Application to cultural heritage
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In collaboration with CCR (Centro Conservazione e Restauro “La Venaria Reale”), we applied HSI to the study of a small (34 cm x 49 cm) Flemish painting on copper, whose authorial attribution is still uncertain. The crowded and analytic composition made of brilliant colours and very subtle brushes typical of the Flemish School – the same characteristics that lead the viewer to dwell on any one detail – has represented the occasion not only for getting information useful for pigments’ identification, by also for evaluating the spatial resolution performance of the instrument. For validating the HSI instrument as diagnostic tool, we selected some interesting points and we used fiber optic reflectance spectroscopy (FORS) analyses as comparison means. At the end, we used hyperspectral data to calculate the RGB image, useful as basis for color rendering elaboration.
Fig.1, Picture of the set-up with the HSI prototype on the left hand side, the map on the right hand and the painting in the background of the picture.
Result:: Comparison between FORS and HSI
The hypercube of the main central scene, displaying Mosè dividing the waters of the Red Sea (Fig. 2). The dimensions of the acquired area is about 10 cm x 10 cm, corresponding to an image size of about 1000 pixels x 1000 pixels, due to the aperture of our F-P interferometer (for acquiring scenes with different spatial resolution we could adapt the optical system). The measurement took 180 seconds, with a 10 THz resolution: the painting was 120 cm distant from the camera and one halogen lamp lighted the scene at 45° angle on the right to the painting, in order to reproduce one of the illuminating/viewing geometry (45°x/0°) recommended by CIE [19] and the same geometry and illumination used for FORS analyses.
In the graph, the red line reports the spectrum measured with the Fiber optic reflectance spectroscopy (FORS), the dashed black line the spectrum extracted from the HSI hypercube of blue p26. The double absorption band at around 595 nm and 650 nm could be related to the presence of the smalt pigment, with the expected intermediate reflectance values in the IR region .
Result:: RGB reconstruction
In this picture we present the RGB obtained from the data of the painting details weighing each spectrum with the CIE1964 colour matching functions rendered with the D65 standard illuminant which has the maximum colour rendering index for this kind of observer.
This RGB reconstruction was not characterized by a colorimetric point of view (since spectra start from 450 nm) but it is intended to show the capability of the system only. The white board on the right hand side id used to normalize the reflectance of each pixel since the spectrum has to normalized due to the trasmittance of the optics and the responsivity of the camera
This RGB reconstruction was not characterized by a colorimetric point of view (since spectra start from 450 nm) but it is intended to show the capability of the system only. The white board on the right hand side id used to normalize the reflectance of each pixel since the spectrum has to normalized due to the trasmittance of the optics and the responsivity of the camera
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