Headlights – Why You Can See Where You’re Going at Night by Roger Sitterly

Roger is making the information available to all via this post. Copied without amendment, statements and opinions are his alone.


This is the first of a periodic series of articles I hope to write that will discuss some of the options available to MINI owners who wish to enhance the quality of their driving experience. We all know the MINI is a great car right out of the box, but because it, like any other car, is the result of numerous compromises in its design, there are a lot of things that can be done to create the perfect MINI that you’ve specialized just for yourself.

Some of what I write may be well known to you already. I hope, however, that at least some of what I present will provide new information to help you decide what, if anything, you would like to change on your MINI to make it, like MINI says, “You-nique”. Possible future topics include auxiliary lights (fog and driving), brakes, tires, wheels, suspensions, aerodynamics, and perhaps some other things.

Right up front, I want to make it clear that I’m not an engineer of any kind, and I’m not a good-enough mechanic to make my living at that trade. I’m simply someone who has been in love with sports cars for more than 40 years, who learned how to synchronize a pair of SUs with a ruler and the cardboard tube from a roll of paper towels, and who has done more than his fair share of reading and research into various topics in an effort to improve the quality of my personal driving experience. If, at any time, you think I’ve blown a head gasket (or worse), put your thoughts down on paper and submit them to the club. If you’ve got a reasonable perspective on a particular point, we’ll post it for others to read and evaluate for themselves.

Now, on to the subject of this article.


The design, effectiveness, and efficiency of headlights have all come a long way since the days of the original (now referred to as “classic”) Mini Cooper. For several decades, the United States lagged behind the rest of the world in the type of lighting technology that was allowed on cars sold in this country. That has now changed, and our MINIs are, thankfully, available with state-of-the-art headlights. After-dark driving has, as a result, become much safer.

Back in 1959, when the original Mini was introduced, sealed-beam headlights were the only legal headlights in the U.S. Cars could have two seven-inch units, or they could have four five and three-quarters inch units, but the construction required by federal regulation was the sealed beam first introduced in 1940. The advantage of this regulation was that replacement headlights were extremely inexpensive and could be found everywhere. The disadvantage was that by the time the regulations were changed in 1978, cars being sold in this country had been stuck for nearly 20 years with lighting technology that had truly become a “dim bulb” idea.

Our new MINIs have much better headlights, and we should all be grateful for this. However, there are some things on the horizon with the potential to render even Xenon HID headlights something of a “dim bulb” idea.

Sealed Beam Headlights

Sealed beam headlights came into being as the automotive industry developed better technology than had graced cars prior to 1940. It was, at the time, a step forward. Essentially, the complete unit was nothing more than a very large light bulb with a parabolic reflector, two strategically-located filaments (high and low beam), and a lens that focused the light to create a coherent beam. That beam, when projected onto a flat vertical surface in front of the car, had an oval high intensity area that was typically a bit wider than it was high, and there was reduced illumination all the way around the high-intensity zone. Both high and low beams had the same illumination pattern, and only placement of the low-beam filament away from the focal point of the reflector was responsible for the “dipping” of the beam pattern. When properly aimed, the low beam pattern’s high-intensity zone typically covered only about three quarters of a lane, and a common practice was to aim the passenger-side beam slightly higher than that on the driver’s side.

Two sealed beam lights low beamTwo sealed beam lights low beam

Two sealed beam lights high beam Two sealed beam lights high beam

While the U.S. lived with an antiquated headlight design, Europeans were moving forward with a design that featured a replaceable bulb and separate reflector. In addition, high beam and low beam patterns were distinctly different. The European high beam pattern resembled the American pattern, but the low beam pattern featured a much wider horizontal spread of light (typically the high intensity zone covered nearly two lanes of the roadway) with an extremely sharp horizontal cutoff of the high intensity zone in the area where the beam spread into the left-hand lane of the road. This allowed for the use of more powerful bulbs in the lights because very little glare was produced to blind on-coming traffic when the lights were properly aimed.

Two Euro-code lights low beam Two Euro-code lights low beam

Two Euro-code lights high beamTwo Euro-code lights high beam

Quartz-Halogen (QI) Headlights

In 1962, the Europeans produced the first commercial quartz-halogen headlight bulb. This bulb, when used in headlights with a separate reflector, produced more light, usually had a much longer life, and was in all ways vastly superior to sealed-beam units. The innovation was the use of a halogen gas (usually iodine, and hence the common abbreviation of “QI” for the bulbs) in the atmosphere surrounding the tungsten filament.

