Stranica 56 [56]
OCR
0.99 0.98 0.97 0.96 0.95 0.90 0.80 0.70 0.60 0.50 0.40 Internal transmittance 0.20 0.10 0.05 0.01 1E-03 1E-04 1E-05 200 300 400 500 600 700 800 900 1000 1100 1200 Wavelength [nm] Figure 2-6. Transmittance curve of a UV/visible-blocking filter (e.g. Schott RG830, which cuts-on at 830 nm). — UV-reflected (UVR) Images Radiation source: The most commonly used excitation sources for imaging methodologies requiring UV radiation are UV-A glass mercury vapour lights (Aex. = 365 nm), also known as Woods lamps (named after Robert W. Wood who in the 1920s, created a glass with nickel oxides which has the property of transmitting UV and IR radiation and blocking most of the visible radiation®) or black-light-blue (BLB) lights. The spectral output distribution of a typical Woods lamp (Philips PL-S 9W double BLB) is shown in Figure 2-7. It should be noted that, although the source has a principal peak at 365 nm, the emission has a tail which extends well down into the visible and IR range. Mercury lights also have a peak at 405 nm. Therefore, a considerable amount of violet, blue and IR radiation is emitted. This parasitic component can easily mask the luminescence of weak or poorly concentrated emitters in this region. Filter in front of radiation source: In general terms, silicon-based camera sensors have low sensitivity to UV radiation, but are very sensitive to visible and IR radiation. Therefore, it may be beneficial to block off the parasitic visible and IR radiation described above. The use of an interferential excitation filter (vapour-deposited heavy metal oxides on both sides), such as the Schott DUG11X filter (see transmittance curve in Figure 2-8) in front of the radiation source has been proposed in order to minimise the transmittance of both violet-blue and IR radiation." It should be noted that, although the DUG11X filter is still available at present, it will soon be out of production. It is important that alternatives to this filter (whether one or a combination of filters) retain the properties exhibited in Figure 2-8, and effectively completely cuts-off IR radiation, as well as visible wavelengths. Note that, in addition to the parasitic light due to the excitation source, sources of ambient stray radiation (Such as apertures or fluorescent specimens present in the studio or Version No. 1.0 48 Date : 14/10/2013
