To understand the lives of ancient human beings, scientific analysis is commonly performed in archaeological research. In the old days, rather destructive methods were applied to reveal a hidden object. For example, mummies had to be unwrapped to see the real bodies. Nondestructive, on-site analysis is now an essential tool for studying cultural heritage objects, and electromagnetic waves have been used extensively. Techniques based on X-rays, including X-ray computed tomography, are efficient for observing the internal structure of most objects. However, X-rays ionize radiation and should be applied only in restricted areas. Radiation with spectra ranging from the ultraviolet to the mid-infrared is also commonly used to analyze materials close to the surface, in particular in the examination of a painting before conservation treatments. In the case of lower frequencies, microwave radar has been used to find artifacts under the soil. When it comes to the THz region, previous works with time-domain spectroscopy (TDS) systems have shown that THz spectroscopic imaging can roughly give material information as well as internal layer structure based on the time-of-flight method. However, limited power makes it difficult to penetrate into thick objects, and the TDS system is usually rather expensive.
Jean-Pascal Caumes and Emmanuel Abraham have developed a transportable THz topographic imaging system by using a Gun diode (110GHz, 20 mW) as a source together with a commercially available photothermal detector. The optical path is carefully designed with a Teflon lens and off-axis parabolic mirrors. The museum object under examination is then placed between the source and the detector, on a rotating stage that is itself placed on an X–Y stage. This configuration limits the size of the objects that can be studied to about 20 cm in each direction. A transmission image in 2D is obtained by moving the object with the rotating stage in the X and the Y directions, and the 3D image is assembled from the 2D images acquired at different angles by use of the Simultaneous Algebraic Reconstruction technique. The authors chose 10-deg steps in this work, resulting in 18 projections. The measurement time in total for a pottery bottle 16 cm high and 8 cm in diameter took approximately 9 h. Although one may think that 9 h is too long, high throughput rate in measurement is not too important for museum objects, if the information is valuable.
The objects introduced in this paper are two, sealed Egyptian pottery bottles from the Egyptian XVIIIth Dynasty, dated 1479–1425 B.C. From the historians’ knowledge, it is estimated that these bottles contain something like food offerings, but X-rays could not provide a clear image. One of the bottles makes a small noise when moved. The authors measured the object by changing its inclination and found that it contains, along with other loose contents, a solid part that is stuck to the bottom. The figures clearly show that the material at the bottom does not change in shape, while the moving contents move following gravity.
The authors estimated the absorption coefficients of these two parts and found that they are the same. Indeed, we must admit that it is rather difficult to determine what is inside. Even if we can estimate its refractive index, it is still almost impossible to identify the substance only by the experimental results at a single frequency within this frequency range. It can be possible to distinguish resin and ceramic, but it may not be possible to distinguish wood resin and animal glue, for example.
However, THz imaging can show that there's something that has been kept in these bottles for more than 3500 years—without the need to open them on-site or at the museum. Given that THz technology, including sources and detectors, is advancing rapidly, I believe that THz imaging as well as spectroscopy will play an increasingly important role in cultural heritage science.
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