Oldal 32 [32]
OCR
CHARISM A I 7 a Figure 1-19 shows an example of a UV-induced luminescence image which has been flatfielded. The top and bottom edges of the image shown in Figure 1-19(a) appear darker than the central section. An image of a uniformly reflective board taken under the same lighting conditions, Figure 1-19(b), shows the inhomogeneous illumination. A flat-fielded image, which removes this inhomogeneity using the procedures described in section i, produces an image which appears more evenly and brightly illuminated, Figure 1-19(c). The above method can also be applied to visible-induced infrared and visible-induced visible images (although the post-processing of this image type is outside the scope of this work) and will be integrated into a workflow for the development of the post-processing software addressing the correction of luminescence images (see Chapter 3). The optimisation of experimental procedures to minimise the spatial inhomogeneities in illumination as well as the data acquisition requirements for post-processing are discussed in Chapter 2. — Ambient stray radiation Under ideal experimental conditions the radiation captured by an image should be due to reflected or emitted wavelengths. In practice, working conditions can often be far from optimal and it is not always possible to exclude ambient or parasitic sources of radiation fully. Parasitic radiation is associated with the inefficient filtration of unwanted radiation from a source. Sources of UV for example, also often emit a considerable amount of violet, blue and IR radiation and while single-colour LED radiation sources do not usually emit IR radiation, commercially available fluorescent radiation sources in the visible range are very likely to emit some IR radiation. Figure 1-20 shows visible-induced infrared luminescence images, acquired with a mixture of radiation from LEDs (R, G and B) and tungsten lamps (Figure 1-20(a)), and with radiation from LEDs only (Figure 1-20(b)). The IR radiation emitted by the incandescent source used in Figure 1-20(a) is visible as parasitic radiation in the image. However, as Egyptian blue is strongly luminescent and the amount of parasitic IR radiation is low, the effect is acceptable and can even be useful, as it can aid the location of luminescent areas on the object under investigation. However, in large amounts or cases with limited luminescence, these parasitic components can easily mask the luminescence of weak or poorly concentrated emitters. Strategies towards the elimination of parasitic radiation arising from these sources by means of filters are discussed in Chapter 2. Version No. 1.0 24 Date : 14/10/2013