In a normal incandescent light bulb (like a sealed-beam headlight or a typical household light bulb) the filament slowly vaporizes as it produces light and heat. The tungsten vapor produced is then deposited more or less evenly on all the interior surfaces of the bulb. That’s why a blown incandescent light bulb from your kitchen has a black area on the inside that’s visible if you look for it (it localizes somewhat because the tungsten vapor floats upward so the upper inside surface gets more heavily coated). Sealed beam headlights have so much interior surface that minimal blackening of the lens and reflector takes place over the life of the bulb and there’s not much degradation of the light output. This was one of the attractions of the technology when it was introduced in 1940.

Quartz-halogen bulbs take advantage of chemistry to overcome the problem of a slowly vaporizing filament. When a hot filament out-gases tungsten vapor in a halogen atmosphere, a chemical reaction takes place with the surrounding halogen gas and the vapor is re-deposited on the filament and nowhere else. In fact, any vaporized tungsten previously deposited on the inside of a bulb’s surface will be scavenged by the halogen gas and redeposited on the filament. This chemical reaction depends on having a truly white-hot filament, one so hot that in close proximity to the bulb’s envelope, it would melt the type of glass used in a sealed beam headlight. To overcome this problem, the bulbs are not made of ordinary glass but instead are made of quartz or alumino-silicate glass (neither of which will melt until they reach temperatures well in excess of 2,500o Fahrenheit). In addition to having a higher luminosity (output of lumens), QI bulbs also have a higher Kelvin temperature. This makes their light appear much whiter than that from a sealed-beam headlight.

The Color Temperature of Light

The Kelvin temperature scale measures the color emitted by a standard black body based on the temperature in degrees Kelvin of that body as it emits light. For example, a sealed-beam unit with a Kelvin temperature of 2,000o is emitting a color of light identical to that emitted by the standard black body when heated to 2,000o K (or 3,140o Fahrenheit). The closer to natural sunlight the Kelvin temperature of a light source, the more natural it appears (assuming it emits light throughout the entire spectrum, as is the case with a tungsten filament). Lights with lower Kelvin temperatures tend to look yellowish to the human eye; as the Kelvin temperature increases, the lights appear whiter. As the Kelvin temperature surpasses that of bright daylight, the lights tend to appear very slightly bluish to the human eye. Keep in mind that Kelvin temperature is the temperature equivalent of the emitted light, NOT the temperature of the filament emitting the light.

High-Intensity Discharge (HID) Headlights

Fast-forward to 1991, and BMW (on the 7-series) introduced the first commercially available Xenon HID headlights. This lighting technology has significant advantages over the use of tungsten-filament QI headlights. HID headlights emit more than twice as many lumens of light as a QI headlight and that light has a Kelvin temperature that is higher than bright daylight (which is what gives them their characteristic slightly-blue appearance).

SIDE NOTE – When you see oncoming headlights that are distinctly bright blue and that produce excessive glare, they’re not HID. Very likely they’re some cheap QI bulb with a blue coating on the surface of the bulb. Because they’re cheaply made, the filament(s) are usually not accurately located, so they tend to create far less usable light for the driver of the car and far more glare for oncoming traffic than they should. I once bought a cheap replacement H-1 bulb for a Carello driving light – the beam pattern went from a well-focused oval that projected about 100 yards down the road to three separate scattershot beams that very nicely illuminated only the tops of all the trees in the neighborhood. Lesson learned about cheap QI bulbs.

Back to Xenon HID lights. An HID light doesn’t use a bulb in the normal sense of the word – the proper term is “capsule”. There is no filament. Instead, it’s an arc light – light is created by passing an electrical current of considerable voltage through metallic salts contained within the capsule’s atmosphere. Instead of a halogen gas inside, HID bulbs use one of the noble gases. Another correct term for the lights is “metal halide”, and a common application is in street lights. Automotive HID lights generally use xenon as the gas atmosphere within the capsule, while streetlights and other applications usually use argon. Argon is not suitable for automotive use because it takes an argon light several minutes between the time it’s turned on and the time it reaches operating temperature. Xenon allows this time to be cut to just a few seconds.

HL 5 Euro code HID low beam lights
European HID low beams
HL 6 Euro code QI low beam lights
European QI low beams

Notice the obvious difference in “whiteness” in the above two illustrations. This is solely a function of the Kelvin temperature of the light, and is in no way related to the wattage or lumens of output.

Because HID lights can’t operate on 12 volts of DC power, they require an igniter and a ballast. If you have HID lights on your MINI, pay attention to what you see the next time you turn them on in the garage after dark. There will be an immediate quick flash of light (the igniter firing), after which there will be a somewhat dim light emitted for a few seconds. As the seconds pass, the capsule comes up to full operating temperature/output, and the emitted light becomes noticeably brighter and more spectrally complete.

You may also notice the automatic self-leveling system adjusting the lights’ aim for the car’s current load and stance. While not required in the U.S., automatic self-leveling of HID lights is required in Europe, so we get it here because it’s less expensive for MINI to provide it than to have two kinds of HID lights on MINIs, depending on where they’re sold. The warning label on the ballast means what it says – MINI HID lights operate at 25,000 volts. You DO NOT want to be playing around with them when they’re lit up.

