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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 cross¬
linked 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-mean¬
square optimisation to produce a 3x3 matrix from camera RGB to XYZ.
Version No. 1.0 56 Date : 14/10/2013