A high craftsmanship feel is generally provided by a hard surface. Therefore, a curved touch-lens over an 8-inch display requires a birefringence-free polymer lens base material thickness of at least 2 to 2.5mm that impacts the selection of the touch technology.
Projected capacitance (pro-cap) screens can be adapted to provide a high craftsmanship continuous curved surface touch-lens desired by automotive styling studios. Integrating the touch screen into a curved surface eliminates resistive touch screen, most infrared LED, and acoustic wave technologies as solutions since these technologies require a flat surface to operate. Technology companies are working on infrared and acoustic wave technologies for continuous curved surface touch solutions that may become suitable for automotive applications in the future.
Audible and tactile feedback augments the verification that the touch has been registered. Providing tactile feedback can be a challenge with the constraint of a relatively large lens. The haptic actuation and lens mounting features need to be carefully chosen to accomplish the desired wave propagation on a rigid curved surface. Adding multi-stage touch sensing or the press-for- intent feature increases the complexity of the mechanical mounting by requiring detection of a slight pressure while maintaining a mounting system suitable for haptic feedback.
Spatial recognition is a new feature gaining popularity and offers some unique challenges to the touch-lens. If the requirements are simple driver vs. passenger hand detection, a projected capacitance solution may suffice. However, for more complex gestures and larger detection areas above the panel, IR LED reflection, or possibly IR camera technologies are warranted.
This solution requires the use of an IR transmissive surface. Other technology used to recognize no-touch movements (such as ultrasound), may result in a decreased craftsmanship level (visible microphone openings).
Focusing on aesthetics
Hiding the display behind a continuous curved surface that provides partial or complete display hiding provides a high craftsmanship appearance that is a trend in the automotive space. This exciting styling opportunity gives the Tier 1 supplier or system integrator one of the biggest challenges in the center stack of the vehicle. Addressing this challenge requires the ability to quantitatively assess the visibility of the display for a given optical system and vehicle packaging, optimize the design of the optical system, and provide the best combination of display and efficient backlight.
Providing a dead-front or black panel appearance requires an understanding of how the human eye works to optimize the light transmission while providing the desired hiding effect. Hiding the display opening when the display is off involves human factor studies. The contrast sensitivity function (CSF) per figure 3 shows that a contrast of about 0.01 is necessary to hide the display opening. Using the CSF as a guide, different optical constructions may be considered to control the reflection level of the display opening versus the non-display area to minimize the contrast (also known as Michelson contrast, Cm) while maximizing display visibility. There are many interrelated factors to consider when selecting the lens optical configuration, such as transmission, polarization, retardation, display luminance and reflected background luminance. Since reflections can be used beneficially to hide the display opening, the lowest reflection lens configuration may not always be the best solution.
Taking the visibility mathematical function into account, various optical configurations may be examined to ascertain the required display luminances. Lower display luminances (higher optical system efficiencies) can be achieved with different optics other than the traditional neutral density filter (ND). However, the background reflection level must be considered to pick the best optical configuration based on actual in-vehicle jury evaluations.
Figure 3: The contrast sensitivity function