Infrared reflectography in the macro- and micro-range: New advances in the analysis of the painting technique of miniatures

Miniatures are distinguished by their exceptionally detailed painting technique. Therefore, optical instruments such as magnifying glasses or microscopes are essential for the analysis of the painting technique and to examine even the smallest of details. However, underdrawings hidden beneath the paint remain invisible to the human eye even with the aid of such instruments. They only become visible using infrared reflectography, a non-destructive imaging technique.
However, previously used camera technology has often led to unsatisfactory results. While infrared cameras with silicon-based CMOS sensors can achieve high spatial resolutions in the macro range, their spectral sensitivity is limited to the wavelength range of the near infrared with a maximum of 1050 nm, which also limits the ability to pass through highly absorbent layers of paint. In contrast, cameras sensitive up to the short wavelength infrared range offer better transmission capability but do not allow macro photography, and their sensors only have a very low spatial resolution which is not sufficient for imaging the fine structures that are typical for miniatures.
Therefore, a new type of imaging setup was developed at the Institute Materiality in Art and Culture (IMIKUK), Bern Academy of the Arts (HKB), which combines a novel type of camera that is sensitive even in the extended short wavelength infrared range up to 2100 nm with macroscopic and microscopic magnification technology. This enables visualisation of underdrawings with a previously unmatched spatial resolution even below layers of paint that strongly absorb in the infrared range. With this method, not only can the presence of underdrawings be detected, but the type of drawing medium can also be analysed.