Moiré patterns result
from superposition of two gratings. The uniform grating is the reference
grating and the non-uniform is the deformed or specimen grating. The moire
pattern thus shows the difference between the two patterns. This helps
among other things to identify the difference between the two gratings.
Subtle differences can be highlighted as in this animation.
Shifting the reference grating moves the moire pattern. If the moire pattern
moves in the same direction as the grating shift, this implies that the
deformed grating has a larger spacing between the lines than the specimen
grating. In this example it helps differentiate between hills and valleys.