Controlling thickness of freeform lens

  • 28 June 2024
  • 5 replies



I’m using two extended polynomial surfaces to define a lens. How do I control the min and max thickness of the lens? Are there operands like FTGT and FTLT for freeform lenses?



5 replies

Userlevel 5
Badge +3

You want to constrain surface normal lengths to the next surface, rather than thickness.  Use the NORD operator, along with OPGT and OPLT, at many places along the surface contour from axis to edge.


Normal distance to the next surface. This operand computes the surface normal vector at the coordinate defined by X and Y on the surface defined by Surf, then returns the distance to the next surface measured along the normal vector.

Userlevel 3

In case of hard aspheric surfaces, it can give non-expected values. Maybe MNEG with a specific zone will be better here.

Userlevel 7
Badge +3

What’s wrong with the FTGT and FTLT operands? Don’t they do precisely what you want?

  • Mark

FTGT measures the thickness only along the Y axis so it works only on symmetrical lenses. Freeform lenses are rarely symmetrical.  

Userlevel 7
Badge +3

Ooff...good point