Asphericity of aspheric and freeform parts

  • 29 December 2023
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Asphericity is a useful value when manufacturing an asphere or a freeform surface.  It is typically defined as the distance between the smallest best-fit sphere (BFS) that touches the surface without intersecting and the ideal surface shape, as sketched below.  (The BFS usually contacts the surface at one radial location; in the part sketched below, the BFS contacts the part at the outer edge.) 

OpticStudio can return both the BFS and the asphericity using the Surface Sag map and/or the DSAG operand in the Merit Function.  Choose Analyze/Surface/Sag to open the Sag map of the surface and then set “Remove” to “Best Fit Sphere” with the “Minimum Volume” option, as shown below.  The peak-to-valley of the resulting plot is the asphericity.  The radius of curvature of the minimum volume BFS is also listed.

Use the Surface Sag map to find the asphericity of a surface.

Both values can also be retrieved using the DSAG operand with Remove = 2 and BFS = 0.  Data = 2 retrieves the P-V of the sag map, equal to the asphericity. Data = 9 retrieves the radius of curvature of the Minimum Volume best-fit sphere.


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