## 3 EM Techniques for CH Diagnosis
### Multispectral and Hyperspectral Imaging  
These techniques capture data across a wide range of the electromagnetic spectrum: the visible region, infrared, and ultraviolet. Although these two terms are often used interchangeably, the bands used for multispectral imaging (MSI) consist of tens of bands, while those used for hyperspectral imaging (HSI) encompass hundreds or thousands. Consequently, the first technique relies on separate and specific wavelengths, whereas the second uses a continuous range of wavelengths [101]. MSI employs photography with lighting at specific wavelengths, and filters are sometimes used to enable fluorescence, which is usually combined with spectral imaging. HSI uses a white light source, and gratings are employed to split the detected light into its spectral components [82]. Sampling is not required, which is a significant advantage when analyzing CH artifacts, as it allows for the visualization of underdrawings. However, image processing is time-consuming and requires a certain level of expertise. Practical Application of MSI and HIS to CHMSI systems entered the CH field in the 1990s to identify and map pigments in paintings. They permit the gathering of the spatial distribution of materials based on spectral emission information over the entire artwork [98]. Image processing methods of multispectral techniques allow for the grouping of materials, pigments, and alterations according to spectral similarities. When used in the short-wave infrared regions, they permit the investigation of the inner layers of paintings, highlighting also underdrawings and pentimenti--thus revealing the artist's technique and non-original retouching. Multispectral imaging is also used as a means of preserving fragile artifacts by creating digital high-resolution records promoting remote access for research purposes. It is extensively used to examine different types of inks on paper and parchment [108; 109; 110]. Figure 4: Images of two paper samples at different magnifications. **(a,b)** paper sample of the XIX century; **(c,d)** paper sample of the XXI century. Credit [PERSON] et al. [107]. Licensed under Creative Commons, attribution (CC BY), [[https://creativecommons.org/licenses/by/4.0/](https://creativecommons.org/licenses/by/4.0/)]([https://creativecommons.org/licenses/by/4.0/](https://creativecommons.org/licenses/by/4.0/)) (accessed on 20 November 2024). No changes were made to figure but to capture. Figure 5 provides a comprehensive example of the results that can be obtained through multispectral imaging, and Figure 6 shows how pigment identification can reveal retouching. As can be easily understood from these figures, multispectral imaging is never used alone; it is always combined with other techniques. According to [111], the documentation of current MSI applications to CH objects is often inconsistent, not allowing for the repeatability of results. Although a great variability in the image data also occurs for HSI [112], the results of MSI are, according to [PERSON]  
Figure 5: _Madonna and Child_, Ingels Collection, Sweden. Probably Sienese School, 14 th century. Example of multispectral imaging documentation. Credit [PERSON] [110]. Licensed under Creative Commons [[http://creativecommons.org/licenses/by/2.0](http://creativecommons.org/licenses/by/2.0)]([http://creativecommons.org/licenses/by/2.0](http://creativecommons.org/licenses/by/2.0)) (accessed on 20 November 2024), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited. No changes were made to the figure but to capture. Figure 6: _Madonna and Child_, Ingels Collection, Sweden. Probably Sienese School, 14 th century. The two different blue pigments reveal retouching. Credit [PERSON] [110].