Stranica 36 [36]
CHARISM A I 7
a
— Camera response (luminescence calibration)
As discussed in the previous section on reflected images, the issue of varying camera
responses and therefore how to calibrate the camera, or apply corrections to address these
variations, is a particular challenge. Approaches for the colour calibration of reflected visible
light images have been addressed (see section above) but the calibration of camera
response with respect to luminescence is much more problematic, as this corresponds to the
direct capture of a light source. As a result, standard colour charts and greyscales used for
reflection cannot be used to solve issues related to exposure and colour correction.
Absolute calibration methods as discussed above, although accurate, are time-consuming
and restricted to well-equipped laboratories. These factors make absolute calibration
methods impractical for most users. However, such methods may not strictly be necessary in
order to produce comparable device independent images.
In this work the focus has been on the use of an ‘indirect’ method to standardise
luminescence images to allow their inter-comparison. The method described below achieves
a solution which will make use of accessible resources and are a viable alternative for more
advanced technological approaches. This new approach considers existing methods used
for the colour calibration of reflected visible light images based on use of a Macbeth target”
and an existing nip2 tool," as discussed earlier, and extends these to the calibration of
luminescence images by considering that:
The colour calibration matrix for reflected visible light images M calculated by this tool, takes
a Macbeth target illuminated with a source of colour temperature T and imaged with the
camera in the camera’s RGB colour space and converts this output into the equivalent CIE
XYZ colour space under a D65 illuminant i.e. D65 CIE XYZ (Figure 1-23).? The colour
Calibration matrix, M, can then be applied to calibrate the VIS image acquired under the
same conditions.
M
T D65
RGB XYZ
A UV-induced luminescence image acquired with a white balance set to D65 and imaged
with the camera RGB would require a calibration matrix M’, to take this target to D65 CIE
XYZ (Figure 1-23).
D65 D65
RGB XYZ
This cannot be determined directly but we can think of M’ as a product of a colour
temperature transform MT’, which takes camera RGB D65 to camera RGB T, and a matrix
M (the colour calibration matrix for VIS images, which is known), which then takes this image
to D65 CIE XYZ (Figure 1-23).
Version No. 1.0 28 Date : 14/10/2013