Creating and Aspheric Lens Element
Lens elements in FRED are defined using two axially symmetric conic surfaces, but can be customized to have the desired surface profiles using the procedure described below.
1. Create a Lens element by right mouse clicking on the Geometry folder and choose, "Create New Lens" from the context menu
2. In the lens dialog, set the following properties:
- Name = Aspheric Lens
- Parameter Type = Focal Len/Bend
- Focal Length = 25
- Bending Parameter = 1
- Thickness = 3
- X/Y Semi-ape = 5
3. Expand the tree view using the "+" icons so that you can see the individual surfaces underneath the Aspheric Lens node. In this construction, the solid Aspheric Lens element is constructed using three surfaces; Surface 1, Surface 2 and Edge. Surface 1 and Surface 2 are both Conic surfaces, while the Edge surface is a simple cylinder. In the image below, Surface 1 is selected on the tree and it becomes highlighted in the 3D view (it's red wireframe bounding volume is also shown in the 3D view).
4. Double click on Surface 1 in the tree view to open up its surface dialog. Note that the surface Type is set to "Conicoid (Sphere, Ellipse, Parabola, Hyperbola, etc.)" in the dialog.
5. In the surface dialog, click on Conicoid to open the drop-list of surface Type options. From the list, select "Standard Asphere (Conicoid plus even order radial polynomial terms)". FRED will prompt you with a confirmation dialog asking whether you want to make the surface Type modification. Press Yes.
6. Leave the Radius and Conic terms alone, but enter the following values for the even order aspheric coefficients and then hit Apply on the dialog.
- r^2 = 0.0
- r^4 = 0.003
7. Although it does not affect the raytrace, the user may also wish to increase the tessleation of the surface representation in the 3D view so that the surface profile appears "smoother". Go to the Visualization tab of the dialog and, on the center right of the dialog, set the Tesselation Scale factors to 0.1 for X, Y and Z. Hit Apply on the dialog. This has the effect of rendering the surface with 10X finer triangle patches in the 3D view.
The Lens element has now been customized with the desired surface shape for Surface 1. In general, this procedure will also need to be applied for Surface 2 if it is not described by a Conicoid surface type. Depending on the type of surface selected, additional trimming operations may need to be applied to the Edge surface in order to ensure a closed volume for the Lens element.
Example file: Aspheric.frd