The collection of cultural relics in the Palace Museum has a complete system. The cultural relics cover ancient and modern times, with excellent quality and rich categories. The total number of existing collections reaches over 1.8 million pieces (sets), of which more than 8000 are first-class collections. The involved cultural relics are very diverse, mainly including calligraphy and painting, ceramics, metalwork, lacquer, wood, inlaid decoration, textiles, watches, murals, ancient architecture, the copying of ancient calligraphy and painting, and traditional packaging. The Forbidden City not only engages in the maintenance, restoration, and upkeep of cultural relics but also studies the production techniques and disease mechanisms of cultural relics to provide a better preservation environment for cultural relics and promote the development of new protection technologies. Cultural relics are the physical retention witnessing historical human development. It is also the most powerful basis for helping modern people explore history and restore historical truths. The protection and restoration of cultural relics is a multidisciplinary job containing archaeology, physics, chemistry, and biology. Terahertz (THz, referring to the frequency range from 0.1 to 10 THz) selected for research on cultural relic protection has the electromagnetic characteristics of wavelength: perspective, low energy, and high spectral time resolution. Enamel plays an important role in palace art and has complex production processes. Therefore it is of significance to apply THz technology to the conservation and restoration of enamel. This study builds a high-speed scanning THz imaging system based on asynchronous optical sampling (ASOPS) and employs it for the reflection imaging of enamel.

A high-speed scanning THz imaging system is built in the Palace Museum, and it is based on the ASOPS technology. It is a new technology that combines ultrafast spectroscopy with femtosecond laser and can solve the problem of long imaging time. The traditional THz imaging system is divided into two paths, the pump and detection sections through a beam splitter respectively. However, the pump and detection sections of the asynchronous sampling THz imaging system are not split through a beam splitter. They are generated by two femtosecond lasers with controllable repetition rates, and the time delay device of the pump path is canceled. Thus, the time delay between the pump and detection paths is completely controlled by the repetition frequency of the two femtosecond lasers. As shown in Fig. 2, the laser adopted in the experimental measurement has a repetition frequency of 100 MHz, a pulse width of 100 fs, and output laser energy of 300 mW. The spectral width of setup system is 5 THz, the scanning speed is 100 Hz, and the signal to noise ratio is 80 dB.. In this system, the THz wave generation is based on photoconductive antennas. The commonly employed photoconductive material is high-resistivity GaAs. The reflection mode is adopted to test the enamel sheet, and the THz wave is almost vertically incident and focused on the enamel sheet. During the measurement, the enamel sheet is fixed on a three-dimensional (x-y-z) electric translation. The system can not only test flat objects but also test complex shapes such as circles.

As shown in Fig. 3, the length, width, and thickness of the enamel sheet are 11, 6, and 0.3 cm respectively. Enamel is an important component of palace decorations, whereas there are few references on the application of THz technology to enamel both domestically and internationally. Through observations from the enamel sheets, the edges and patterns are made of yellow metal. The thickness of the leveraged metal lines varies in different parts, and the reflection of THz pulses exists both in the glaze and bottom layers. In Fig. 4, the enamel surface is enveloped and not flat, which indicates that the thickness of each position is different. The surface of the metal substrate is flat, which can be estimated as 0.23 cm according to Ref. [2]. As shown in Fig. 4 (a), the grayscale intensity image is the optical contrast of the enamel sheet with different materials in the THz frequency band. The yellow metal part, especially the metal part inside the box, has higher reflectivity, while the reflectivity of the edge pattern is not as high as the metal part inside the box. Although the internal filigree pattern is relatively thin, its shape can also be displayed in THz images. Additionally, no differences in the color of different enamel glazes are observed. The data is further optimized to obtain false color images of the enamel sheet, as shown in Fig. 5 (b). The reflectivity of the box and some edge parts is not the same as the edge part. The reason may be that the metal composition of the two parts is different.

We study the enamel sheet by utilizing the setup system in the Palace Museum. This system is utilized to investigate the enamel sheet. The experimental results show that the reflectivity of the carcass and filigree structure is higher, but the reflectivity of the edge pattern is not as high as that of the intermediate metal. Although the metal wire of the filigree structure is relatively thin, its morphology can still be displayed in THz images. There are no observed differences in the color of different enamel glazes, and the interior of the enamel sheet has at least two layers of structures respectively, including carcass layer and filigree glaze layer. The surface of the glaze layer for enamel is not flat but presents an envelope shape. It may be due to incomplete polishing during the final step of making enamel. The underlying carcass is flat and regular.