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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
