Fig. 8. (a) Raw SORS signals from KClO₄ powder filled in 25 mm square HDPE container with a wall thickness of 3 mm with 0 mm (a), 6 mm (b) and 8 mm (c) offsets respectively. Laser beam was focused on to the container. As the offset is increased to 8 mm, the signal of the sub-surface material improves.