Scanning multispectral vis-nir reflectography
Spectral imaging, widely used in remote sensing applications, such as satellite or radar imaging, has recently gained importance in the field of artwork conservation. In particular, multispectral imaging in the visible (VIS) and near-infrared region (NIR) has proved useful in analyzing ancient paintings because of the transparency of most pigments in the NIR region and their varied reflectance changes in the VIS region. A variety of systems, with different detectors and filtering or dispersing technologies, have been implemented. Despite the recognized potential of multispectral imaging, which provides information on both spectral and spatial domains (thus extending the capabilities of conventional imaging and spectroscopy), most of the systems currently used in art diagnostics have limitations. The technology is still in its early stages of development in this field.
Spectral reflectance characterization of the paint layer has received a great attention in recent years because of its several applications ranging from diagnostics (monitoring of conservation state and recognition of pigments) to digital documentation, in an absolutely non-destructive way.
The technique is based on irradiating the painting surface with broadband continuous sources, such as halogen lamps, and detecting with a suitable detector the back-scattered radiation within narrow spectral intervals.
Up to now, either CCD or Vidicon cameras have been used as a detector. In order to select suitable spectral bands in the spectral range of interest, the sensor is coupled with filters (a filter wheel or a LCD tunable filter) or is equipped with an imaging spectrograph.
These systems entail calibration procedures to correct the non-uniform irradiation of the investigated area as well as the chromatic and geometrical aberrations and the prospective distortions due to the camera lens. Moreover, as the spatial resolution is related to the measured area, in case of large panels several images must be acquired and combined into a mosaic.
To overcome the above mentioned problems, related to the use of spatially-extended sensors, we propose a innovative device based on a fast single-point spectrophotometer, specifically developed for multispectral imaging in the VIS 380-800 nm and NIR 800-2300 nm spectral region, opportunely combined with an high-resolution scanning system.
The lighting system, firmly settled with the collecting optics, and the single-point measurement ensure uniform illumination for all the sampling points and minimize the heating of the painting surface. Moreover, single-point detection avoids off-axis aberrations and the mechanical scanning entails distortions free images. The beam divergence is 10°, according to CIE specifications for the 45°/0° illumination/observation geometry.