In order to perform a stray light analysis on this lens, we need to convert it to a Non-Sequential Component. This is a simple and speedy process in ZEMAX.

The first thing we need to do is to realize that ZEMAX requires that the Aperture Stop of the system be located before any Non-Sequential Component in the Lens Data Editor. Remember: our plan is to make the outer surface of the flat window a scattering surface, so the window must be part of the Non-Sequential Component. Looking at our lens, we see that the Aperture Stop is actually buried deep inside the lens.

To make the Aperture Stop occur before the window in the Lens Data Editor, we need to trick ZEMAX just a little bit. We’ll add the aperture stop before the window, such that its location and size coincide with the Entrance Pupil of the lens. Then we’ll have the Lens Data Editor step backwards in space to the location of the window, and then the rest of the system can follow as usual.

We need to know the Entrance Pupil’s location and size, so let’s go to the Merit Function Editor and insert two new operands: ENPP and EPDI. Update the merit function, and you’ll see these values calculated automatically:



{Alternatively, use File|Preferences|Status bar and add these two items to ther status bar at the bottom of the main ZEMAX window.} The entrance pupil location is 86.063994mm after the outer window surface, and its full diameter is 25.0mm.

Click on Surface 1 in the Lens Data Editor and hit the Insert key once. Set the following values for this new surface:

Surface 1
Surf:Type = Standard
Comment = Aperture Stop
Radius = Infinity
Thickness = -86.063994

Now right click on the Surf: Type cell of Surface 1 and, under the Type tab, check the box next to “Make Surface Stop” and then click OK. Note that since this system’s Aperture Type is Entrance Pupil Diameter, we don’t need to set the Semi-Diameter of this surface: it’s already set for us to 12.5mm.

We’re almost ready to convert the lens to a Non-Sequential Component. Go to the last lens surface (Surface 14) in the Lens Data Editor, and make its comment “last surface.” This is not a necessary step, but it’s helpful when we go to find this surface when we convert to NSC, in the next step.

Now go to Tools|Miscellaneous|Convert to NSC Group. For the First Surface choose “2 – Window-outer” and for the Last Surface choose “14 – last surface” and click OK.

You’ll notice that the MTF of the system is identical to what it was before conversion to NSC: this is a good check that everything converted over correctly.

Open a 3D Layout window and set the First Surface to 2, and you’ll see that the system looks nearly identical to the original sequential layout:



Note that the only difference is that the edges of the first group of lenses are bevelled: if this is not desired, just add apertures to these surfaces prior to conversion, or edit the file by hand after conversion.