INFORMATION ABOUT BLUE HEADLAMP BULBS:

Various companies and individuals are selling halogen bulbs that have a coating that makes them light up with a bluish color. There are lots of claims made for these bulbs, and lots of myth and misinformation surrounding the bulbs, their performance and their legality.

ARE BLUE HEADLAMP BULBS DANGEROUS?

Yes, they are. Here are the nuts and bolts of why blue bulbs are a bad idea:

The output spectrum of halogen headlamp bulbs includes *very* little light in the blue frequency range. These blue bulbs have a filter coating on them that allows only the blue frequencies through the filter. Because very little light is produced by a halogen bulb in this range in the first place, it is only this very small amount--a tiny fraction of the total amount of light produced by a halogen bulb filament--that ever reaches the road. This can be confirmed this with a good-quality non-chromosensitive light meter; even a very apparently-bright blue bulb actually throws very little light.

Recent tests by the US Department of Transportation's Office of Crash Avoidance Standards found that a standard-wattage 9004-type blue headlamp bulb reduced the road lighting ability of a standard headlamp by 67%, and increased glare for oncoming and preceeding trafic by 33%.

This illustrates the difference between the signal image, which is what you see when you look at an illuminated light, and the emitted luminous flux, which is the light that is thrown from the bulb to illuminate surrounding items, either directly or via a reflector and/or lens.

Aside from the disadvantage of throwing less light onto the road, another aspect of blue light creates another road hazard when blue headlamp bulbs are used:

Blue is the shortest wavelength/highest frequency color of visible light, and, as such, scatters the most readily. This is why the sky is blue rather than any other color from the sun's white output spectrum. And, you may find it interesting to find a dark blue storefront sign or something else that's a dark, pure blue against a dark background in the absence of white light. From any appreciable distance, it's almost impossible for your eyes to see the blue lighted object as a sharply defined form...the edges blur significantly.)

SO HOW DOES THIS SCATTERING TENDENCY OF BLUE LIGHT AFFECT HEADLAMP EFFECTIVENESS AND ROAD SAFETY?

In two ways:

•Blue light scatters very readily in water droplets (rain, fog, snow), causing increased backglare for the driver of a car equipped with blue headlamp bulbs, and

•Blue light creates increased glare for oncoming traffic. That's because blue light does not trigger a strong pupil-closing response in human eyes. due to the comparatively weak pupil response to blue light, the human eye is very glare-sensitive to a blue signal image. This is especially a problem with blue-tinted halogen headlamp bulbs that are also over stock wattage. So although the same amount of blue is emitted by a halogen bulb whether it's got a blue-filter coating or not, in the "no filter" case, the remainder of the output spectrum triggers a pupil-closing response in the eyes of oncoming traffic, helping to reduce the short and long term effects of headlamp glare. This glare-protection response is severely compromised when the oncoming signal image is blue.

> Wait a minute -- how blue are these bulbs, anyway? Blue like a new

> Mark-whatever Lincoln or Mercedes? Or blue like the spotlights at a

> Broadway play?

It's a difficult question to answer (and brings to mind an old BlackAdder exchange about the blueness of the sky vs. the blueness of the Blue Stone of Galveston...) But here goes:

Genuine arc-discharge (also called metal-halide HID) headlamps run with a very purplish-white character that reminds of the color of the electronic flash on your camera, because the same technology is at work (an electrical arc jumping through an atmosphere of Xenon gas). But despite the purplish appearance, this light is actually white with a discrete blue component. That is to say, the vast majority of the output light from an HID headlamp is a good, solid *white* that is closer to the white of the sun than most halogen bulbs' output spectra can reach. And, in addition, there is a separate output spectrum of blue-green to blue-violet frequencies that is a byproduct of metal-halide lamps such as the HID lamps currently used in cars. That blue-green to blue-violet frequency band is "throwaway" light in an HID headlamp. The signal image of an automotive HID headlamp has a distinctly blue cast, but if you drive behind them you are struck by the very white characteristic of the light.

You may remember looking through a diffraction slot box in highschool physics to view the output spectra of Sodium, Boron, Xenon, etc. in those funny discharge tubes...this is what I'm talking about. If you had one of those diffraction slot boxes, you might have pointed it at the overhead incandescent bulbs and seen a contiguous "rainbow". If you were to allow only the blue frequencies of that incandescent bulb into the diffraction slot box, you'd see a short blue band and not much else.

