Stranica 53 [53]
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