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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
