What colour light should headlamps emit? White, of course, is the global consensus. But what means "white"? Which white, exactly? The white box in the UN and US regulations is identical—and quite large. This topic is heating up fast as LEDs' practical applications become broader. For the most part, past light sources have come with tight constraints on the spectral power distribution of headlamps; selections were driven largely by available intensity, cost and efficacy. A particular light source (such as a tungsten bulb, a halogen bulb, an HID bulb) produce a particular kind of light, variable only slightly by design characteristics of any particular bulb.
But now white LEDs are giving us an unprecedented amount of freedom to adjust the SPD (spectral power distribution) to optimise the quality of light for whatever task is at hand—including headlighting. For the time being, mostly we don't yet know what "optimal" really means. Scientists' findings seem to conflict, though much of the apparent conflict is probably just because there's a great deal we have yet to figure out; once the knowledge gaps start getting filled, many of the apparent conflicts will probably resolve. For example, it's been found that LED vehicle headlamps create the sense (feeling, impression) of "brighter" light, and it's also been found that they give poorer seeing performance compared to halogen lamps of equal intensity. It's been found that for equal intensity, higher-CCT white headlights create significantly higher discomfort glare without an accompanying improvement in seeing, and that lower-CCT white headlamps create significantly lower discomfort glare without an accompanying degradation in seeing. What is the relative size of of each of these effects' influence on real driver seeing performance and glare reaction? Our community's best scientific minds are hard at work seeking to find out.
Right now the science is young and there are more questions than answers, though that does not hinder