Stranica 18 [18]
i. Reflected images
The factors affecting reproducibility and device dependence of reflected images are
discussed in the sections below. Three main effects are considered:
1) Spatial inhomogeneities of the radiation source;
2) Spectral density of the radiation source and non-linear camera response;
3) Camera response (colour calibration for visible-reflected images only).
In addition, the need for image registration of the various reflected images in order to
produce false-colour images is discussed.
— Spatial inhomogeneities of the radiation source
The spatial distribution of radiation reflected from objects and captured by a camera
depends, among other factors, on the position of the radiation source(s) and the geometry of
the object. Non-uniform illumination can lead to spatial inhomogeneities introduced by the
illumination system.
Figure 1-6(a) and (c) show two images of a uniformly reflective board taken under different
illumination conditions: Visible and UV illumination, respectively. Although these images look
relatively uniform, it is clear from the enhanced contrast images (iso-grey level values)
shown in Figure 1-6(b) and (d) that the distribution of incident radiation in each case is very
uneven.
Visible-IR illumination UV illumination
WB visible: Enhanced WB UV Enhanced
min RGB = 126 contrast Min RGB = 160 contrast
max RGB = 198 max RGB = 231
A= 28% A= 28%
Figure 1-6. Images of a uniformly reflective board taken under different illumination conditions: (a)
Visible and (c) UV illumination. Images (b) and (d) are enhanced contrast images showing the iso¬
grey levels corresponding to the distribution of incident visible light in (a) and UV radiation in (b) on
the surface under investigation. "
An example of the impact of non-uniform illumination on the resultant reflected image is
shown in Figure 1-7(a), where the upper section of a visible-reflected image appears to be
Version No. 1.0 10 Date : 14/10/2013
NTH FRAMEWORK