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0.1% % Transmittance 2 Aysuag esnyig 10% 100% 200 300 400 500 600 700 Wavelength (Nanometres) Figure 2-11. Transmittance curve of a new (right) and well used (left) Kodak Wratten 2E filter demonstrate that these ultraviolet absorbing filters gradually lose their effectiveness and should be replaced regularly." — Visible-induced infrared luminescence (VIL) Images Radiation source: The choice of the radiation source depends on the luminescence properties of the pigment under investigation. Incandescent radiation sources have been used in the investigation of cadmium-containing pigments.’® It should be noted that such radiation sources are very likely to emit some parasitic IR radiation and as such will require filtering. Light emitting diodes (LEDs) have been used in the case of Egyptian blue, Han blue and Han purple. *”’® LEDs are good visible light sources that generally emit minimal amounts of infrared radiation, however the spectral distribution of LEDs should always be checked to evaluate how much, if any, infrared radiation is emitted. Filter in front of the radiation source: None if LEDs with spectral distributions in the visible are used. Filter in front of the camera: For VIL images a filter is required which will block light in the visible region (400-700 nm). The filter selected should allow IR radiation to be collected between 700 and 1100 nm, the end of the sensitivity of most DSLR cameras but 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, which requires a filter with properties similar to the Schott glass RG830 filter (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). Version No. 1.0 52 Date : 14/10/2013