Here’s a table comparing various sources of light for a driver. I couldn’t find wattage numbers for LED headlights.

Headlight Type Watts (at 12V) Typical Lumens/Watt Kelvin temperature
Sealed beam, low 40 17 2,000o
Sealed beam, high 50 17 2,000o
Quartz-halogen 55 28 3,600o
Xenon HID 35 91 5,400o
LED 100 6,500o
Bright sunlight 4,900o

Reflector Design

In addition to technologically improved sources of light (from normal incandescent to QI to HID), reflector technology has also improved, which puts more of the emitted light where it needs to go. Sealed beam lights used a more-or-less standard parabolic reflector and depended on the optics of the lens to create a usable beam pattern. Beginning in the mid-1980s, manufacturers both in Europe and the U.S. began working on headlights with a perfectly clear lens and a complex reflector – the reflector did all the focusing work and the lens merely protected the unit from the elements.

Complex reflectors utilize multiple three-dimensional segments, each with different complex contours, so that in unison they project the beam where the engineers want it to go. The standard QI headlight on a MINI is a perfect example of a complex reflector. A variation on this theme is known as the “free-form” light. Free-form lights have complex reflectors, but there are no obvious segments. Instead, the “segments” transition smoothly from one to the next, giving the impression of a “normal” parabolic reflector while in fact yielding a carefully-designed beam pattern. The stock fog lights on at least the first generation MINIs are an example of a free-form light. I haven’t looked closely at the OEM fog lights on a second-gen car.

The Next Steps and the Future

Initially, HID headlights were available only for low beam applications. My 2004 MINI “S” is an example. This limitation is a direct result of HID lights needing at least a few seconds to come up to full operating output, and that makes it impossible to switch between high and low beams without incurring a delay during which the driver would have great difficulty seeing where he/she was going. In my car low beams are always illuminated, even when the QI high beams have been selected.

The problem with having high and low HID beams in one headlight unit has now been solved with the introduction of a movable shade inside the unit. With the shade in the light path, the light operates on low beam because the shade blocks part of the beam pattern. With the shade out of the way, high beam is projected. This gives drivers the best lighting available on both low and high beams.

Adaptive headlights are also becoming available (MINI now offers them – they weren’t available for my 2004). These headlights mimic the Cyclops light on Preston Tucker’s 1948 “Torpedo” that pivoted left and right with movement of the steering wheel. This pivoting headlight allowed drivers to see further into corners, though the Tucker’s mechanical system was cumbersome. Today’s technology makes it more feasible to execute the idea – microprocessors and small electric motors can accomplish the desired pivoting movement more easily and more reliably.

Coming soon to a car in the next lane may be LED headlights. Beginning in 2004, LED headlight application has been seen as one path to the future. In 2007 (for 2008 models) Lexus became the first to use LEDs for headlights on vehicles sold to the public, and the 2010 Toyota Prius offered them as an option. Some Audis and the Cadillac Escalade also have LED headlights. Many more are using LEDs for daytime running lights. However, for headlight use there are still significant design problems to overcome, primarily due to the large amount of heat that must be removed from the units (you only think LEDs aren’t hot because you haven’t felt the back of a bank of them putting out enough light to be a headlight) and the fact that warm LEDs emit noticeably less light than do cold ones. The units are also very expensive relative to the cost of HID or QI units, and there apparently are some regulatory grey areas that remain to be clarified. I think it’s doubtful that your next MINI will have LED headlights.

Sounding more like Buck Rogers than scientifically possible is the use of lasers for automotive headlights. Before you chuckle and agree, however, you should know that BMW demonstrated what it calls “Laserlight” headlights at the 2011 Frankfurt Auto Show. Three small blue lasers are mounted in a triangle to constitute one headlight. The lasers shine onto small mirrors, which direct the beams into a lens. The lens is formulated with yellow phosphorus, which produces an intense white light when excited by the blue lasers. The white light is focused by the lens on a reflector that generates and projects the beam pattern down the road. Because it’s the white light generated by laser energy on the phosphorus, and because that light is reflected by a mirror, oncoming traffic is not looking directly at a laser, so there is no danger to other drivers’ eyes. BMW says the emitted light is more than 1,000 times as bright as LED headlights while using half the energy, though one published magazine report says the laser headlights produce “only” 170 lumens per watt of input, approximately double that of current HID headlights. Another report says BMW hopes to have them in production by 2013. If laser headlights do reach production, Cibié’s old tag line “portable daylight” may become reality.

I discuss auxiliary fog and driving lights in additional articles also available in our Technical Talk section.

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