Periodic structures are of great importance in optical instruments. Actually, the periodic structures in
optical instruments are not perfect and this may result in the negative effect on the function of the
instruments. It is necessary for us to study the specification of periodic structures. In this paper, a new
meaningful parameter, named the total of dark points, is defined based on the characteristic of the
overlay of the images of two repeating elements in a periodic structure. Furthermore, a new method
with high measure accuracy, named dark point algorithm, is introduced to test the periodic structures.
Some testing examples and testing results using dark point algorithm are provided as well. Quantitative
description of the quality of periodic structures can be made taking this new method, and it can help to
analyze the differences between the structures and their designs. The new method can also be easily
realized in many physics processes and with some equipment, it can indeed be used in large scale
production and bring economic value to us.