Stranica 53 [53]
OCR
at — Visible-reflected (VIS) Images Radiation source: The three most commonly used radiation sources for visible light photography in cultural heritage applications are: fluorescent light banks, incandescent light sources (Such as tungsten or tungsten-halogen lamps) and strobe or flash sources. Each has associated advantages and disadvantages and these are discussed in some length in the numerous manuals available on the photography of works of art.? For the purposes of this work, the most important consideration is the spectral output distribution (SPD) of the chosen source. A well-characterised SPD will allow the appropriate selection of filters to eliminate the unwanted wavelengths of radiation, e.g. many sources of visible radiation may include some UV and/or IR radiation in the form of heat (see the SPDs of a typical tungsten lamp and a xenon flash tube shown in Figure 2-3). Tungsten lamp Relative spectral power 200 00 400 500 600 700 800 100 | Xenon flash tube lative Intensity a R 300 400 500 600 700 800 900 Wavelength (nm) 350 400 700 1100 Wavelength range of a typical camera CCD/CMOS sensor Figure 2-3. Schematic representation of the spectral output distribution (SPD) of a typical tungsten lamp and xenon flash tube compared to the wavelength range of a CCD/CMOS sensor. Shading shows wavelengths to be excluded (filtered) in the acquisition of visible-reflected images. In addition, knowledge of the SPD of the source, its associated colour temperature (or correlated colour temperature for non-incandescent sources) and colour-rendering index is valuable in the evaluation of its colour rendering capabilities and in the subsequent colour Version No. 1.0 45 Date : 14/10/2013
