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CHARISM A I 7 ij 1. Introduction to Multispectral Imaging This chapter describes the most commonly used multispectral imaging techniques that are currently available to the scientist, conservator, archaeologist and art historian, at a relatively low cost, for the non-invasive investigation of works of art. It will focus on the use of inexpensive and widely available equipment to provide qualitative, non-invasive, affordable and portable tools for the spatial localisation of specific materials or material types. In addition, it will aim to offer an understanding of the nature of the image sets produced from these techniques as well as the limitations which lead to the need for image correction procedures. Multispectral imaging is the procedure used to observe an object using selected ranges of wavelengths in the electromagnetic spectrum! that include and extend beyond the Capabilities of the human eye. This study will concentrate on the wavelength range that can be observed using modified commercially available cameras, which typically employ siliconbased sensors sensitive from approximately 350 nm to 1100 nm. Cameras based on InGaAs sensors, which can record infrared radiation from approximately 700 nm to 1700 nm, are also used regularly in cultural heritage applications,” but these shall not be addressed here as this specialized technology is often prohibitively expensive for the average user. A generic setup for multispectral imaging is composed of three main components: 1) Incoming radiation, which is generated by a radiation source and travels towards the object; 2) The object, which interacts with the incoming radiation; 3) Outgoing radiation, which, following the interaction between the incoming radiation and the object, travels from the object to the recording device. Both the incoming and outgoing radiation are typically in one of three ranges (Figure 1-1): ultraviolet radiation (UV 200-400nm), visible light (VIS 400-700nm) or infrared radiation (IR 760-1700nm).*" INFRARED 200 400 700 1700 Wavelengths in nm Figure 1-1. Wavelength ranges in the electromagnetic spectrum commonly used for multispectral imaging in cultural heritage applications. Version No. 1.0 Date : 14/10/2013
