HEADLIGHTS: BULBS
A bulb is an airtight glass globe filled with an inert gas. Inside, suspended between two steel rods, is a filament made of wolfram. The steel rods conduct the current between the socket and the filament. When voltage is applied, the filament heats up and glows, thereby emitting light. The light intensity is a compromize between how much light we want (as much as possible), and how long the bulb should last (forever). By increasing the filament temperature, it will emit more light, but the lifetime will be reduced drastically. Nobody buys bulbs that only last a month or so. Once maybe, but the next time you will buy another brand.
In order to get two kinds of light out of one lightunit, two filaments
are needed. In 1925 the BILUX bulb was invented, which provided
both dip, and full beam from one bulb. The full beam filament
is in the reflector's focal point. The dip beam filament is located further
forwards than the focal point, so the lightrays emitted from the TOP end
of the filament leave the reflector, tilted downwards. The light-distance
is reduced, but this will not blind oncoming traffic.
Of course, the lightrays emitted from the BOTTOM end of the filament would
be tilted UPwards, (and we dont want that),
so a shield is placed under the dip filament.The shield is not straight,
the left half is tilted down 15 degrees. This provides an asymmetric
dip beam, illuminating the right half of the road further ahead. In RHD-countries
(UK, Australia, etc), the shield is
of course tilted the other way. This is why visitors from RHD-countries
has to blank out a sector of their headlights with black tape. (And
vice versa)
When the bulb is switched on, the wolfram filament gets so hot that fragments from its surface vaporises, and then condensates at the inside of the glass, leaving a dark soot deposit. This process goes on during the bulb's lifespan, and in the end the filament is so thin that it melts. By now, the glass is very dark. The side-effect is of course, that the light intensity gradually decreases, during the bulb's lifespan. In an effort to minimize the filament vaporising effect, various gas-fillings were experimented with. This lead to the development of the halogen bulb, introduced in the early sixties.
The HALOGEN bulb is filled with an active halogenous gas, usually
iode. When fragments of the filament vaporize, they chemically react with
the active gas. Instead of condensating at the glass, this mixture "bounces"
back and chemically bonds to the filament. A recycling effect! This gave
headroom to increase the filament temperature, so a halogen bulb emits
2,5 times more light than a conventional bulb with the same watt rating.
It lasts longer too, and the glass is crystal clear until the very end.
A quartz glass is used instead of normal glass, due to the increased temperature.
Older lightunits were not designed to withstand the raised heat either,
so new ones with reinforced glass and reflectors were designed.
One thing the bulb manufactorers tell us, is: Never touch the bulb-glass!
I've heard that the greasy fingermarks will make the bulb blow prematurely.
This, I cant explain, but I do know one thing: The greasy fingermark will
be warmed up until it vaporizes. It will then condensate on the reflector
and glass, leaving them not-very-shiny-and-bright anymore.
In 1962 came the H1 bulb, followed by the H2 in 1966. They
both have longitudinal filaments. In 1967 the H3 emerged, with
a transverse filament.
These bulbs only had one filament each, so getting both dip- and full-beam
meant back to the pre-1925 days solution: Two separate lightunits each
side. This was no problem for a car with twin 5 ¾" uni-size
units, but practically all other cars were left in the dark.
Until MARCHAL came up with the "AMPLILUX".
This
is a 7" uni-size SAE-fit halogen unit with both dip- and full-beam.
Like all other lightunits, only the upper half of the reflector is used
at dip beam. But instead of wasting the lower half (as
all other dip beam units do), there is another smaller reflector
placed there, with a H3 bulb in its focal point. This one provides full
beam. Dip beam is by a H1 bulb.
"-Two separate reflectors in one uni-size lightunit, and TWO bulbs?
Sounds expensive". Said the bean counters, who were reluctant
to apply this unit to a broad range of car models. "-Its more
cost-effective to stick to the non-halogen bilux units" they
said.
But this became a very popular aftermarked unit. Until the "bilux"
H4 halogen bulb was invented in 1970, and OEM fitted to cars from
1973. Finally we all could have decent illumination.
And so it was for the next two decades, until someone with the opinion that normal halogen bulbs were not enough, introduced the H7 bulb (1992). They had been experimenting with the gas mix again, and found that an added dose of xenon under higher pressure, gave more headroom to increase the filament temperature. The result was a good 30 % increase in light output, over a standard halogen bulb. During the nineties, the H7 bulb became the choise of an increasing number of car manufacturers. Equipped with only one filament, it requires separate dip and full beam lightunits, but today this is haute couture.
After the introduction of this new gas recepie, the bulb manufactorers made versions of the older halogen bulbs with it. From 1997, Philips have had their PREMIUM H1, and H4. Osram can supply H1, H3, and H4 SUPER. Others too have joined the bandwagon, with their versions of "+30%" bulbs. A 55 w "+30%" bulb has the light output of a 71 w standard bulb.
