Stranica 20 [20]
The uniformly reflective board is ideally a grey Lambertian reflector (a surface showing the
same radiance when viewed from any angle - see Chapter 2 for examples). An image of the
board represents an RGB map of the distribution of the radiation source which is incident on
the surface under investigation. The board is placed parallel to the object and as close to the
plane of the object as possible. An image of the board, which must cover the entire capture
area, is taken. If the illumination is uniform, the image of the board should also have
consistent RGB values throughout.
The reflected image of the object is divided, channel by channel and pixel by pixel, by the
image of the uniformly reflective board. Subsequently, the reflected image is normalised
NTH FRAMEWORK
reflectance standard (for a description of the 99% Spectralon diffuse reflectance standard
see Chapter 2). This procedure avoids the normalisation to hot pixels. The result of the
division theoretically corresponds to the image of the object uniformly illuminated.
Figure 1-8. Compensation of spatial inhomogeneities of the radiation source for VIS, IRR and UVR
images of a wall painting fragment from the British Museum (Winged youth from the Tomb of the
Nasonii, 1883,0505.5). Each image is divided by its corresponding image of the uniformly reflective
board. The latter represents the distribution of the incident radiation. The result of the division
theoretically corresponds to the image of the object uniformly illuminated.
Figure 1-8 shows some examples of images which have been corrected for the spatial
inhomogeneity of the radiation source using this approach. The integration of this method
into a workflow for the development of the post-processing software addressing the
correction of reflected images is discussed in 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.
Version No. 1.0 12 Date : 14/10/2013