A recent post described how to read optical production drawings, but an additional parameter found on optical drawings - "Datum Surface" deserves extra attention. The datum surface symbol looks like black triangle with letter inside square near triangle apex (see image below).
The key function of the datum surface is to define "reference point" for all other geometrical requirements for the lens (or for another mechanical or optical element which was drawn on the drawing). What does it mean?
In the drawing above the front face and back face of the lens are spherical surfaces. The side face is cylindrical surface. The axis of cylinder formation is datum axis. All other geometrical requirements are defined relative to this datum axis. In our case this is centering of the surfaces (see "How To Read An Optics Production Drawing" - "Table of parameters area" - "Column with surface parameters").
There are many ways of defining of the datum surfaces - on spherical surface, on chamfer surface, many more complex variants are possible. Only optical engineer in cooperation with mechanical engineer can estimate requirements for optical system or optical element and define the right method of datum surface assignment.
Using the wrong datum surface can effect on cost of the optical element production and on optical system quality. So we advise you to check your drawings definitions with your lenses producer.