Michael!! You made all this up, didn't you?
Kidding, of course. For years I've known not to finger the H2s, but never
had it explained so elegantly. Actually never had it EXPLAINED at all.
Thank you, thank you, thank you.
Just one question - should I go ahead and replace my H1 & H4s with them
Xenon thingys?
Mad Mike (hoping Brennan and others post this puppy for future reference)
...
-----Original Message-----
From: Michael Daily [SMTP:mdaily@accessone.com]
Sent: Thursday, March 18, 1999 5:33 PM
To: rally-l@scifi.squawk.com
Subject: [R] The big Xenon fib, a long tech note
If you don't care about light bulbs at all, press DELETE now.
If you care just a little, you could skip some of this and find the
punchline near the end.
If you want to know more, read on...
> >Err, I think John was talking about the Hella Rallye 4000 Motorsport
HIDs
> >which use Xenon bulbs. As mentioned a few posts ago they are ~$700 each
> >and include the ballast supply thingy and it automatically shuts
> >off in the event of a collision.
> >
> >I assume they'd be rather bright.
>
> I'm sure they are! But I _was_ referring to the new H1 and H4 bulbs that
> have Xenon gas in them to facilitate a hotter filament and thus more
light
> output.
Time to drag out the soapbox.
I know a thing or two about light, light bulbs, light fixtures, and
marketing.
Lighting 101:
In basic form, there are two kinds of "light bulbs" -- filament sources and
arc sources. A filament source produces light by passing current through a
wire. The current flow heats the wire to incandescence. An arc source
produces light by creating an arc (like a tiny lightning bolt) between two
or more electrodes. In some cases, this arc is the actual light source.
Arc sources like regular flourescent tubes that we have all seen are a
variation of this. In these tubes, the light you see is not from the arc,
but is a result of electrons flowing though the ionized gases within the
tube. These electrons produce photons that are absorbed by the whiteish
phosphor coating on the inside of the glass. The phosphor releases other
photons, which is the light that you actually see.
By changing the recipe of the phosphor coating, the color of the light can
be changed. This is why there are lots of different tubes on the market.
"Cool White" is what you normally get the cheapest, perfect for a warehouse
or shop when color perception is not important. In a retail store though,
color perception must be very good. One of the ways an arc source in
evaluated is by CRI or Color Rendering Index. Remember that the color of
an object you see is the light that is reflected off of that object. If
the light source produces only blue light, only blue light will be
reflected.
Filament sources are called "continuous spectrum" sources. The light they
produce can be defined as having a color temperature (stated in degrees
Kelvin). Photography buffs know about this. "Tungsten" film is optimized
for light sources of 3200K (a little higher than lamps in your home) or
"Daylight" which is above 5000K (5100-6500, varies with weather). A
continuous spectrum source produces photons throughout a range of
wavelengths, which could be graphed into a typical bell-curve. Not equal
output at all wavelengths, but smoothly decreasing away from a central
peak.
Arc source are "composite spectrum" sources. The photons they produce are
of specific wavelengths determined by the chemical composition of the
phosphor coatings. The more exotic coatings will have a very complex
composition, so that they emit photons of a greater variety of wavelengths.
Light reflected by an object will accurately describe its color if the
light source has good spectral content. A composite spectrum source is
said to have a CRI of X, where X is the percentage of spectral content
compared to a continuous spectrum source of similar color temperature.
Blech, sounds like math and physics. What does this mean. An arc source
cannot have CRI greater than 100, which is a "perfect" continuous spectral
content. A really good fluorescent tube used in retail merchandising might
have a CRI as high is 93. That is about as good as tube manufacturers can
practically achieve. Most are 88-91. Your el-cheapo tubes are in the
60-80 range. This is why things look different in the store versus at home
versus outdoors.
Why do we care? Filament sources must, by their very definition, produce
heat. In fact, it is not the electricity that produces the light. The
current flow produces the heat required to achieve an incandescent state.
An arc source typically produces more light for the same amount of energy,
with less heat produced.
Lighting 102:
First we had the filament, but making it incandesce caused it to burn up.
By placing the filament in a vacuum (sealed inside an evacuated glass
capsule) it could in reach incandescence for an extended period. Without
oxygen, the filament would not burn up right away. Ta-da! the light bulb
is born. The filament could still be damaged by vibration/physical shock
(very fragile when hot), and would wear out eventually. The "wear" was due
to evaporatation of the filament. This is seen as gradual darkening of the
glass as the evaporated material is deposited onto the glass. In some
bulbs, the glass enclosure was made very large, so that the surface area
was greater. With more area, the darkening was less dense, so the bulb
maintained brightness longer. The conventional incandescent light bulb
usually has a color temperature around 2600-2900K.
Then somebody had a great idea.
What if you pressurized the bulb? Theory suggested that a pressurized
enviroment would slow the evaporation of the filament. This eventually
became the Quartz/Halogen bulb that we all know and love. It was not a
direct path, and several things were discovered along the way. Briefly,
the Q/H bulb is elegant. The heat of the filament causes an increase in
pressure of the inert gas in the bulb. The pressurized gas reduces the
evaporation rate of the tungsten filament. Some tungsten does evaporate,
but it does not get deposited on the quartz glass. It remains suspended in
a gaseous state while hot, and most is redeposited on the filament when the
filament cools. This is called the Halogen Cycle. Side note: if your Q/H
bulb glass is tinted grey, the bulb is not operating at the design
temperature intended, probably due to incorrect voltage. Halogen cycle
works only within a narrow gas temperature range. Q/H lamps typically have
a color temperature of 3050-3250K.
