Stranica 20 [20]
OCR
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
