Why do high end makers use HID which is "bluish" yet produces a whiter
light?
That's a good question, and the answer isn't what you think it is.
The SAE and ECE "white" boundaries were codified many decades ago, and are
enormous. "White" light is allowed to show significant blue, yellow,
orange or green casts and still be considered "white". There was, at time
of codification, no incentive to promulgate a more restrictive "white"
boundary, for the only light sources used on cars were tungsten filaments,
all of which produced a comparatively similar light color.
When the first automotive HID headlamp was demonstrated by a major
European lighting manufacturer to automakers, in the early mid 1990s, it
was a very well-designed optic, given the infant state of the art at the
time. It handily outperformed most halogen lamps, and of course consumed
less power. It was based on modified HPS (high-pressure Sodium) arc
chemistry, and had a very similar operating appearance when warmed up to a
halogen headlamp.
The automakers reacted favourably to the increased performance and reduced
power consumption, but rejected the lamp on the grounds that customers
would be unwilling to pay any premium for a lamp that looked the same as
the ordinary kind, regardless of increased performance.
Now, back to that very large "white" boundary: it was a very simple matter
to rework the arc chemistry in the auto HID lamp to create high spikes in
the blue and blue-violet. This created a markedly new/different
appearance, which the automakers' marketing boffins pounced on. Here was
something they could sell on appearance, something non-owners would notice
and come in to the dealers to ask about. No visual benefit to the blue
spikes, and the resultant colorimetry still fit within the legal "white"
boundary: Voila.
Now, of course, NHTSA is getting snowed under with complaints specifically
about blue light. Rigourous studies of the matter (e.g. Sivak and
Flannagan) show that for any given intensity level, bluer light is
significantly more glaring than white light without a blue tint, and that
there is *no corresponding visual benefit* to the bluer light. So they're
looking at ways of reducing the blue spikes in the SPD of auto HID
burners. European ECE regulations have already been modified to limit the
degree to which HID headlamps may emit blue light. Progress is slow,
though, because the new EU EVOL directives phasing out the use of Mercury
in automobile parts means HIDs are going to have to lose their Mercury.
Amongst other effects on the beam, this makes the output spectrum...bluer!
DS