Stranica 58 [58]
OCR
200 450 500 550 600 650 700 750 800 Figure 2-8. Transmittance curve of a UV bandpass filter (e.g. SchottDUG11X)." — UV-induced visible luminescence (UVL) Images Radiation source: See UV radiation sources equipped with visible and IR-blocking filters, as described above. Filter in front of the camera: Collection of UVL images requires a filter with properties similar to the IDAS-UIBAR filter (bandpass, c.380-700 nm, see Figure 2-4 for transmittance curve). However, a narrower bandpass which would also eliminate the parasitic light at c. 420 nm would be preferable. If unavailable, an additional filter with properties similar to the Schott KV418, which cuts-on at 418 nm (see Figure 2-10 for transmittance curve) could be added to the bandpass to remove these additional wavelengths. Kodak WRATTEN 2 Optical Filters’* (particularly the 2E filter) are also very commonly used as UV-blocking filters in the capture of UVL images. Kodak WRATTEN 2E is a pale yellow filter which cuts-on at c.430 nm.” Although this would eliminate the parasitic light, its use is not ideal, as the longer cut-on gives images a yellow cast which must be considered when Carrying out image calibration. In addition, some studies have suggested that this class of filters fade with exposure to UV (Figure 2-11) and luminesce slightly when illuminated by short-wave UV radiation and should be replaced regularly if used." Version No. 1.0 50 Date : 14/10/2013
