Hello Amit Nodal Named Selections are tricky, glad to see you figured it out.
Optical surfaces are typically defined using a Sag equation.
A spherical surface is defined by the Radius of Curvature R and the Diameter D. I would replace D/2 with r in the equation below because if you have the X,Y coordinates of a node in the coordinate frame as I described above, then the value of r is simply the square root of the sum of the squares, and the Z coordinate is the SAG.
CAD systems can create a perfect spherical surface, so it is likely that the nodes in the model lie exactly on the surface defined by this equation.
Many optics use an aspheric surface
The optical design engineer would provide the coefficients in that equation to the mechanical design engineer/analyst who can recreate the surface precisely.
The CAD geometry you have may not precisely match this sag equation if it is an aspheric surface. It is a good idea to check the deviation of the sag of each node location from the sag value calculated from the equation and provided coefficients. The error should be less than a small fraction of a nanometer.
I don't know how to export nodal coordinates in a local coordinate frame in ANSYS (I know how to do it in Nastran). I expect there is a way. That might be a good question for a new Discussion and perhaps an ANSYS staff member will reply. It would make sense if you only have two surfaces, to align them so the global coordinate frame is on the vertex of one of the surfaces with the Z coordinate pointing to the other vertex, then the local coordinates of the other surface is easily obtained by subtracting the lens thickness from the z coordinate of the second surface.
I tested a Directional Deformation result and you get to specify the local coordinate system in which to measure the deformation, which is good. However, when you export the data, the nodal location is given in Global coordinates, which seems silly. What is worse, the text file doesn't even name the coordinate system in which the deformation is measured, so you need to do careful bookkeeping.