Average colour ratios versus phase angles: (a) R905/R750; (b) R905/R633; (c) R905/R458; (d) R750/R633; (e) R750/R458; (f) R633/R458. The number markers 1 to 5 that represent particle size distributions are indicated in (b). The ratio curves are obtained by averaging the ratio values of pixels of a region of interest (ROI) within the sample area for each sample. The inset in (c) shows an example of how the ROIs were chosen for measurement at e = 0°. The ROI shape changes with the variation of viewing angles because the square sample boxes will look like trapezoids at a viewingangle far from nadir. We note that some of the curves are too close to show the number markers.