Stranica 52 [52]
OCR
i. Camera (detector) For a general discussion and recommendations on the selection of cameras and lenses for the capture of high-quality photographic documentation in art historical and conservation contexts, the reader is referred to the AIC Guide to Digital Photography and Conservation Documentation’ and references contained therein. In addition, the following recommendations are made for the purposes of the multispectral imaging techniques described in this work, which as discussed, employ commercially available digital cameras with a silicon-based sensor modified by removal of the IR-blocking hot mirror to provide sensitivity from approximately 350 nm to 1100 nm: 1) A high native sensor pixel count in order to produce high resolution images; 2) Ability to shoot RAW formats; 3) Live view capability. Note that the sensitivity of the camera sensors is typically 200 — 1100 nm and that the range is attenuated to the quoted 350 — 1100nm (in the modified cameras described in this work) because of the widespread use of glass lenses. The use of quartz lenses would make this range approximately 225 — 1100 nm, although it should be noted that the sensitivity of the sensor drops off significantly at the extremes of its wavelength range (see spectral response curves for a typical silicon-based sensor shown in Figure 2-2). 1.0 0.9 Bor 0.7 ~ 06 0.5 0.4 2 0.3 2 02 0.1 0 400 500 600 700 800 900 1000 1100 1200 Wavelength (nm) Figure 2-2. Schematic representation of the spectral response of a typical silicon-based (CCD/CMOS) sensor with and without an IR-cut off filter (as in the modified cameras described in this work). li. Radiation sources and filters The appropriate choice of radiation sources in order to produce sufficient incident radiation of the desired wavelength range, together with the efficient filtration of this radiation from the detector/camera, is paramount to the optimisation of the experimental set-up. In this section the output of typical sources for each of the imaging techniques is considered, in combination with the properties required by a filter (or filter set) to block this from entering the camera, whilst allowing the transmittance of radiation in the wavelength range under study. Version No. 1.0 AA Date : 14/10/2013
