The coordinate breaks for the first pass have been defined, but now we have to introduce the coordinate breaks to model the tilt/decenter of the lens in the return path. 

The concept is the same, but in the return path, the same exact tilts must be maintained, so some extra pickups are required.  The return path may be modeled by performing the following:

• Add a dummy surface after the mirror.  Add a Position solve on the new dummy surface (surface 7) thickness to locate the following surface (which will eventually be a coordinate break) at the location of the very first coordinate break (change the “From Surf:” to 1 and the “Length” to 0).

• Under the Draw tab of the Surface Properties dialog for surface 7 (the new dummy surface), check the box “Do Not Draw This Surface.”

• Highlight surfaces 8 and 9 and use the Tilt/Decenter tool with the following settings:

• Set positive pickups for the parameters of the first coordinate break (surface 8) to pickup from that of surface 1. 
• Add a Position solve on the new coordinate break surface 8 so that the following surface is located at the physical back of the lens.

• Delete surface 12 (the dummy surface) and change the Position solve on surface 11 to locate the IMA surface at the OBJ.

At this point, the Lens Data Editor should look like the one below.  Note all of the pickup and position solves required to model a simple double pass system!



The layout, although we have added several surfaces, should appear to be identical to the layout on page 2. 



How do we know whether or not we have made a mistake in the setup?  One quick test is a qualitative measure of how the layout looks as we tilt/decenter the lens by modifying the parameters on the very first coordinate break.

First make sure that the OBJ surface is set as the Global Coordinate Reference:



Now, adjust some of the parameters on the first coordinate break surface.  No matter what combination of values is entered, there should only appear to be 1 physical lens in the 3D Layout.  If not, then the return path is not properly mapping the initial beam path, and you need to double check your setup!



Note that in addition to the Layout, the Global Vertex option in the Prescription Data Report can also accurately confirm if the global positions of the front and back faces of the lens in both paths are identical.