Oldal 55 [55]
a CHARISMA I 7
are not appropriate. Glare-free and uniform illumination across the entire surface is essential
as shadows, reflections or uneven illumination could be misconstrued as variations in
infrared absorbency, although some of these issues can be dealt with in post-processing as
discussed in Chapter 1.
Filter in front of radiation source:
None
Filter in front of camera:
For infrared-reflected images, a filter is reguired which will block radiation in the UV and
visible region (200 - 700 nm). The filter selected should allow IR radiation to be collected
between 700 and 1100 nm, the end of the sensitivity of most DSLR camera sensors (Figure
2-5). However, the choice of the filter depends on the type of investigation which is
undertaken, as different materials have different reflective properties in the IR range. The
most commonly used range is approximately between 800 and 1100 nm, requiring a filter
with properties similar to the Schott glass RG830 filter which cuts-on at 830 nm (see Figure
2-6 for transmittance curve). Note that the sensitivity of the sensor drops off significantly at
the extremes of its wavelength range (See Figure 2-2).
Tungsten lamp
Relative spectral power
200 300 400 500 600 700 800 900
Xenon flash tube
Relative Intensity
300 400 500 600 700 800 900
Wavelength (nm)
350 400 700 1100
Wavelength range of a typical camera CCD/CMOS sensor
Figure 2-5. 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 infrared-reflected images.
Version No. 1.0 47 Date : 14/10/2013