OCT sees through painted canvases

23 Aug 2005

The preliminary sketches made before an artist starts painting are being revealed thanks to optical coherence tomography.

Optical coherence tomography (OCT) is helping a team from the UK and Canada probe and uncover the different layers found in paintings. The interferometry-based technique not only reveals how the structure and thickness of the paint and varnish layers vary across the painting but also gives the scientists an insight into the preliminary drawings made on the canvas. (Optics Express 13 6144)

OCT cross-section

"OCT is the only technique that can produce cross-section images or 3D images of the inner structures of a painted object non-invasively and at a safe distance," Haida Liang from Nottingham Trent University told Optics.org. In comparison, traditional methods involve removing a small sample from the painting and studying it under a microscope.

Using OCT is said to give the highest resolution and dynamic range images of the so-called under-drawings which are covered by layers of paint. It also gives the team a non-invasive way to find out which layer contains the under-drawings.

Liang has been working with researchers from the University of Kent, UK; Canadian firm Ophthalmic Technologies and the UK's National Gallery. The team has put together two OCT systems and studied painted samples and varnish-on-glass as well as a 50-year-old and a 18th Century painting.

Both OCT systems use a superluminescent diode (SLED) to illuminate the sample. One setup uses an 850 nm SLED with a bandwidth of 20 nm while the other relies on a 1300 nm source with a bandwidth of 55 nm.

"The light from the SLD is coupled to an optical fiber," explains Liang. "The light from the fiber goes through a lens and is reflected off a set of scanning mirrors. The light is finally focused en-face on to the painting through an objective lens. We estimate a power of 500 µW incident on the painting."

According to Liang, the 1300 nm system has a transverse resolution of 25 microns while in the 850 nm system this improves to 15 microns. The team operates the systems in C-scan and B-scan modes which allow them to acquire 2D constant depth and 2D cross-section images respectively at typically 2 frames per second.

"We have started using OCT for quantitative analysis of optical properties of paint and varnish layers," said Liang. "So far we have succeeded in measuring the refractive index and physical thickness of varnish layers. We are also extending our studies to explore the full potential of the technique for non-invasive examination of historic objects."

Author
Jacqueline Hewett is technology editor on Optics.org and Opto & Laser Europe magazine.