Without the CB return, we can return to the coordinate system of a prior surface using “dummy” propagation.  However, as the number of coordinate systems within the dummy propagation increase, it becomes more and more difficult to “back-track”, and it’s easy to make mistakes.  The CB Return on the other hand, works no matter how many coordinate rotations or decenters are involved, and no matter what their ordering is.

In the diagram below, a y-offset is created at S2 and S3, simply because we have propagated some non-zero z distance in the coordinate system of S1 (which is tilted about the X-axis by 20 degrees).







If we would like to define the position of S3 in the OBJ space coordinate system we could do one of a few things:

• Calculate by hand what the magnitude of the y-offset is due to the z propagation along the tilted coordinate system, and decenter the surface appropriately.
  
• Use dummy propagation back to Surface 2 (the first CB), restore the tilt, and then specify the appropriate z thickness to the following surface.
  
• Have ZEMAX automatically restore to the coordinate system of Surface 1 (the dummy surface prior to the first CB).

Any of the three options above would be relatively easy to do in this simple sample file, but what if we had several nested layers of Coordinate Breaks, and wanted to restore back to the OBJ space coordinate system?  This is where the Coordinate Return comes in really handy.