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

Miniatures are characterized by their exceptionally detailed painting technique. Therefore, optical instruments such as magnifying glasses or microscopes are essential in the analysis of the painting technique and to examine even the smallest details. However, underdrawings hidden beneath the paint remain invisible to the human eye even with the aid of such instruments. The underdrawings only become visible using IR reflectography. The infrared radiation penetrates the paint and is reflected by the support so that underdrawings that absorb the infrared radiation can be visualized with IR cameras. However, the camera technology used so far has often led to unsatisfactory results. One shortcoming of the current technology is the lack of spatial resolution to image the fine structures typical of miniatures. The other weakness of most IR cameras is that they are only sensitive in the near infrared (NIR) which cannot penetrate highly absorbent paint layers as this requires shortwave infrared (SWIR). Therefore, we have developed a new type of camera setup at the HKB in Bern, which combines macroscopic and microscopic magnification technology with a camera that is sensitive up to the SWIR. This allows visualization of underdrawings with up to 9x magnification, even under highly absorbent layers of paint. This method not only detects the presence of underdrawings but can also characterize and describe the drawing media underneath.