In
this bulb-chart I have only included part numbers for standard-bulbs to
show the difference in light intensity. If there is a "+30%"
version available, the standard bulbs are now obsolete and I never
use them.
WHITE
BULBS
A "white" bulb does not emit only white light. It starts
with yellow (long wavelength / low frequency),
in one end of the color scale, goes through white, and ends up with blue
(short wavelength / high frequency).
The total sum of these colors looks white to the human eye.
YELLOW BULBS
Until recently, French legislation prescribed all new cars to have yellow
bulbs. In all other countries this was optional. Under many driving conditions
yellow bulbs makes sense, but forcing someone to have them at all times
is taking it a bit far… But then again, the French have never been
afraid to do things controversary. Until the early nineties, yellow light
was provided by either a tinted glass globe around the bulb, a tinted
reflector, or tinted glass in the lightunit. The drawback with these three
solutions is a slight reduction in light output, but the benefit of yellow
light under harsh driving conditions more then counter for the loss. In
snow, contours impossible to see with white lights, emerge clearly. Backglare
from wastelight, is notably less tiresome. It is a known fact that yellow
light enhances visibility in poor weather conditions, and yellow is perceived
as a comfortable light by the human eye.
In the early nineties, a new tecnique (old technique, but new to cars) was introduced to create yellow light. A dichroic yellow-pass filter subtracts blue, and leaves the yellow light, nearly loss free, from the output spectrum. To begin with, the Philips "GOLD" bulbs gave a very yellow light. Today's yellow bulbs are labelled ALLWEATHER, and are yellowish/white. Osram can even supply H1 and H4 bulbs that are both ALLWEATHER and SUPER (+30%).
BLUE BULBS
The poseur's choice! Blue halogen bulbs are doing their best to ape the
color of the new Gas Discharge Lights. But with only a fraction of the
light output. A halogen bulb emits very little blue light to begin with,
and then they've added a dichroic blue-pass filter that takes away yellow
and most of the white light. Despite emitting less light than a white
bulb, they look brighter/harsher to anyone having the misfortune of beeing
in an oncoming car. This is because short wavelength/high frequency blue
light does not trigger your eye pupills to contract sufficiently. On top
of this, the manufacturers have the nerve to charge ridiculous prices
for them. More about blue bulbs in my "sum up" below.
Oh by the way, life length is highly compromized too.
UV-CUT BULBS
Like the sun, a bulb emits ultra-violet light. It doesn' bother us, we
can't see it. Why bother?
A lot of modern cars has the headlights made of plastic. Even the lense
is made of Poly-Carbonate (lexan),
a very impact resistent/unbreakable plastic. Ultra-violet radiation
makes plastics turn yellow… All new Philips and Osram bulbs has
UV-BLOCK quarts glass.
OTHER
COLORS
What other colors? Well, poking around the internet, I stumbled across
a website with references to a bulb manufacturer. Their color chart was
…eh…stunning. Superwhite, blue, green, amber, and yellow H4
bulbs. Remember, any kind of tint will reduce light output. You really
ought to see this one!
http://www.e-lamp.co.jp/vestec/f2.html
And German car-accessory warehouses sells heat resistent paint and dyed
silicone condoms for headlight bulbs, park bulbs, etc. Green parklights???
Give me a breake!
XENON, GDL (GAS DISCHARGE LIGHT),
HID (HIGH INTENSITY DISCHARGE )
The final solution! Totally superior compared to old halogen technology!!!
…… Or is it?
Let's see… Xenon light is not a new light-unit technology. It's a
bulb-tecnology. They still have to choose between the three available
headlight-types. (Parabolic reflector, DE,
or FF)
A xenon bulb emits 3,5 times as much light (Luminous
flux) as a halogen bulb, with the same watt rating. (These
figures vary, depending on the halogen-bulb's efficiency.) For
this reason, they've decided that 35w is sufficient. (So
far). According to my calculator, this equals a 122w halogen bulb.
In real life, it appeares that this "3,5 times" figure is ...
a bit optimistic. If we compare the Luminous flux ratings of the D2R (35W
xenon) and the H2 (100W halogen),
we get the numbers 2800 and 2850 respectively. Compare the Lumens of the
D2S (35W xenon) and the H3 (130W
halogen), we end up with 3200 and 3650.
Since these bulbs emit so much light, ECE regulations prescribe that all xenon dip beam units must have an automatic height adjusting mechanisme, to prevent them from blinding oncoming traffic. As if the xenon lights weren't expensive enough…
The light source in a xenon bulb is NOT a glowing filament. Suspended inbetween the two steel rods is NOTHING. But if we put these steel rods closely together, and apply a very high voltage, an arc will continuously jump over the gap. Almost like a spark plug, and very much like an electric welder. High voltage is supplied by a lunchbox sized hide-away unit. (No, not one box, one box for each bulb)
Using
an arc discharge as a light source is indeed a very old technique. My
hometown's "Historic Aviation Society" has restored one
of those search-lamps the Germans used to illuminate allied bomber planes
with, during the war. The reflector has a diameter of one
and a half metres, and it weighs nine tons, including a generator.
