Understanding the Best Focus Point Result
In some case, HOA simulation can return no focus point even if light rays are crossing the eye pupil, which leads to an incorrect HUD system.
Description
The location of a focus point on HOA relies on a set of rays conjugating a point of the PGU and a set of support rays passing through the contour of the eye pupil (let's call him a research cone). The HOA best focus location is then defined as the location along the research cone where the section of the cone is the smallest and most circular.
From that definition, the geometry of the research cone, which depends on the imaging mirror and windshield, can lead to the three results.
Focus Point Cases
Case 1: Virtual Focus Point
This is the expected behavior if the HUD mirrors are correctly designed.
_HUD_Virtual_Focus_Point.png)
Case 2: No Focus Point
The research cone rays are in this case parallel and no focus position can be determined (actually an infinite number of solution relatively to the HOA best focus definition).
_HUD_No_Focus_Point.png)
Case 3: Real Focus Point
The point is typically behind the eyebox or between the windshied and the eyebox.
In this case the focus point is determined but incorrect respectively to a consistent HUD design.
_HUD_Real_Focus_Point.png)
Conclusion
In case 1, the virtual point is well displayed at the end of HOA simulation, while in case 2 and case 3 no best focus is displayed since no result/inconsistent result is found with respect to the HOA best focus search criterion.
If you encounter the case 2 or 3, make a luminance analysis of your system to see what human eyes would see. Then, modify your HUD system with HOD and analyze it with HOA by iterating until you get a correct system.