A head up display shows speed and navigation on the windscreen so the driver does not look down. Making that image usable requires solving several optical problems at once.
The image must not sit on the glass
If the display appeared on the windscreen surface, the driver's eyes would refocus from the road to a point a metre away and back again for every glance.
Refocusing takes time and effort, which would defeat the purpose of not looking down at the instrument cluster.
The optics are therefore arranged so the image appears to float several metres ahead of the car, close enough to the road's focal distance that refocusing is minimal.
Producing a distant virtual image takes space
Creating that apparent distance requires a folded optical path with mirrors, at least one of which is a large curved element.
The assembly occupies a considerable volume beneath the windscreen, competing for space with ventilation ducts, wiring and the structure of the dashboard.
Larger apparent image size and greater apparent distance both demand a bigger mirror, which is why augmented displays that overlay the road require substantial packaging.
The windscreen itself is part of the optical system
A windscreen has two surfaces, and a projected image reflects from both, producing a doubled or ghosted result.
The fix is a wedge-shaped interlayer within the laminated glass, slightly thicker at the top, which brings the two reflections into alignment.
This makes the windscreen a specific part matched to the display, and a replacement without the wedge produces ghosting that cannot be corrected elsewhere.
Brightness has to cover an enormous range
The display must remain legible against bright sunlit road ahead and must not dazzle the driver on an unlit road at night.
That requires a wide dimming range, an ambient light sensor and careful control, since the eye adapts to the surroundings rather than to the display.
Solar loading is a further problem, because sunlight entering the aperture can be focused back onto internal components and damage them.
Content choice matters as much as optics
Placing too much information in the driver's forward view creates clutter that obscures the road, which is the opposite of the intended benefit.
Successful implementations show a small set of items that change slowly, with transient elements such as a turn instruction appearing only when relevant.
Augmented systems that draw guidance onto the road itself raise the difficulty considerably, since the overlay must track the scene accurately or it becomes a distraction.