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[Quantum Owners] Re: Car headlights barnacle Thu Feb 23 14:00:14 2012

Fluorescent lights emit UV light internally, which is converted to to
visible light by persuading phosphors on the inside of the tube, er,
fluoresce. That means you can't have a point source, or anything close
to it, so even with the mechanical issues sorted you'd still need an
enormous reflector/lens to get a decent light pattern.
White LEDs also fluoresce, but they're a smaller source - but usually
only bright in bulk so again, they're a flood light rather than a
spot. Fluorescents also have issues with slow starting and more
subtly, reduction in light output over time rather than just fading.

HID lights require a high voltage (a few thousand volts) to strike the
initial spark; thereafter they run at about 80v or so to maintain it.
However, the spark can be interrupted by something as simple as earth
bounce by turning on main beams, and it won't restart unless you have
smart electronics which can recognise that - not always found on the
cheap eBay units. On the other hand, the light is a small point
source, so ideal for 'projector' type headlights (H1 and H7 types) but
because H4 bulbs require both sources in the same shell, you have to
use complex mechanical systems to go between main and dip. HID lights
also take a few seconds to come to full brightness after an initial
flash from the strike. HID lights as new fit require levelling
devices, light washers, and also require that the dips remain on when
the mains come on. They're also something of a legal grey area;
they're not expressly forbidden on new vehicles since they're allowed
in Europe, but they're not type approved so they're a constructions
and use offence as a retrofit. It *may* be that you could get one
through an SIV... but i'd hate to gamble on it.

That said - HIDs are the best lights I've come across for projector
lamps (I wouldn't use them on reflectors e.g. in H4 fittings or
standard main beams); Osram Nightbreakers are the next best bet in a
normal halogen H1/H7 fit.

Neil

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