Wedged Lens

The wedge or tilt of a lens can be modelled in FRED by following these basic steps:

Convert nominal lens to a custom element
Apply (maximum) tilt (or decenter) to the wedged lens surface(s)
Oversize the surface trimming volumes
Apply surface trimming

As demonstrated in the example below featuring a simple plano-vex lens where the front convex surface is wedged.

Example

In a new FRED document, add a new lens with the following parameters:

If we edit surface 1 of the lens and try to add a rotation (tilt) about X-axis via the location/orientation tab it can be seen that the Lens Primitive will not allow us to add a tilt to the surface. Any parameters we change on the child surfaces are ignored (if we close and reopen the dialog we will see our changes are gone because they are set by the parent primitive, which does not support wedge).

To give us control over the child surfaces we need to right-click on the lens in the object tree and choose the option to "Convert to Custom Element".

With the lens now represented as a custom element we can edit surface 1 and in the location/orientation tab specify a tilt of 5 degrees about the x-axis.

Figure 2 - Custom element lens with wedged front surface creates gaps and overlaps in the geometry

Note - it is recommended to start with the maximum wedge or tilt you expect to use.

We can see that the tilt (wedge) of the surface is now in place, but the geometry is no longer closed, we have a gap at the bottom of the lens between surface 1 and the edge, and a similar problem at the top of the lens where edge surface extends past surface 1.

We will need to close these gaps and trim overlaps between the surfaces.

The first step to do this is to make the trim volumes larger so that the first and edge surfaces extend such that they intersect each other.

Figure 3 - extending the trim volumes to allow the surfaces to intersect

Note that in making this change to the Z-extent of the edge, the edge now also goes past the rear plane surface.

Now we can trim the surfaces... first the edge is trimmed by both surfaces 1 and 2:

Figure 3 - Edge surface trimming to prevent the edge extending beyond the front or rear lens surfaces

Next we can trim surface 1 with the edge surface:

Figure 4 - Front surface trimming to prevent extending beyond the edge of the lens aperture

Which leaves us with our final wedged lens:

Figure 5 - Final Wedged Lens with 5 degrees of tilt

Because the trim volumes were arranged to overlap at our maximum expected tilt of 5 degrees, values smaller than this will also work with the lens.

Figure 6 - Final Wedged Lens with 2.5 degrees of tilt