Evaluation of luminance vs. brightness for automotive RGB LED light guides in autonomous cars

Abstract

Colored light for visual communication in cars at daylight conditions has recently started: A light guide with RGB LEDs is placed at the bottom of the windshield. Usage is safe visualization of autonomous mode and warnings beside e.g. wayfinding support, directional highlighting and turn signals. Today’s white luminance of light guides is about 1,250 cd/m² for daylight with a RGB luminance ratio of typically 28:65:7. A new proposal from an OEM requires the same luminance just for blue. Therefore we started an evaluation with different aspects as basis: The luminance of blue is low due to V(λ) curve but its color perception is high by Color Matching Functions, the lightness L* (perceived brightness) is related to luminance L by L* ~ L^0.44 and the RGB luminance ratio of traffic signs (acc. EN12966) is 35:50:15. We designed our car mock-up for a white luminance of 13,000 cd/m² using a RGB LED stripe reaching 1,500 cd/m² for blue. The surrounding simulates night to sunlight conditions with additional 100,000 cd/m² blinding sun. We tested 18 subjects (pandemic restrictions). The luminance of blue for “annoying” (= safe recognition) reached 900 cd/m² (sunlight + blinding). Additionally, the subjects had to adjust two neighboring colors to the same brightness or judge (brighter, same, and darker) for different colors. Our survey resulted in a reasonable luminance of 500 cd/m² for blue (emphasized by blinking) and a RGB luminance ratio of 35:50:15. The portion of blue is nearly doubled which forces a redesign of today’s automotive RGB LEDs.