Standoff Mid‐Infrared Emissive Imaging Spectroscopy for Identification and Mapping of Materials in Polychrome Objects

Abstract

Microscale mid-infrared (mid-IR) imaging spectroscopyi su sed for the mapping of chemical functional groups.T he extension to macroscale imaging requires that either the mid-IR radiation reflected off or that emitted by the object be greater than the radiation from the thermal background. Reflectance spectra can be obtained using an active IR source to increase the amount of radiation reflected off the object, but rapid heating of greater than 4 8 8Ccan occur,which is aproblem for paintings.Rather than using an active source, by placing ah ighly reflective tube between the painting and camera and introducing al ow temperature source,t hermal radiation from the room can be reduced, allowing the IR radiation emitted by the painting to dominate.Thus,emissivity spectra of the object can be recovered. Using this technique, mid-IR emissivity image cubes of paintings were collected at high collection rates with al ow-noise,l ine-scanning imaging spectrometer,whichallowed pigments and paint binders to be identified and mapped.

Imaging secondary reaction products at the surface of Vermeer’s Girl with the Pearl Earring by means of macroscopic X-ray powder diffraction scanning

“…The main limitation of MA-XRF is that it cannot differentiate between compounds with a similar elemental but different molecular composition, as is often the case in degradation processes. More recently, a macroscopic scanning variant of FTIR in reflection mode (MA-rFTIR) has been developed, providing information on the molecular level [12,13]; however, its application is often limited to paintings without varnish, since the latter absorbs most of the infrared radiation. Visible/near infrared reflectance imaging spectroscopy (RIS) has also been frequently used for the investigation of oil paintings and is capable of delivering distribution images based on molecular features from both organic and inorganic compounds [14,15].…”

Near-UV to mid-IR reflectance imaging spectroscopy of paintings on the macroscale

“…The mock-up (Fig. 1A) has been previously described and analyzed ( 13 ). In brief, it was prepared using three sets of commercial tube paints.…”

“…Commercial imaging systems operating at these rates in the visible to NIR now exist. Hyperspectral cameras also exist for portions of the mid-IR, and we have used a high-throughput low-noise mid-IR camera operating from 1240 to 760 cm −1 to collect image cubes of paintings at room temperature in emissive mode at scan rates of 0.5 m 2 /min at 1-mm 2 spatial sampling ( 13 ). However, the drawbacks of commercial mid-IR imaging systems include the much higher costs due to increased complexity of the spectrometer and the mid-IR focal planes.…”

“…Extensive analysis of the distortions in mid-IR spectra has led to an improved capability to assign spectral features to specific vibrational transitions ( 10 , 12 ). Second, macroscale emissive mode mid-IR imaging spectroscopy has been shown to provide mid-IR spectra that are as good as what can be obtained with mid-IR reflectance mode point spectroscopy without the use of a hot thermal source that risks heating the painting ( 13 ).…”

Optical Spectroscopy of Surfaces, Interfaces, and Thin Films: A Status Report

“…They describe the power of this technique for providing critical information in the documentation of artworks and in situ identification of materials without sampling, in addition to improved visualization of underdrawings. Gabrieli et al recently proposed a solution to the thermal heating problem through the use of macroscale emission-based IR . Using a cooled aluminum tube between the aperture of the detector and the painting sample (maintaining an air gap between the tube and the painting so that it was not cooled too rapidly), these researchers obtained IR emission spectra comparable in sensitivity and selectivity to reflectance IR spectra taken of the same painting.…”