Head-up displays (HUD) technically belong to augmented reality ( AR). This technology dates back to the 1940s and was used in military aircraft. Currently, head-up displays are used in civil aviation and on a test basis in passenger cars.
The difference between head-up displays and head-down displays is that with the head-up display, important driving information such as the current speed, warnings or navigation information is superimposed on the windshield and thus directly in the driver's field of vision. The driver can continue to look at the traffic situation and at the same time see the driving information. With head-down displays, on the other hand, the driver must direct his gaze to the instrument panel.
Head-up displays are projected directly into the driver's field of vision on the windshield. Technically, the vehicle information is shown on a TFT display, by means of lasers, mini-projectors or other high-intensity display technologies. From there, they are projected onto the windshield via lens and mirror systems. The mirror optics compensate for the curvature of the windshield and extend the path that the light travels from the display to the projection surface. This path is decisive for the distance at which the driver perceives the display so that eye focus does not have to change while driving.
An important aspect for the projection is the nature of the windshield. Since it consists of two separate layers of glass with a PVB (butyrene polyvinyl) layer between them, two minimally offset displays would appear during projection. This is compensated for by a newly designed windshield with minimally different curvatures of the two glass layers, so that the projection images of both panes are superimposed. Another point is the projection brightness, which is controlled depending on the external brightness and the incidence of light. The outside brightness is detected by ambient light sensors and readjusts the projection brightness.
An alternative to projection technology is transparent OLEDs, Transparent OLED( TOLED), which are attached to the windshield. With all technologies, it must be ensured that the HUD display is still clearly legible even in strong sunlight.