The visual and photometric properties of LED rear position and stop lamps in cars

• The signal lamp design has evolved significantly due to the adoption of LED sources. • The apparent luminous area of position lamps has decreased and often lacks coherence. • LED signal lamps more effectively meet fundamental luminous intensity requirements. • Most LED stop lamps cause visual discomfort, even under daylight conditions. • Luminance regulations need to be introduced to road improve traffic safety. Automotive signal lamps are now primarily manufactured using LED technology, allowing for more complex and compact designs than traditional incandescent lamps. LED lamps’ high luminance and flexible shape raise concerns about their compatibility with lighting regulations under road safety standards. This article presents research on the geometry and photometric properties of ten modern LED rear position and stop lamps using twenty-year-old data from incandescent signal lamps as a reference. The luminance distributions and the apparent areas of the individual samples were precisely determined based on the measurements and image analysis. Luminous intensity along the reference axis was calculated, and visual comfort was assessed for assumed daytime and nighttime conditions. The results identified trends and best practices in the design of automotive LED signal lamps. All LED rear position lamps tested had emission areas below the Borderline between Comfort and Discomfort (BCD) curve in daylight conditions, indicating acceptable comfort levels. However, four LED rear position lamps exceeded the comfort threshold under nighttime conditions. Moreover, all the LED stop lamps analyzed may cause discomfort at night, and about 70% during the day. Therefore, it seems necessary to promptly introduce mandatory and verifiable requirements for luminance levels and the size of emitting surfaces in signal lamps, in addition to luminous intensity requirements. Further research is also necessary to refine these guidelines, considering the subjective nature of light perception and factors such as observer age and viewing conditions.