Exterior displays for autonomous cars: techniques, challenges and solutions

Abstract

Autonomous cars should communicate by exterior displays with other road users to increase safety. Examples are the visualization of the driving mode and a pedestrian waiting at a crosswalk being informed that the autonomous car will stop. Such displays must be sunlight readable and smoothly integrated into the front and rear of a car. We prototyped a full-scale mock-up to evaluate RGB LEDs and e-paper regarding optical performance. The display size of 80 cm x 40 cm was chosen accordingEN 12966. We evaluated the reasonable LED pixel pitch as 6 mm, which is about half of the minimum for variable traffic signs. RGB LEDs (~ 100 W/m²), black/white and color e-paper (zero power) were measured with simulated ambient light and judged by subjects regarding legibility, color inversion and color perception. The measurements show a large contrast ratio and gamut for the LED displays. The reflectance of the e-paper was 40% (color) and 50% (monochrome), it's a contrast ratio of about 10:1. The reflectance shows a strong dependency of specular included and excluded measurements. However, the color gamut of e-paper was measured as being small and rated as poor. RGB LEDs with attention-grabbing blinking and high luminance (5,000 cd/m²) as well as high contrast ratio and large gamut were rated best. The best exterior application for e-paper is a digital license plate. The foreground of the e-paper should be black or colored, and the background white. However, this is challenging for car design