Title: Windshield with Enhanced Infrared Reflectivity Enables Packaging a Driver Monitor System in a Head-Up Display
text: Integration of a driver monitor system (DMS) in a head-up display (HUD) gives the monitor camera a continuous view of the driver’s face, since the driver always faces the road ahead. However, with both infrared (IR) illuminator and IR camera packaged in the HUD, reflectivity of the windshield is important at IR wavelengths used by the camera. Not only is windshield IR reflectivity important for a clear camera image of the driver’s face, but increasing windshield reflectivity also decreases the effect of ambient sunlight on the camera image of the driver’s face. We describe a method to measure windshield reflectivity, both for the 940 nm band used by a DMS, and for visible light for the HUD. The measurement method uses a fiber-optic spectrometer, two collimating lenses, and a method to compensate for sample tilt. The lenses are mounted on a stage that adjusts the height above the sample. As an example, this method was used to characterize an IR reflecting windshield, prepared for a prototype automotive HUD. At 940 nm, and 45° angle of incidence, the measured reflectivity is > 85% for unpolarized incident light. For visible light at 550 nm, and 62° angle of incidence, the measured reflectivity is 13.9% for both an IR reflecting windshield and for a reference windshield, for unpolarized incident light. The prototype windshield gives a good reflected image for the DMS IR camera and a good HUD image as seen by the driver. The method used to prepare this prototype windshield is suitable for high-volume production.