FIG. 7. The calibration procedure for digital imaging aims to equalize the grey-level values of pixels corresponding to the same mineral species wherever they are located in the field of view. A line drawn through the image of a single crystal illustrates how the original values are distributed (a). The use of a time averaging of successive frames drastically reduces the punctual noise (b). The additional correction for uneven illumination and uneven pixel response leads to an almost constant rendering of the uniform mineral surface (c).