FIG. 6. Plot of reflectance standards of 4.3%; 14.7%; 25.1% and 87.3%, respectively, at 550 nm against grey levels measured with a CCD sensor fitted with a 550±10 nm interference filter. The grey levels are average intensities from a 20x20 pixels region taken in the centre of the field of view. The quality of the regression demonstrates the perfect linearity of the CCD response under the chosen imaging conditions.