Windshield Wedge Angle and Layer Thickness Measurements Michael A. Marcus and Filipp Ignatovich Lumetrics, Inc., 1565 Jefferson Rd #420, Rochester, NY 14623 mmarcus@lumetrics.com Introduction In the February 2016 issue of Photonics Spectra, Heads-Up-Display Systems (HUD) for the automobile windshield were reviewed 1 . The focus of this article was on the image display unit and on the types of data and features which are displayed. The present article describes the types and layer structures involved in the construction of the windshield itself and the most common methods of getting ghost free images from automotive HUD displays 2 . This article then describes windshield metrology and more particularly methods to measure the thickness profiles of all of the layers of the windshield simultaneously. As described below, a small wedge angle has been found useful in minimizing ghosting in HUD displays. The metrology system described is also shown to be capable of determining the wedge angle as a function of location on the windshield with a high degree of accuracy as well as determining how much wedge is in each layer. Presently, automotive windshields are constructed of laminated Safety Glass. In the laminating process two layers of glass are bonded together with one or more plastic interlayers. The most common plastic used for the laminated interlayer is Polyvinyl butyral (PVB). The lamination process is now required by law to be used in all windshields because it minimizes the chance of passengers being injured by the windshield during a collision. The plastic layers cushion the impact during an accident and prevents the windshield glass from separating into shards as a result of an object hitting the windshield, thus reducing the severity of lacerations and other hazards as a result of an automobile accident. Today, consumers demand quality from every automotive purchase so that windshield manufacturers, vehicle designers and carmakers are being driven to improve safety, aerodynamic and fuel efficiency, driving comfort, electronics connectivity and aesthetics. In practice this means introduction of increasing windshield sizes, reduced thickness to gain component weight loss for improved fuel efficiency, decreasing installation angles, as well as complex windshield surface geometries, coatings for reduced UV and Infrared (IR) radiation and acoustic interlayers for sound insulation. Many windshields today include interlayers composed of multilayer plastics. When acoustic interlayers are used the acoustic absorbing layer is usually sandwiched between a pair of PVB layers. Many windshields today also include coatings for de-icing and demisting, have built-in heaters, integrated antennas for radio, TV, cell phone, navigation etc. integrated rain sensors for automatic wiper activation, as well as other sensors, such as forward obstacle detection, and hydrophobic coatings for improved visibility. Recently, automotive windshields, which are specifically designed for use with Heads-Up-Displays (HUD) to provide ghost free images, are becoming more and more prevalent.