Stranica 32 [32]
CHARISM A I 7
a
Figure 1-19 shows an example of a UV-induced luminescence image which has been flat¬
fielded. 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