Last new years eve, the lads towed it up on the nearest mountain and used
it to illuminate various buildings in the city, the mountain on the other
side of the fjord, and such. (Almost like
giving a five-year-old a torch for christmas) It was stunning!
This search lamp uses an arc as a lightsource.
As a young kid, I sometimes used to watch the light poles in the late
afternoon, when the photocell switched them on. At first, there was a
glimpse of light, then the bulb was glowing weakly for a minute or three,
and suddenly it burst into light. Not at full effect, though, it took
an additional five minutes for it to "warm up".
This long start-up-time made the arc discharge bulbs unsuitable for cars.
Imagine having to wait several minutes for the lights to come on?? But,
in the late nineties, the engineers at Hella, Osram and BMW had finally
worked their way around this culpit, and proudly presented the light of
the future. For silly money, you could now order your BMW 7 series with
Xenon lights. (Dip beam only, so
far)
Trouble is, the color of the light is… blueish. It contains a lot
of white light, thank you very much, but it still looks blueish.
Especially to oncoming traffic. Why dont they put on a dichroic filter
or something, to take away this blue light?
They tell us that the color of xenon light is closer to daylight color.
(The type emitted by the sun) Thats
all very nice when everything is illuminated, but at night we carry our
lightsource with us, and only illuminate fifty to a few houndred metres
in front of the car. I'd like to hear a scientific explanation to why
daylight color should be better at night. Besides, it's not exactly
daylight color, but closer than halogen bulbs. (Look
at the bulb-color chart above)
And why is it that ECE authoroties, banning everything over 65w, suddenly embraces a bulb type with the equivalent of 122w? We can easily acheive a similar light output using halogen bulbs, and the white light would be less irritating for oncoming traffic.
Next question is what to do with all this extra light. Should we "give"
this newfound brightness to the designers, and let them downsize the lightunit
until it equals the the previous halogen unit? Or should we keep the size,
and have more light? Where should it be distributed? The easy solution
would be to keep the same light-pattern as with the previous halogen unit.
Then the light-pattern will be the same, but the light intensity will
be stronger. Or, using various reflector techniques, one could widen the
light-pattern. This will put a lot more light in front of the car and
to the sides, but it won't increase the range.
Or (again using various reflector techniques)
one could increase the range. Let the dip beams illuminate the road further
ahead.
Whichever solution the car manufactorers have chosen, they're not likely
to tell you. The only thing I've heard from them, is "-Look! We've
got the new xenon high performance headlights, -much better than halogen,
and bla bla bla!" They seem to be unable to tell us how
it is better.
As with halogen lights, there are good and not-so-good xenon lights. The Opel Omega (Vauxhall Carlton, Cadillac Catera) is praised for its effective long range DE xenon dip beams. The Nissan Maxima has got FF xenon dip beams. But only the upper half of the reflector is used! And on top of this they've made a hole in the upper half of the reflector, to let the park light bulb peek out!?! Despite having a wastelight-shield, it emits plenty of it. And finally, the light output is not in the class of the Omega.
The manufacturers also claim that the life-length of the bulb is so long, that they'll last (almost) as long as the car. This is not entirely true, they lasted a year-and-a-half (or 155 000 Km) in a Mercedes W210 taxi, in my neighbourhood. Another Mercedes blew a bulb after 85 000 Km (seven months). And for the ridiculous amount of money they charge for a new bulb, you can buy a LOT of halogen bulbs. You'd probably still have bulbs left, when the car is sent to the crusher. (The taxi got new bulbs for free. Warranty claim. Had it been a year older…)
The real benefit of the xenon bulb lies in it's efficiency, it's ability to transform current consumtion into light output. With less current draw, the need for a heavy duty wiring harness is reduced. Heat radiation depends directly of the wattage, so the temperature of these bulbs is lower. Regarding the color of the light, I believe they're working on it, and the next generation xenon bulbs will be more pleasant. The lunchbox-sized high-voltage transformer isn't a real problem. On the next generation xenon bulbs they've probably integrated a smaller version of it in the bulb base. Remember the first generation NMT-450 mobile phones? (They weren't portable, they were luggable!) Look at mobile phones now!
MORE ABOUT BULBS
I strongly recommend you to visit Daniel Stern's bright site, and read
his thoroughly illuminating explanations.
<http://lighting.mbz.org/>
ECE R37 bulb descriptions.
PDF
file. Lengthy and detailed. Enjoy!
