The key function of the datum surface is to provide a reference point for all the other lens geometrical requirements. Its symbol looks like black triangle with letter inside square near triangle apex. There are many ways of defining the datum surfaces such as on a spherical surface and many more complex variants. An optical, together with a mechanical engineer can estimate the requirements for an optical system or element and define the right method of datum surface assignment. A wrong datum surface can affect production costs the cost and system quality.