By Brett Liem and Michael Robertson
Last year we published a short article in Active History where we described optical techniques for recovering the contrast from faded documents. A range of light sources from ultraviolet (UV) to near-infrared (NIR), filters, and a camera adapted to form images with light outside or the normal visible spectrum were used to reveal residual ink that was no longer visible due to damage or aging. This year, we extended the work to investigate the use of similar optical techniques for imaging a pencil sketch underneath a painting. The inspiration for this work was a paper by Delaney et al  where three layers of underdrawings were imaged beneath Picasso’s The Tragedy.
In order to understand the optical properties of the acrylic paints used in this study, optical transmission spectra were obtained from 8 colours of acrylic paint applied to a glass slide as well as from the glass slide itself. The paint spectra were divided by the spectrum from the glass slide to give optical transmittance values scaled between 0 and 1. The thickness of the paint varied between samples and comparisons of absolute transmittance values between the curves are not appropriate. However, above a wavelength of about 600 nm there is a general trend of increasing transmittance of all the paints up to the spectrometer limited wavelength of 1700 nm. Thus, the paints become more transparent as the wavelength of the light becomes longer.
The ability of these paints to mask one another is presented in Figures 2 & 3 where the vertical lines have been painted thicker than the horizontal lines to improve opaqueness of the overlying paint with respect to the underlying paint. A thin line drawn by a graphite pencil is present at the bottom of both images. Figure 2 was recorded using visible light from a white LED source and the right-hand image was acquired using 880 nm NIR light with a digital camera that had its internal filters removed to extend optical sensitivity out to 1100 nm.
Under normal white-light conditions, the various paints covered the underlying paint and pencil line quite effectively. However, using NIR light, the paints become more transparent and the pencil line is clearly observable for all the paints except for the black and white paints. This particular black paint is very good at absorbing a wide range of wavelengths of light. The reason we were able to obtain an optical transmission spectrum for the black paint in Figure 1 was because the paint layer was very thin. The white paint blocked the underlying pencil line by a different mechanism; it reflected nearly all the incident light regardless of wavelength up to about 900 nm. Thus, the conclusion that the paint becomes more transparent with increasing wavelength is supported by these images.
From the general trends of the curves in Figure 1, if a pencil becomes more visible underneath paint using 880 nm as opposed to shorter wavelength visible light, then visibility should improve for wavelengths of light further into the infrared regime. In fact, Delaney et al  found wavelengths in the 1400-1550 nm range useful for imaging drawings underneath paint.
Unfortunately, our camera was only usable up to wavelengths of 1100 nm and cameras that operate in the 1400-1550 nm range are fairly specialized and more expensive than normal visible light cameras. In order to perform a proof-of-concept experiment of forming an image using 1550 nm light, we built a scanning imaging system using parts available scavenged from teaching and research labs and the setup is shown in Figure 4.
An inexpensive x-y scanner kit equipped with two stepper motors was used to scan an optical fiber over the surface of a painting where the image was built up point-by-point by measuring 1550 nm reflectance for each pixel. A 1550 nm LED was positioned at about a 45°angle beside the optical fiber to provide the NIR illumination. Light reflected from the painting entered the optical fiber and was measured using a photodiode detector. The experiment was controlled with a laptop computer and LabVIEW software. Although the apparatus was capable of a minimum step size of 11.4 microns, the increment between measurements was increased to 182 microns to reduce the time required to form an image. Even with a relatively larger step size, the scan time was about 37 hours.
We commissioned a local artist to prepare a pencil drawing covered by a painting in order to test these NIR imaging techniques in a realistic situation. The pencil drawing was a cardinal as shown in the top left image of Figure 5.
This drawing was then covered by a painting of a nature scene where the paint thickness was increased until the pencil drawing could not be observed using a human eye under incandescent lighting conditions (Figure 5 – top right). The bottom left image was captured using 880 nm light and the modified digital camera. It is observed that contrast from the head, some of the back and tail of the cardinal is visible through the painting. However, in the 1550 nm scanned image, most of the cardinal and much of the underlying branch and needles can be observed – a marked improvement over the image recorded with the 880 nm light.
In conclusion, the ability to see drawings beneath paintings is sensitively dependent on the wavelength of light used to illuminate the painting and improves as the wavelength increases, at least up to 1550 nm as demonstrated in this study.
Brett Liem is an Acadia University undergraduate honours Physics student presently in his 4th year of studies and this work is part of his honours thesis. Michael Robertson is a Professor in the Dept. of Physics at Acadia University whose research interests include the application of optical imaging and spectroscopy techniques to a variety of materials and fields. The authors gratefully acknowledge artist Barbara Martin of Wolfville, Nova Scotia for providing the paint samples, pencil sketch and painting used in this work.
 J. K. Delaney, M. Thoury, J. G. Zeibel, P. Ricciardi, K. M. Morales and K. A. Dooley, Visible and Infrared Imaging Spectroscopy of Paintings and Improved Reflectography, Heritage Science, 4:6 (2016). DOI 10.1186/s40494-016-0075-4