19 Feb 2008
An imaging system that uses terahertz pulses is giving scientists their first glimpse of lost paintings.
Researchers in the US are using terahertz radiation to evaluate underdrawings that have been covered and hidden over time by plaster or additional art work. The process uses a pulsed terahertz reflectometer and imaging system to detect metallic and dielectric paint patterns through additional layers of paint and plaster. (Optics Communications 281 527).
"We can clearly resolve images that are buried beneath layers of plaster or paint," John Whitaker, a researcher from the University of Michigan, told optics.org. "We can determine the size and shape of drawings made with a variety of different materials and we hope to extend this to eventually distinguish different colours."
Whitaker and his colleagues are collaborating with researchers from the world-famous Louvre Museum in Paris and US firm Picometrix. The group is using the T-Ray system developed by Picometrix, which can penetrate up to 1 cm of plaster. Whitaker's team has already used the system to detect coloured paints and a graphite drawing of a butterfly through 4 mm of plaster.
Imaging historical artefacts requires a non-destructive, non-invasive and precise method that can be applied on-site. Unlike X-rays and other highly energetic beams, terahertz is non-ionizing making it safe to humans and appropriate for use in open environments. It can also reveal depth and detail that other techniques cannot.
"X-rays do not provide good depth resolution and microwave techniques may give some depth resolution, but their spatial resolution is severely limited," explained Whitaker. "Pulsed terahertz beams, however, provide spatial and lateral resolution down to less than a millimeter, the ability to see through most non-metallic materials and provide information on the composition of an object."
The team's set-up uses a Ti: sapphire laser emitting at 810 nm and a repetition rate of 80 MHz. Pulses with a duration of 100 fs are fired onto a photoconductive switch, which converts them into terahertz pulses with a duration of around 1 ps.
"A simple antenna structure helps to radiate the terahertz pulse into free-space, and we use optical elements to focus the pulse onto our object," explained Whitaker. "Because the terahertz pulse is so short, we can separate out each of the reflections from the object."
As the object is scanned from side-to-side through the beam, the amount of reflection changes depending on the material the pulses are hitting. "From each point we get a time-domain series of pulses with information about the reflection from each discrete layer," explained Whitaker. "The characteristics of the measured terahertz pulses are then correlated with the position that they hit on the mural, and an image is created."