Oldal 38 [38]
a
CHARISM A I 7
Mes T D65 T
RGB XYZ XYZ
This procedure has been incorporated into a post-processing workspace for UV-induced
luminescence images using nip2. It applies the colour calibration matrix M (determined for
the calibration of VIS images, captured with an illuminant of colour temperature T), to the
UVL image acquired with a white balance set to D65 and imaged with the camera in RGB
colour space. This creates an image in ‘D65’, which requires a colour temperature transform,
MT, to compensate for the difference in colour temperature between D65 and T, and move
the result back to the equivalent CIE XYZ colour space under a D65 illuminant.
In the post-processing workspace this approach to the white-point adjustment has been
developed into a slider, so that the user can select the calibration illuminant colour
temperature T (used to acquire the VIS image and to determine M), from a source with
known colour temperature e.g. a D50 flash, or as measured with a colour temperature meter
(see Chapter 3). However, the approximation assumes that the changes in colour
temperature between T and D65 will not be very large. This is likely to be the case, since the
overwhelming choice of illuminant used in the acquisition of VIS images are flash lights
which are approximately D50, and hence close to D65. Trials have shown that for other
illuminants such as tungsten lamps (D32), the change in colour temperature is too great and
the approximation breaks down.*?
Currently the above method has only been applied to UV-induced luminescence images but
not to visible-induced infrared luminescence images as these are monochrome. The method
could possibly be applied to visible-induced visible images but the post-processing of this
image type is outside the scope of this work. The approach will be integrated into a workflow
for the development of the post-processing software addressing the correction of
luminescence images (see Chapter 3). The optimisation of experimental procedures, as well
as the data acquisition requirements for post-processing, are discussed in Chapter 2.
Version No. 1.0 30 Date : 14/10/2013