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> Everyone I know hates those super-bright high-intensity-discharge
> headlights. Even people who have them on their own cars hate them on
> other people's cars. "They blind oncoming drivers," they complain.
> "How come they're allowed?"
Agreed.
> First, some facts. It's not just your eyes: High-intensity-discharge or HID
> lights really are blue.
They produce what is considered white light; the "white" definition used
by SAE and ECE regulators to define "white" headlamp light is VERY wide
and contains significant overlap areas into yellow and blue areas. The
*SIGNAL IMAGE* of an operating HID headlamp (some designs more than
others) is quite blue-appearing. Blue light stimulates a glare reaction
in human eyes, because it does not stimulate the pupil to close down.
(remember "signal image" is defined as "what you see when you view an
operating headlamp", while "beam pattern" is defined as "what your own
headlamps let you see".)
> Instead of using a glowing filament, they use high voltage to generate
> a bluish spark which arcs from one electrode to another through xenon
> gas, lighting it up.
The Xe gas is there only so that the arc provides SOME light right when
you turn it on, because in a headlamp application, when you hit the
switch, you need light RIGHT NOW. The term "Xenon headlamp" is something
of a misnomer; these are actually metal-halide HID (High Intensity
Discharge) lamps. The heat from the arc passing through the Xe gas
vapourises metal salts inside the arc capsule; the metal halide vapours
produce an environment for the arc which causes the arc to emit LOTS more
light than a pure-Xe arc. Once the halides are vapourised (about 4
seconds after initial cold startup of lamp) the Xe gas does nothing.
> The advantages of HID? Twice as much light as halogen bulbs.
Closer to 3x. With power consumption of 63% of a halogen bulb.
> Up to five times the life span of halogen.
True, but at greater than 10x the cost of a halogen bulb *for the HID
capsule only* (omitting the supporting ballast), this advantage is pretty
well "marketing only".
> A wider spectrum of light, so that road signs and pavement markings
> appear more vivid.
No. The spectrum from an automotive HID capsule is quite a bit more
uneven and peakier than that from a halogen bulb. The result of this is
that automotive HIDs have a much poorer Colour Rendering Index (CRI) than
automotive halogen headlamps. A halogen headlamp produces a CRI of about
98 (perfect is 100), while even the best-designed automotive HIDs have
CRIs of only 73 to 77. Signs and pavement markings do NOT appear "more
vivid" (or more anything) because of HID light; they appear more vivid
because these headlamps pump three times the luminous flux ("amount of
light") as halogen lamps. A halogen bulb produces a continuous spectrum
through the entire visible-light band; HID capsules are extremely heavy on
blue and purple wavelengths and extremely poor in red and yellow.
Ironically, yellow to yellow-green is the chief component that is
important for your ability to see at night, and blue-to-purple is the
chief component of glare and poor-weather scatter. A halogen bulb that
produces the same amount of light as an HID capsule makes a better
headlamp (with a nearly perfect CRI of about 99 to 99.5) than an HID
headlamp; the problem is that such bulbs consume a LOT of power (90 to 130
Watts!).
> problem of blinded oncoming drivers (definitely uncool). Yet every
> researcher I've talked to says it ain't so. Even though they're twice
> as bright as halogen, HID lights still fall within government
> standards. The problem, it seems, is that we're just not used to them.
> As we weren't when halogen lights were introduced, and people
> complained of being blinded by them. Now, I have no reason to doubt
> the experts when they tell me it's all in my mind.
You should; this is spurious dismissal of a real problem (glare) based on
a faulty assumption (that these "government standards", contained in
Federal Motor Vehicle Safety Standard 108, adequately control glare
light). "Glare light" in a headlamp is defined as light emitted ABOVE
horizontal and to the LEFT of centre (except in countries where they drive
on the LH side of the road, where glare light is above horizontal and to
the RIGHT of centre). Oncoming drivers are located in this region (their
eyes are above horizontal, and they are to the left of the centre of your
car). Drivers in front of you are above the horizontal.
US headlighting standards have for MANY decades permitted far higher
levels of glare light than the counterpart regulations in Europe, where
glare is very strictly controlled. The reason why glare is now becoming a
problem? With old headlamp technology (old sealed beams, halogen sealed
beams, early halogen bulbs), the light source did not emit a lot of light,
so even though the glare LIMIT might have been high, these light sources
did not emit enough light for the headlamp to get very close to that
limit, so glare levels were generally below the legal minimum, and
therefore frequently not a problem.
