At the 59th session of GRE, Japan’s lighting expert proposed that regulations be amended so two bulbs of different colours, producing different lighting functions, aren’t allowed behind a common lens of a single colour.
The idea is to prevent so-called “phantom coloured light” which from certain angles causes one lighting function to mimic the appearance of another function. An example would be a red brake lamp bulb and an amber direction indicator bulb, placed side by each, behind a colourless clear lens.
From certain angles, the illuminated amber direction indicator would produce red light, creating the appearance of a brake lamp. All parties agree in principle with Japan’s precautionary proposal; future GRE work will focus on how to write the rule so as to prevent problematic implementations without banning safe designs.