As discovered, froth images collected from industry
field show that: 1) a froth image is fully occupied by
bubbles, in normal case, without gaps or background
region between bubbles; 2) the illumination on bubble
surface is uneven; 3) each bubble has a convex shape,
which leads to the appearance of white spots, generally
on the top if the incident ray is right from the vertical
direction of froth layer (Fig.2); and 4) edges between
bubbles are weak, while edges caused by spot highlight
are obvious.