ELSEVIER Chemometrics and Intelligent Laboratory Systems 39 (1997) 103-114 Chemometrics and intelligent laboratory systems Principal component analysis of visible and near-infrared multispectral images of works of art S. Baronti, A. Casini, F. Lotti, Simone Porcinai * lstituto di Ricerca sulle Onde Elettromagnetiche “ Nell0 Carrara” - C.N.R., Via Panciatichi, 64, 50127 Firenze, Italy Received 15 October 1996; accepted 20 February 1997 Abstract Principal component analysis (PCA) was applied to a very simple case of a tempera panel painted with four known pig- ments (cinnabar, malachite, yellow ochre and chromium oxide). The four pigments were spread pure as well as dilute with carbon black (5% w/w, 10% w/w) thus creating 12 homogeneous areas of the same size. The panel was imaged by a Vidi- con camera in the visible and near-infrared regions (420-1550 nm) resulting in a set of 29 images. PCA was applied by taking various subsets of the input data. From the analysis of this simple and predictable case study some guidelines are synthesized and proposed for the application to actual work of art. Results are presented for the painted panel. Preliminary results are also reported for the Luca Signorelli’s “ Predella della Trinitl” . The multivariate image analysis results in the visible and near-infrared regions show that it is possible to use the multispectral image data in order to get a segmentation and a classification of painted zones by pigments with different chemical composition or physical properties. 0 1997 Else- vier Science B.V. Keywords: Principal component analysis; Imaging spectroscopy 1. Introduction can be easily handled to collect data from as many After ten years of experience at our institute, we are in a position to affirm that fiber-optic reflectance spectroscopy (FORS) in the visible and short-wave- length infrared is a powerful technique for attaining good knowledge of the constituent materials and of the degradation products of paintings, without caus- ing any damage to them [ 1,2]. Among non-invasive spectroscopic techniques, it is also the most functional, as it can be performed with transportable equipment. Its fiber-optic probes * Corresponding author. Tel.: + 39-55-4235273; fax: + 39-55. 410893; e-mail: porcinai@iroe.iroe.fi.cnr.it. points as needed, without limits to the size of the ob- ject. However a collection of local 1D spectrograms, though rich in information to a chemist, cannot be readily correlated to the visual appearance of the work of art. A technique which promises to be even more effi- cient than punctual reflectance spectroscopy is its 2D extension: imaging spectroscopy or multispectral imaging, which supplies very readable data by retain- ing the visual aspect of the objects. Although suit- able imaging spectroscopes are not yet available, valid test data can be collected using a set of optical filters which select the wavelength of the radiation detected by a camera. 0169-7439/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PIZ SO169-7439(97)00047-6