But now we have HID capsules pumping three times the light of an efficient
halogen bulb, AND we have new, highly-efficient halogen bulbs, so it is
now possible for headlamps to approach and even to reach the legal glare
limits. The US glare limits have always been too high. It's just that
now we can reach those limits. So saying "Gosh, the glare limits haven't
changed...HID headlamps meet the same glare limits as other
headlamps...therefore it's all in your head" rests upon the faulty
assumption that the glare limits are low enough. They are not.
> those double binds where cold reason says one thing and everyday (or rather
> every-night) experience says the opposite.
"Cold reason" based on a faulty headlamp standard. Go drive for a week in
Europe. There are FAR more HID headlamps in use there, but they do NOT
blind you, nor do their halogen headlamps, because their beam pattern
controls glare much more strictly than ours. In addition, European
regulations have for years required that headlamps be equipped with
levelling devices so that a car with a load in the rear can have the
headlamp aim "dialled down" from the driver's seat so the lamps aren't
pointing up in the air. This regulation has been extended such that cars
with HID headlamps in Europe are required not only to have the lower-glare
beam pattern, but also to have AUTOMATIC headlamp levelling that senses
vehicle attitude and adjusts the headlamp aim accordingly, to prevent
glare. In addition, all European-spec vehicles are required to have
headlamp lens cleaning equipment--wipers, high pressure washers, or both.
Dirt on a headlamp lens diffuses the light from the "seeing light" area
into the "glare control" area, greatly increasing glare light for oncoming
and leading traffic. The latest vehicles have automatic in-headlamp
sensors that detect dirt buildup and activate the cleaning system, but
this is not compulsory.
Headlamp cleaners are NOT required in the USA.
Headlamp levellers, driver-controlled OR automatic, are NOT required in
the USA.
The USA beam pattern allows MUCH more glare light than the European beam
pattern.
All of the above add up to a high-glare US driving experience.
> Perhaps HID lamps should be placed only on the high beams,
There's no reason to do this. Proper beam control and proper headlamp
control (see above) will get the job done just fine, and US driving
conditions do not lend themselves to frequent use of high beam headlamps,
therefore the high cost of HID lamps isn't generally justifiable for most
people on the high beams.
> So perhaps we should light up road signs, as they do in Europe and Japan,
> where headlamps hug the road more instead of being directed upward as they
> are here in the States. But in this tax-averse society, who's going to pay
> for that?
Many US headlamp pundits claim that the upward stray light required in the
US beam pattern is necessary for illumination of unlit overhead road
signs. This comes from these researchers spending their days examining
headlamps' performance as evaluated by an optical measuring machine.
Most of the US headlamp pundits (there are relatively few and I know most
of them) have shockingly little on-road time with any but a very few types
of headlamps, and base their statements and recommendations on laboratory
conditions that are not representative of real-world usage factors. The
main difference between the goniophotometer in the lab and the car on the
street is that the goniophotometer fails to take road-bounce into account.
Even with a beam pattern that produces relatively little upward light,
such as a typical European-specification (ECE) headlamp with a standard
60/55W H4 bulb, there is angular bounce of light from the road up onto the
(universally-reflectorized) overhead signs. Sign visibility is not a
problem with most ECE headlamps.
There was a thread on these same newsgroups some months ago in which a
well-respected automotive lighting tester posted that overhead signs
cannot be seen with European ECE headlamps. Numerous people followed up
to his post saying that they had installed ECE headlamps on their own
cars, aimed them properly, and experienced no such difficulty. The tester
responded with a guess that these folks were benefitting from the upward
light produced by the US DOT beam patterns of headlamps of the cars around
them on the road; I and several others responded they had tested this by
driving alone on dark roads with only their European ECE low beams and
STILL had no trouble reading overhead signs. It is truly a question of
what the book says "must be" the case versus what "IS" the case in the
real world.
Anybody who doubts this is more than welcome to come over to my place some
dark night, by prearrangement. We'll head out to some VERY dark unlit
roads out here with many unlit overhead signs, which you'll be able to
read with ZERO difficulty with my properly-aimed European-spec headlamps
with stock-wattage bulbs (which, remember, put out about 1/3 the light of
an HID capsule! 3x the light = 3x the road bounce = even MORE of a
non-issue reading road signs.) The overhead sign problem DOES NOT exist.
