Hyperspectral Imaging: A non-destructive detection tool for unlocking the "invisible codes" of Renaissance masterpieces

To commemorate the 500th anniversary of Raphael's death, the Galleria Borghese in Rome utilized reflective Hyperspectral Imaging (HSI) combined with Macro X-ray Fluorescence (MA-XRF) to complete a full-frame, sub-millimeter non-destructive inspection of the Renaissance masterpiece "The Deposition" (Baglioni Entombment). This technology is like giving a famous painting a "non-invasive spectral CT scan," penetrating the pigment layers to reveal underdrawings, traces of modification, and pigment codes hidden for over 500 years, allowing us to understand the master's entire creative process.

I. What is Hyperspectral Imaging?

Hyperspectral imaging, simply put, is a "two-in-one" of "imaging + spectroscopy." It doesn't just capture a picture; it records the complete spectral information of every pixel from visible light to short-wave infrared (400–1700 nm), turning an ordinary photo into a three-dimensional data cube available for deep analysis.

The Visible-Near Infrared-Shortwave Infrared hyperspectral scanner used in this study was specifically designed for cultural relics: it adopts push-broom scanning with extremely high resolution, and the illumination is concentrated only on a narrow area, causing almost no damage to the painting; even when facing curved wooden panels, clear imaging can be guaranteed through optical correction. The research team scanned the entire painting in 8 segments and then precisely stitched them together to obtain ultra-large spectral data, achieving full-frame, zero-dead-angle analysis, completely moving away from the limitations of traditional single-point sampling.

II. Seeing Raphael's "Invisible Creations"

The greatest capability of hyperspectral imaging is seeing underlying information invisible to the naked eye. With the help of algorithms such as Principal Component Analysis (PCA) and Minimum Noise Fraction (MNF) to process spectral data, "invisible content" within the frame emerges one by one.

In the background sky, spectral processing unexpectedly discovered covered early landscapes: originally clearly outlined trees and vegetation were later softened by Raphael to blend into the blue sky, making the space feel more profound; the shapes of the mountains also changed from sharp to rounded. These traces of modification in the middle pigment layers are key evidence difficult to capture with traditional infrared or X-rays, directly restoring the master's composition adjustment process.

Even more startling is the underlying sketch. Traditional infrared reflectography can only see carbon-based lines clearly, while hyperspectral imaging—by selecting optimal infrared bands and synthesizing false-color images—clearly presents finer underdrawings: hatching on the male characters' faces and heavy outlines on the Virgin Mary's cheeks and lips, which were previously completely hidden. This proves that Raphael's underdrawings were completed in multiple stages using different materials, making the creative process far more complex than imagined.

III. Hyperspectral + XRF Cracks the Red Pigment Code

Hyperspectral imaging alone cannot fully determine pigment components; when used in conjunction with MA-XRF, they form a "molecular spectroscopy + elemental analysis" golden duo, precisely cracking the core red code of this painting.

Researchers used Spectral Angle Mapping (SAM) to divide the red into three types of spectral characteristics: two types corresponding to red lakes and one type corresponding to vermilion. Then, by cross-referencing the element distribution map from X-ray fluorescence: mercury (Hg) signals only appeared in the vermilion areas, potassium (K) signals confirmed the red lakes, and iron (Fe) was unrelated to the red, excluding iron oxide red.

Ultimately, it was confirmed: Raphael used only two red materials, vermilion and red lake, and used three techniques—single-layer thick application, multi-layer glazing, and red lake over vermilion—to create rich layers. Only the core figure, Grifonetto, used "vermilion base + red lake glazing" to highlight his status. This rigorous yet ingenious way of using color was revealed completely for the first time.

IV. The Future Core Technology of Cultural Relic Protection

This cross-border cooperation between technology and art fully demonstrates the unique value of hyperspectral imaging in cultural relic protection: completely non-destructive, deep penetration, global analysis, and data archivability.

It requires no sampling and no damage to the painting to excavate underdrawings, layering, pigments, and restoration traces, becoming a standard tool for museum research, restoration, and digital protection.

From invisible underdrawings to covered compositions and then to precise pigment formulas, hyperspectral imaging lets masterpieces "speak" their creative stories. It is not just a cutting-edge technology, but a bridge connecting art history and materials science, protecting and decoding mankind's most precious cultural heritage in the gentlest way.