covers the farthest part of the infrared spectrum that the digital cameras can’t reach.

InGaAs camera VERSUS Digital Camera

Can we get the same results with infrared photography (IR) or does the IRR camera make a difference? We compare 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. However, the advantages of the InGaAs camera, even if real, are limited to specific cases.
Pigments become considerably more transparent at longer infrared wavelengths (i.e. using an InGaAs camera). This statement is true for some historical pigments, but not for all of them. And in many cases the increase in transmittance is negligible.

A digital camera can record light until about 1100 nm while an InGaAs camera until about 1700 nm. Besides their cost, the first issue with the InGaAs camera is their low pixel count. With detectors in the order of 320×256 pixels (for the most affordable), they deliver an image that is just a fraction of what we get with Nikon D850 (8256 x 5504 pixels). This translates into a lot of time-consuming stitching of hundreds of IRR images to have the same pixel dimension of one D850 single shot. That said, let’s see what are the actual advantages in terms of pigments’ transparency in the infrared.

Pigments that look the SAME under an InGaAs camera or a digital camera

This illustration shows some examples of pigments that have the same opacity with both IRR and IR but for different reasons. Lead white represents those pigments, mostly the whites, that just reflect so much the infrared that they remain always opaque. On the other hand, lamp black, like all the other carbon-based pigments, is opaque to both IR and IRR because it absorbs all the infrared. Vermilion represents those pigments that become a bit transparent to the IR and maintain the same transparency in the IRR image.

Pigments that have DIFFERENT transparency under an InGaAs camera or a digital camera

This illustration shows azurite, as an example of those pigments that are transparent just in the IRR. Raw umber, in the second line, is representative of the earth pigments, which feature a slightly increasing transparency in the IRR. Eventually, in the third line, smalt shows the behavior of the cobalt-containing pigments (i.e. cobalt blue, cobalt cerulean blue, cobalt violet) that have the peculiar property of becoming transparent in the IR and dark in the IRR.

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-based pigments, such as cobalt blue, cobalt green, and smalt are less transparent in the far infrared (InGaAs) than in the closest infrared (Digital camera). So, the bottom line, a digital camera can better see through them!

Panoramic Infrared Reflectography

Panoramic Infrared Reflectography (PIRR) is a valid alternative to the much more expensive scanners for Infrared Reflectography (IRR) The PIRR method consists of taking a series of images of a scene with a precision rotating head and then using panoramic software to align and stitch the shots into a single, seamless panorama. It can be implemented with consumer panoramic imaging tools, which can be upgraded following technical developments; as opposed to infrared scanners, which are products that cannot be modified. Self-assembled, modular equipment can be modified for specific tasks and upgraded with comparatively little funding, following technical and scientific developments in the consumer market, e.g. upgrading to an InGaAs camera with higher pixel count. The stitching software is easy to use; the overall panoramic method does not require specialized personnel or intensive training and, for these reasons, the method is appealing to medium‐small museums and private conservators who want to implement an affordable method to professionally document their collections.

Our Free online, IRR Training course

The Panoramic Infrared Reflectography (PIRR) Training module provides technical insight on hardware and software tools for PIRR using budget equipment already available commercially for panoramic photography along with an InGaAs camera. Click HERE

Read our resources on Infrared Reflectography

Publications on Infrared Reflectography
A. Cosentino “Panoramic infrared Reflectography. Technical Recommendations ” Intl Journal of Conservation Science, 5(1): 51–60, 2014.
A. Cosentino “Type II Super Lattice (T2SL) imaging technology for infrared reflectography of polychrome works of art” e-conservation Journal 5, 2016 Available online 15 March 2017.
A. Cosentino “CHSOS Application note #7: IRRF – Infrared Reflectography Fluorescence” 2022.