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OCR
iii. Standards and Calibration targets The correction methods detailed in Chapter 1 and the workflows proposed for the development of the post-processing software, make use of a number of calibration tools in order to apply these correction procedures. These are discussed in some detail below. — Uniform reflective board Flat-fielding, the mathematical operation of compensating multispectral images for the Spatial inhomogeneities of the radiation source and establishing uniform illumination conditions described in Chapter 1, uses an image of a uniformly reflective board to record the distribution of the radiation incident on the surface under investigation. Ideally, such a board should be a grey Lambertian reflector: a surface showing the same radiance when viewed from any angle. However, Lambertian reflectance targets made from materials such as Spectralon’® which have near-perfect Lambertian diffuse reflectance can be prohibitively expensive, especially when considering that a large board is often required since: 1) The image of the board must cover the entire capture area; and 2) The board must be placed parallel to and as close to the plane of the object as possible. Commercially available white foam board, such as Plastazote LD45”° (a closed cell crosslinked polyethylene nitrogen expanded foam), which is commonly used in the storage of museum objects, can provide affordable alternatives to such products. The flexible foam board can be affixed to a rigid support of any dimensions. The foam is non-luminescent in the wavelength range investigated and any means of affixing it to a rigid support should also be tested for it luminescence properties in this region. — Macbeth (X-Rite) ColorChecker The colour calibration procedures for both visible-reflected and UV-induced visible luminescence images and the methods proposed for correcting reflected images for the spectral density of a radiation source, both employ existing methods based on use of a Macbeth target and an existing nip2 tool." The X-rite ColorChecker Color Rendition Chart” (often referred to by its original name, the Macbeth ColorChecker, as it was formally produced by Gretag Macbeth) is a colour calibration target consisting of a cardboard-framed arrangement of 24 squares of painted samples in a 4 x 6 grid. Six of the patches form a uniform greyscale, and another six are primary colors typical of chemical photographic processes — red, green, blue, cyan, magenta, and yellow. The remaining colors include approximations of medium light and medium dark human skin, blue sky, the front of a typical leaf, and a blue chicory flower. The rest were chosen arbitrarily to represent a gamut "of general interest and utility for test purposes", though the orange and yellow patches are similarly colored to typical oranges and lemons.”° Other such targets are available? but this work is based on the use of the X-rite ColorChecker, as the nip2 software referred to above has built into it the Lab values for these patches under D50 and D65 illuminants, which it uses in carrying out the least-meansquare optimisation to produce a 3x3 matrix from camera RGB to XYZ. Version No. 1.0 56 Date : 14/10/2013
