Quantitative Hyperspectral Reflectance Imaging

Abstract

Hyperspectral imaging is a non-destructive optical analysis technique that can for instance be used to obtain information from cultural heritage objects unavailable with conventional colour or multi-spectral photography. This technique can be used to distinguish and recognize materials, to enhance the visibility of faint or obscured features, to detect signs of degradation and study the effect of environmental conditions on the object. We describe the basic concept, working principles, construction and performance of a laboratory instrument specifically developed for the analysis of historical documents. The instrument measures calibrated spectral reflectance images at 70 wavelengths ranging from 365 to 1100 nm (near-ultraviolet, visible and near-infrared). By using a wavelength tunable narrow-bandwidth light-source, the light energy used to illuminate the measured object is minimal, so that any light-induced degradation can be excluded. Basic analysis of the hyperspectral data includes a qualitative comparison of the spectral images and the extraction of quantitative data such as mean spectral reflectance curves and statistical information from user-defined regions-of-interest. More sophisticated mathematical feature extraction and classification techniques can be used to map areas on the document, where different types of ink had been applied or where one ink shows various degrees of degradation. The developed quantitative hyperspectral imager is currently in use by the Nationaal Archief (National Archives of The Netherlands) to study degradation effects of artificial samples and original documents, exposed in their permanent exhibition area or stored in their deposit rooms.

Use of imaging spectroscopy, fiber optic reflectance spectroscopy, and X-ray fluorescence to map and identify pigments in illuminated manuscripts

“…Spectral imaging for art conservation is an active field (Delaney et al, 2005;Fischer & Kakoulli, 2006;Klein et al, 2008;Zhao et al, 2008, Delaney et al, 2009a, b, 2010Padoan et al, 2010) and an important distinction still exists between radiometrically un-calibrated spectral imaging and imaging spectroscopy (Delaney et al, 2005;Klein et al, 2008). The former mainly uses the spectral differences of artist's materials to enhance the visualization of specific features by creating false color composites, difference images, or principal component images, to separate for example iron-based from carbon-based ink writing (Mrusek et al, 1995;Havermans et al, 2003;France, 2011).…”

Hyperspectral Imaging for Ink Mismatch Detection

“…The system provided high spatial and spectral resolution from near-UV through visible to near IR range. The only limitation of the system was its extremely slow acquisition time (about 15 minutes) [7]. Contrary to that, our proposed system captures hyperspectral images in only a fraction of that time.…”

Advances in multispectral and hyperspectral imaging for archaeology and art conservation

“…The other advantage is that any off-the-shelf monochrome camera and lens system can be used without modification. However, in practical terms an average camera lens has significant chromatic aberration such that images collected at widely different wavelengths will have different focal length resulting in significant difference in magnification [18] which will then need to be corrected in a post-processing software. While such a system is compatible with a wide range of cameras, it can be less flexible and portable in terms of the lighting component of the system.…”