You will hear the terms Tungsten-Halogen, Quartz-Iodine, Halogen, and such,
all of which convey the same concept of marrying quartz glass, halogens
(inert gases, like iodine, krypton, and yes, xenon) and the tungsten
filament. All of these terms are essentially correct. It is not correct
to call them Tungsten, which is a frequent mistake. Light bulbs used
tungsten for filament before this advent. Q/H bulbs can explode because
they are pressurized when hot. The handling requirements for Q/H bulbs are
different from regular light bulbs. The quartz glass envelope is very
compact to keep to gas within a specific temperature range, and as a result
the glass is much hotter being so close to the filament. Finger prints on
the quartz glass will cause it to overheat at that spot and melt the glass.
Because of the internal gas pressure, the glass will bubble outward and
rupture.
Lighting 103:
Arc sources of the short and medium arc length families (as opposed to long
arc like common fluoresecent tubes) can be incredibly bright. HMI's
(actually a trademark of Osram/Sylvania, but so generic like Xerox) are
used for making movies. The bright light coming in through the windows in
you favorite flick. Sunlight? I don't think so. It was a set. The
interior light was probably brighter than you would expect, and the source
"outside" the window has to be enormously brighter to make you believe.
HMI's have a color temperature around 5500K and a CRI above 90. Not bad
for some thing the size of a tennis ball that is rated at 18,000 Watts.
Has approximately 3.5x the lumen output of a filament source. Yeah,
equivalent to a 63,000W Q/H source. HMI's were a great development for
Hollywood. In other applications, the shortcomings are more apparent.
When first turned on, the output is a hideous color and not very bright.
Takes 3-5 minutes to warm up to correct operation. Color temperature
creeps down about one degree for every hour of use. Won't "hot restrike"
meaning that if you turn it off, it has to cool down before it can be
turned on again.
Then xenon came along. Except that xenon has been around before.
Xenon arc sources are stuff dream are made of. The searchlight on the
Coastguard boat on the news. The searchlight on the helicopter you saw
last night on COPS. The latest compact aerial sweepers that show up for
every premiere, bowl game, and Oscar night. The really powerful spotlights
in your local arena. The really trick giant flashlights that Scully and
Mulder use to find the TRUTH. The Luxor Pyramid.
They turn-on to full brightness immediately. The color temperature is
stable. One little issue that comes up now and again is the lamp design.
The gas in the globe is under pressure. Well above one atmosphere when
cold, and the structure of the glass deteriorates as lamp reaches end of
its "life". Improper handling during lamp changes can cause them to
explode. The protective gear used when changing lamps is kevlar. In many
applications the easiest way to address this saftey concern is to make a
modular housing with the bulb inside. Then you replace entire module
rather than just the bulb.
The lighting industry loves xenon arcs. Xenons are really bright.
Marketing tip #1: Use the word Xenon a lot.
Lighting 104:
Let's go back to filament sources. Heat is the key. Hot filaments produce
light. A filament of a given size at the right temperature will produce a
certain amount of light. How do we improve the output? Bigger filament
(higher wattage). That is the obvious answer. Some years ago, General
Electric played with another option. What if you change the temperature?
Well, it screws up the halogen cycle. How about creating the same amount
of heat with less energy? That sounds good in principle, doesn't it.
Eventually they developed a nearly transparent coating to go on the inside
of the quartz glass. The coating reflected some of the infrared energy
(heat to you and me) back towards the filament. This causes the filament
to reach the incandescent state using less electrical energy. GE called
their new trick HIR and put it in their "Watt-Miser" line of energy saving
lamps. Typical specs were same light output, using roughly 20% less
electricity. Imagine.
Well whatever patent rights were once held by GE are now open market.
Several bulb manufacturers have the same or similar coatings and nobody
thinks twice about it.
So here we are in an automotive equipped society. What does the law say
about headlights. Wonder of wonders, it is outdated. 60W bulb maximum.
Hmmm, doesn't say much about lumens (actual light output). So, if you make
a light bulb using coating technology and the bulb uses the maximum energy
allowed, it will be brighter. As the technologies have developed, the
color temperatures have crept upward. The whiter light of halogen
headlamps used to be easy to spot. They stood out from all the other cars
on the road that still had conventionals. Today, it is easy to spot the
Mercedes or Lincoln that is equipped with arc source headlamps. I have not
actually seen any in use up close, but the Hella 4000 HID are sure to be
amazing. High Intensity Discharge is the next wave. HID may be another
trademark, but it applies to all of the short and medium arc flavors.
Xenon arcs are part of this family.
Punchline:
A tungsten filament in a quartz glass envelope filled with Xenon (one of
the halogens) gas is a quartz halogen lamp. It may be coated for higher
efficiency, but it is not unique. The coated lamp will have a color
temperature of 3300-3400K, making it even "whiter" (more blue = less
orange). You _can_ see better with the whiter light, with the exception
below.
PIAA got cute and gave us "crystal ion beam" lamps which is another type of
coating. PIAA's purpose was to change the color of the light to yellow for
fog use. This was for their H4 PIAA 80 which gave you a white high beam
and yellow low (fog) beam with a single lens and single reflector. Have
your cake and eat it, too. Very sweet.
Don't even get me started on UV, I need a towel.
Mike Daily
* no insects were harmed in the making of this diatribe.
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