# Why do rays trace behind a fold mirror surface?

This article explains why rays sometimes appear to trace behind fold mirror surfaces. It then shows how to fix this common issue and presents the benefits of using the “Add Fold Mirror” tool.

Authored By Andrew Locke

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## Introduction

If you have modeled fold mirrors without using the Add Fold Mirror tool, you may have produced systems where the rays appear to propagate behind the fold mirror. Although this is not strictly a problem, in that the ray-tracing is still correct, it looks ugly, and is not ideal for presenting your setup.

This article explains what the most common cause of this problem is and how to fix it. As you will see, proper understanding of the coordinate system transformations involved in modeling fold mirrors is essential to avoiding problems like this one. Using the tool we provide for you to add fold mirrors is recommended.

## Analyzing the problem

This is a simple infinite conjugate system with a 90 degree fold mirror followed by a lens:

This file, at first glance, may appear to be setup correctly. In reality, a closer look at the fold mirror indicates otherwise:

Rays appear to be tracing behind the fold mirror which, given the geometry of our system, is physically impossible. So why, then, are rays tracing as shown?

The answer is in how the surface following the fold mirror is setup in the Lens Data Editor. Take a look at the Thickness column in the Lens Data Editor:

Note that Thickness of surfaces 3 and 4 is zero. This means that the next surface, surface 5, is co-located with surface 3.
Click anywhere on the row corresponding to surface 5 in the Lens Data Editor and observe the 3D Layout:

Note that surface 5 is co-located with the fold mirror, because there is no thickness change between the two surfaces. However, surface 5 has a different orientation than surface 3 due to the Coordinate Break (surface 4) between the two surfaces. This Coordinate Break completes the coordinate system transformation required to model a fold mirror correctly. Thus, surface 5 has the proper orientation but it is not positioned at the correct location. This results in the virtual propagation of rays behind the mirror.

How can this be fixed?

## Fixing the problem

Surface 5, the problematic surface, is a “dummy” surface—it has no optical properties. Thus, the surface is simply a placeholder surface. As such, moving the thickness value of surface 5 to surface 4 has no effect, from an optical perspective, on our system. Try this now.
Copy the Thickness value of surface 5 to surface 4 and then set the Thickness of surface 5 to zero:

Update and unzoom the 3D Layout window. Note that the system is optically equivalent but the dummy surface has been moved beyond the fold mirror surface location and the virtual ray propagation is no longer present:

Since surface 5 is a dummy surface, this problem is purely a visual one. The optical performance of this system is not affected by the original location of surface 5. As a result, there is another way to fix this problem.
Re-open the original starting file. In the Lend Data Editor expand the Properties for surface 5 and check the “Skip Rays To This Surface” and “Do Not Draw This Surface” check boxes:

Update the 3D Layout. Once again, the problem has been addressed. Rays are no longer propagating behind the fold mirror and the dummy surface co-located with the fold mirror is hidden from view:

The problem described above will only occur when fold mirrors are added manually to an optical system. Why add such components manually to a system when there is a tool to do so automatically?

Re-open the original starting file. Delete surfaces 2 through 4. You will also need to honor the Thickness sign convention in OpticStudio by setting the Thickness of all of the surfaces to be positive (since there no longer is a mirror in this system). An easy way to reverse the sign of a parameter is to type in *-1 in each cell that you wish to reverse the sign of. Here is the resulting Lens Data Editor:

Update the 3D Layout. Notice the rays are still focusing at the image surface just as they were with the fold mirror in the system. We are now modeling our system as if there had never been a fold mirror in it:

Now use the “Add Fold Mirror” tool via the ribbon in the Lens Data Editor:

Set the “Fold Surface” to “2” (our dummy surface) and click “OK”:

Note that OpticStudio has done all of the work for you! The Coordinate Breaks have been setup and the fold surface has been assigned Material type “MIRROR”. Note also that the Thickness of each surface is setup correctly such that the layout is drawn correctly:

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