Although the use of correction masks is a standard technique for improving thickness uniformity, an alternative method, namely, the technique of terracing, can prove more effective and versatile.
Employing terracing, we reposition the substrates in the z-direction (which coincides with the rotation axis of the planet) and pivot the substrates to achieve a more desirable thickness distribution. To illustrate this, here we reposition and pivot three of the four convex lenses in Case 2 for different amount. (No correction mask is present.) In the new result, shown below, all the four lenses have very similar average thickness and the thickness distributions on all the lenses are symmetric to the lens centers.
Please view also 'Get the most out of you PVD coater' for more details.
To find the best substrate terracing is easy with V-Grade 5. The "Surface Coated" panel of V-Grade 5 allows you to select any substrate (by selecting its associated points) and apply translation and/or pivoting at will.