It is worth noting that the "overhead sign problem" got a lot of
discussion in the late 1980s when BMW began equipping their US vehicles
with projector-beam, halogen headlamps that produced very little upward
light. Owners of such cars began to complain that they were having
problems reading overhead signs. So why didn't road bounce light light
work? Because the US beam pattern, in addition to specifying a certain
amount of upward light, also specifies a certain MAXIMUM on foreground
light. Foreground light is "light on the road", the carpet of light that
lets you see to avoid potholes, see the edges of the road, see the lane
markings, see creatures about to step into the road, etc. US regulators
have long held that anything more than a bare minimum of foreground light
is illegal because it's bad because if you light-up the foreground, the
driver's eyes will be drawn, moth-like, to the well-lit foreground rather
than out in the distance where they ought to be.
The interesting part is that there is ZERO research, here or abroad, to
show that this is the case. Its origin can be traced to the minutes of a
1957 meeting of the Society of Automotive Engineers in which this was
posed as a possible guess at how things might work. The opposing theory
is that if you light the foreground well enough, the driver will trust his
peripheral vision with the task of avoiding foreground obstacles
(potholes, etc.) and of staying within the lane, so he can comfortably
keep his eyes out in the distance to scan for approaching hazards and road
changes. Of the two theories, there is no evidence to support the theory
that leads to low foregorund illumination levels, while there is some
evidence that the second theory (light-up the foreground) is correct.
Certainly European headlamps, which provide higher levels of foreground
illumination than US headlamps, have not been linked to any particular
pattern of nighttime "I didn't see the approaching hazard" type crashes in
Europe. There must be SOME limit to foreground illumination in order to
avoid creating SUCH a bright area that the driver's eyes close down and
*physically* limit his distance seeing ability, but there is a
sufficiently wide window between "enough foreground light" and "too much
foreground light" that there is no reason to have headlamps with
"insufficient foreground light". The fact that US drivers use their fog
lamps full time to fill in the foreground "black hole" left by US
headlamps is pretty strong evidence that US drivers want and get more
foreground illumination than is provided by most US headlamps.
The BMW headlamps with very little upward light also had very little
downward light; most of the light was thrown in a "hot spot" off into
space in front of the car. The worst of all possible worlds; of COURSE
they couldn't see overhead signs! Now, of course, you can't just strongly
illuminate the foreground and not have any light going off into the
distance or else you've a FOG lamp, not a headlamp. But the US beam's
traditional emphasis on a central "hot spot" and some upward stray light,
with minimal foreground light, makes a poor headlamp.
> Clearly, the feds need to re-examine their standards for headlamp
> brightness, reach, and direction
"Brightness" is not the issue. European headlamps use bulbs that produce
the same amount of light as bulbs in US headlamps, whether those bulbs are
halogen or HID.
"Reach and direction" is closer to the mark. It's the beam pattern that
matters. The headlamp with a bright bulb and a poor beam pattern will
lose to the headlamp with a relatively dim bulb and a good beam pattern,
EVERY time. One way to prove this would be to drive a Lincoln Mark 8
(poor beam pattern, HID source) and then drive, for instance, a brand-new
Nissan Maxima which, even in US-spec trim, has headlamps that produce a
beam *similar* to a European beam, and use standard halogen bulbs of a
design first marketed in 1972. It's ALL about the beam pattern. US
headlamps, US turn signals, US brake lamps, and most other aspects of
lighting in the US have been behind the European ECE standards embraced by
most of the rest of the world, for decades. Although recent beam pattern
standard changes have *allowed* better headlamp performance (more
foreground light, less upward stray), this better performance is not
mandatory. US headlamps are still permitted to be really lousy and still
meet those "government requirements" the "experts" you questioned were
telling you about.
> manufacturers need to refine it.
Manufacturers could have produced HID capsules with a very even output
spectrum and a neutral-white signal image rather than today's blue signal
image. I was discussing the colour issue with a Sylvania engineer two
years ago. His comment: "We could have made HID headlamps with a centric
white output spectrum and much less peakiness in the blue and purple
areas, but who would pay 10x the cost for a headlamp that looked the same
as every other headlamp, even if it worked 3x better? So we made 'em look
blue because we're allowed to."
> The only answer may be to respond to high-tech with old-fashioned low-tech.
The only answer is for US regulators to (a) stop pretending they know
better than the rest of the world, and (b) stop bowing to pressure from US
automakers to keep safety regulations cheap and easy to comply with and
(c) adopt the ECE standards that produce headlamps that WORK and DO NOT
GLARE.
Now you know the rest of the story.
--Daniel
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