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Decided to post it here for all to enjoy:
MJ: What's your opinion on the many HID retrofit kits...
DS: Junk. Every last one of them.
[more from Daniel:]
But *NONE* of these HID retrofits really works. Some of them produce beams that are in one fashion or another pretty to look at on the wall -- sometimes most of the cutoff is even maintained -- but what is *below* the cutoff is invariably all wrong (wrong amounts of light heading in the wrong directions).
[...and here's went he *really* explained it:]
There are many unsafe, illegal and noncompliant products on the market,
mainly consisting of an HID ballast and bulb for "retrofitting" into a
halogen headlamp. Often, these products are advertised using the name of a
reputable lighting company ("Real Philips kit! Real Osram kit!") to try
to give the potential buyer the illusion of security. While some of the
components in these kits are sometimes made by the companies mentioned,
the components aren't being put to their designed or intended use.
Reputable companies like Philips, Osram, Hella, etc. NEVER endorse this
kind of "retrofit" usage of their products.
Halogen headlamps and HID headlamps require very different optics to
produce a safe and effective -- not to mention legal -- beam pattern. How
come? Because of the very different characteristics of the two kinds of
light source.
A halogen bulb has a cylindrical light source -- the glowing filament. The
space immediately surrounding the cylinder of light is completely dark,
and so the sharpest contrast between bright and dark is along the edges of
the cylinder of light. The ends of the filament cylinder fade from bright
to dark.
An HID bulb has a crescent-shaped light source -- the arc. It's
crescent-shaped because as it passes through the space between the two
electrodes, its heat causes it to try to rise. The space immediately
surrounding the crescent of light glows in layers...the closer to the
crescent of light, the brighter the glow. The ends of the arc crescent are
the brightest points, and immediately beyond these points is completely
dark, so the sharpest contrast between bright and dark is at the ends of
the crescent of light.
When designing the optics (lens and/or reflector) for a lamp, the
characteristics of the light source are *the* driving factor around which
everything else must be engineered. If you go and change the light source,
you've done the equivalent of putting on somebody else's eyeglasses --
they may fit on your face OK, but you won't see properly.
Now, what about those "retrofits" in which the beam cutoff still appears
sharp? Don't fall into the trap of trying to judge a beam pattern solely
by its cutoff! In many lamps, especially the projector types, the cutoff
will remain the same regardless of what light source is behind it. Halogen
bulb, HID capsule, cigarette lighter, firefly, hold it up to the sun --
whatever. That's because of the way a projector lamp works. The cutoff is
simply the projected image of a piece of metal running side-to-side behind
the lens. Where the optics come in is in distributing the light (under the
cutoff). And, as with all other automotive lamops (and, in fact, all
optical instruments), the optics are calculated based not just on where
the light source is within the lamp (focal length) but also the specific
photometric characteristics of the light source...which parts of it are
brighter, which parts of it are darker, where the boundaries of the light
source are, whether the boundaries are sharp or fuzzy, the shape of the
light source, etc.
There are more "gotchyas" when pondering halogen-to-HID "retrofits". The
only available arc capsules have an axial (fore and aft) arc, but many
popular halogen headlamp bulbs, such as 9004, 9007, H3 and H12, use a
transverse (side-to-side) and/or offset (not directly in line with the
central axis of the headlamp reflector) filament, the position and
orientation of which is physically impossible to match with a "retrofit"
HID capsule. Just because your headlamp might use an axial-filament bulb,
though, doesn't mean you've jumped the hurdles -- the laws of optical
physics don't bend even for the cleverest marketing department, nor for
the catchiest HID "retrofit" kit box.
The latest gimmick is HID arc capsules set in an electromagnetic base so
that they shift up and down or back and forth. These are being marketed as
"dual beam" kits that claim to address the loss of high beam with
fixed-base "retrofits" in place of dual-filament halogen bulbs. What you
wind up with is two poorly-formed beams, at best. The reason the original
equipment market has not adopted the movable-capsule designs they've been
playing with since the mid 1990s is because of the near-impossibility of
controlling the arc position accurately so it winds up in the same
position each and every time. There are single-capsule dual-beam systems
appearing ("BiXenon", etc.), but these all rely on a movable optical
shield, or movable reflector -- the arc capsule always stays in one place.
The OE engineers have a great deal more money and resources at their
disposal -- if a movable capsule were a practical way to do the job,
they'd do it. The "retrofit" kits *certainly* don't address this problem
anywhere near satisfaction. And even if they did, remember: Whether a
fixed or moving-capsule "retrofit" is contemplated, solving the
arc-position problem and calling it good is like going to a hospital with
two broken ribs, a sprained ankle and a crushed toe and having the nurse
say "Well, you're free to go home now, we've put your ankle in a sling!"
Focal length (arc/filament positioning) is ONE issue out of several.
The most dangerous part of the attempt to "retrofit" Xenon headlamps is
that sometimes you get a deceptive and illusory "improvement" in the
performance of the headlamp. The performance of the headlamp is perceived
to be "better" because of the much higher level of foreground lighting (on
the road immediately in front of the car). However, examining isoscans of
the beam patterns produced by this kind of "conversion" reveals *less*
distance light, and often an alarming relative minimum where there's meant
to be a relative maximum in light intensity. When you *think* you can see
better than you can, you're *not* safe.
It's tricky to judge headlamp beam performance without a lot of knowledge,
a lot of training and a lot of special equipment, because subjective
perceptions are very misleading. Having a lot of strong light in the
foreground, that is on the road close to the car and out to the sides, is
very comforting and reliably produces a strong *impression* of "good
headlights". The problem is that not only is foreground lighting of
decidedly secondary importance when travelling much above 30 mph, but
having a very strong pool of light close to the car causes your pupils to
close down, *worsening* your distance vision...all the while giving you
this false sense of security. This is to say nothing of the massive
amounts of glare to other road users and backdazzle to you, the driver,
that results from these "retrofits".
HID headlamps also require careful weatherproofing and electrical
shielding because of the high voltages involved. These unsafe "retrofits"
make it physically possible to insert an HID bulb where a halogen bulb
belongs, but this practice is illegal and dangerous, regardless of claims
by these marketers that their systems are "beam pattern corrected" or the
fraudulent use of established brand names to try to trick you into
thinking the product is legitimate. In order to work correctly and safely,
HID headlamps must be designed from the start as HID headlamps.
The only safe and legitimate HID retrofit is one that replaces the
*entire* headlamp -- that is lens, reflector, bulb...the WHOLE shemozzle
-- with optics designed for HID usage. It IS possible to get clever with
the growing number of available products, such as Hella's modular
projectors available in HID or halogen, and fabricate your own brackets
and bezels, or to modify an original-equipment halogen headlamp housing to
contain optical "guts" designed for HID usage. But just putting an HID
bulb where a halogen one belongs is bad news all around.
[He also has an accurate and amusing take on maintenance:]
...but what's worse on the bank balance is "feature
creep". You're fixing XXX, so you'd better overhaul YYY too, because it'll
just break with the improved power transfer due to the repaired XXX. And
as long as you've got it apart, it'd be dumb not to rebuild NNN, but why
use stock parts when for just a little more you can get the Viggen items,
and all you have to do to use them in a non-Viggen is add RRR, TTT and
SSS, which you can get from the dealer...
posted by 66.136.17...
, Tue, 14 Sep 2004 17:10:10
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