While useful, the FTGT and FTLT operands do have their own limitations. These operands only check the thickness between the vertex and the +Y radial aperture. As such, FTGT and FTLT can only be used with rotationally symmetric surfaces such as radial aspheres. For polynomial aspheric surfaces that are not rotationally symmetric, the more general STHI operand can be used. The STHI operand can be used to constrain the thickness of a surface measured from any location on the surface. When using this operand, you specify a surface number as well as an X, Y location:

Other useful operands for constraining the thicknesses of non-rotationally symmetric aspheric surfaces include:

            XNEA/XNEG/XNET (minimum edge thickness constraints)

            XXEA/XXEG/XXET (maximum edge thickness constraints)