Hi, 

I am currently working on the design of HUD system, including free-form mirror (xy polynomials), following the topic: https://support.zemax.com/hc/en-us/articles/1500005577782-Which-tools-to-use-when-working-on-a-Head-up-Display-.

The forward design for me is fine (though the specifications like distortion and MTF are not that perfect but acceptable). However, after reversing the system using the way provided in the article (make double pass), many rays especially in x-axis are not collected, and the difference between the forward and reverse design is significant.

Does that mean my design has big problem? Or it is common?

Forward design:

After reversing: (the windshield size is the same)