1. What was the motivation/aim of this research?
The motivation for our research was to demonstrate a new method for nondestructively and nonintrusively revealing drawn or painted mural art-work (i.e., underdrawings and paintings on walls/ceilings) that has been covered and thereby obscured by plaster and/or additional layers of paint.
2. Why is terahertz imaging important for studying art?
The terahertz radiation is non-ionizing (unlike X-rays and other highly energetic beams) and thus essentially safe to humans, opening it up to many applications that involve inspection or evaluation in an open environment. Pulsed terahertz beams also combine the following capabilities: they can resolve lateral dimensions down to less than a millimeter; they can see through most non-metallic materials; they typically can resolve the depth of an object or a surface; they can be used in a spectroscopic mode so that they not only image an object, but also provide information on its composition. We have demonstrated that we could clearly resolve drawn images that were buried under either a layer of plaster or a layer of paint. The implication is that one could look for wall-art that may have been covered up because of a regime change, because it was deteriorating, because someone in power disliked it, etc.
3. Why has this never been achieved before?
Terahertz beams have been used to look through walls, boxes, brief cases, clothing (homeland security), etc. In addition, a German group has explored THz inspection of paint-on-canvas art, and a Japanese group has produced a catalog of pigment behavior in the THz spectral range.
4. What are the advantages of using terahertz radiation to evaluate paintings instead of other technologies such as Raman, X-ray fluorescence and near-IR spectroscopy?
Other modes of inspection do not typically use pulses of radiation, and thus they may not have the ability to distinguish the properties of buried interfaces and objects as easily. That is, they lack depth resolution. For example, an X-ray image looks two-dimensional; you cannot tell if a feature is on the surface, just under the surface (say, a layer of paint), or buried under 5 millimeters of plaster. Microwave techniques, similar to radar, may give you some good depth resolution, because one can easily measure phase changes in the return signals, but their lateral spatial resolution will be severely limited. There are also some crucial pigments used in underdrawings (e.g., sanguine) that IR reflectometry cannot readily distinguish.
5. How does the pulsed-terahertz reflectometer and imaging process work, step-by-step?
In the Ultrafast Microwave Photonics lab at the Univ. of Michigan, we start with a laser that produces short pulses of light (approximately 100 femtoseconds in duration) and two customized optoelectronic devices, a photoconductive switch and a photoconductive sampling gate. A continuous voltage bias on the switch is converted into a short pulse of electromagnetic energy (the terahertz pulse; roughly 1 picosecond in duration) when the laser pulse hits the switch. 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 (e.g., a flat piece of plaster with a drawing/painting facing away from the incoming T-ray beam). Part of the terahertz pulse’s energy is reflected whenever it encounters an interface (air-plaster; plaster-paint; paint-graphite, etc.). Because the terahertz pulse is so short, we can separate out the reflections from each other in time. As we scan our object from side to side through our terahertz beam the amount of reflection changes depending on what the terahertz pulses are hitting, and thus from each point at which we stop we get a time-domain series of pulses with information about the reflection from each discrete layer. Finally, the reflected terahertz pulses are directed onto the photoconductive-sampling-gate receiver, which is triggered to measure the terahertz pulses by a part of the short-pulse optical beam that has been split off from the beam that helped generate the terahertz beam in the first place. The characteristics of the measured terahertz pulses are then correlated in a computer with the position that they hit on the mural, and an image is created. We subsequently need to process the image, often with considerable effort, in order that it will yield useful information on the embedded layers.
6. What information can you gather about the artwork by using terahertz radiation?
Currently, size and shape of drawings made with a variety of different materials. In the near future we hope to report on the ability to distinguish different colors (pigments) for embedded paintings.
7. What materials and how thick can this system be used to penetrate?
As far as art is concerned, basically any non-electrically-conductive material that does not have too much water in it will have a relatively low absorption in the terahertz range. We have been using modern and historical plasters, and once they dry for a few days after mixing, we can see through them fine. There is still some loss of the terahertz energy as it passes through the plaster, and each plaster is a bit different. I would safely say we could examine art through a centimeter of plaster, but that’s pretty thick for a heavy covering over a hidden piece of wall art.