That's why the question "which one is bluer?" can't be answered directly; the blue signal images from HID and from blue-tinted halogen lamps arise from two wholly separate phenomena, and therefore can't be directly compared. The main thing is to keep in mind that the blue signal image of an HID headlamp is a throwaway byproduct of a light source that emits a great big lot of very nice white light, while the blue signal image of a blue-tinted halogen lamp is the meager blue ouput left when you've cut-out all the rest of the usable light.

WHAT ARE THE EXACT LEGAL ASPECTS OF BLUE HEADLAMP BULBS?

They are ILLEGAL under the light-color requirements for Europe, Japan, Australia and Canada and in every state of the USA.

WHY ARE THEY ILLEGAL?

US, Canadian, European and Japanese regulations all call for "white" light. There is no one specific light color that is defined as "white" light; rather, there is a range of output spectra that are considered "white".

The US standard of "white" is based on an SAE (Society of Automotive Engineers) specification, while the ECE (European regulatory body) sets their own standard of "white". Both standards of "white" are very similar to each other.

The width of the standards is primarily due to a large allowance in both definitions of "white" for light that tends towards yellow or Selective Yellow. Under the same regulations that define "white" light, "yellow" is a yellow-to-amber color such as one sees from amber turn signals and from white lamps operated at less than their rated voltage, while "Selective Yellow" is the pure yellow hue that used to be required for all car headlamps in France. (More discussion about Selective Yellow is found below.)

The point that is central to this discussion is that neither standard of "white" includes the blue-tinted light that comes from blue-tinted halogen light bulbs.

I FOUND BLUE-TINTED HALOGEN BULBS AT MY LOCAL RETAIL AUTO PARTS STORE. THE BULBS WERE MARKED AS BEING DOT APPROVED. ARE THESE OK?

No. This marking is fraudulent for two reasons: •There is no such thing as "DOT approved". DOT does not "approve" products as the European regulatory body does.

•Semantic questions aside, the relevant regulations (US Federal Motor Vehicle Safety Standard 108, Canadian Motor Vehicle Safety Standards 108 and 108.1, and ECE Regulations 5, 6, 8, 20 and 37, all call for "white" light, defined as discussed above, so the statement of DOT compliance itself is false.

THEN WHY ARE SO MANY MOTORISTS USING THE ILLEGAL BLUE BULBS IF THEY'RE SO BAD?

There are several reasons I have learned by talking to users of these bulbs: •Because they have been confused by marketing claims for the blue bulbs which falsely and incorrectly equate the blue bulbs' performance with the very expensive arc-discharge headlamps found on top-line luxury cars. They have been led to believe that by replacing their car's proper headlamp bulbs with the blue-coated bulbs, their headlamps' performance will be increased. In fact, quite the opposite is true; their headlamps' performance is decreased by the use of blue bulbs.

The placebo effect is alive and well, however, and the motorist who pays $35 or more for a set of these bulbs often will continue to insist that his headlamps have been made "better". Some motorists have continued to insist that the blue bulbs' performance was better, even when the low emitted-light values are shown to them on a non-chromosensitive light meter in comparison to the readings from proper non-blue bulbs.

2) Because they believe that the blue light makes their car look "cool". This would fall into the same category as the dark plastic headlamp and taillamp covers that are snapped-up by certain drivers for their appearance "enhancement" value, despite the fact that these covers, like the blue bulbs, are illegal and dangerous.

BUT IF THE BLUE BULBS DON'T PERFORM WELL, WHY DO SOME DRIVERS FIND AN IMPROVEMENT?

The effect is almost completely psychological...this is the "I just spent $39.99 (or whatever) for these bulbs; of COURSE they're better!" effect. The reality is that it's best to use bulbs with regular clear-glass envelopes.

SO HOW CAN I TELL A GENUINE ARC-DISCHARGE HEADLAMP FROM A REGULAR HEADLAMP THAT HAS THESE BLUE HALOGEN BULBS?

One clue is the type of car. As mentioned at the top of this message, there are only very few models at this time in North America being offered with the arc-discharge headlamp system, and all of them are top-end/luxury models. If you spot the characteristic purplish-blue light color coming from an everyday-joe type of car, you've found the illegal bulbs.

Another way to tell at a glance is to observe the color of the light. Genuine arc-discharge headlamps run with a very purplish-white character that will remind you exactly of the color of the electronic flash on your camera, while blue-tinted halogen bulbs give headlamps a turquoise/blue/green coloring. Once you have this information in mind, it is easy to spot a user of these bulbs at quite a distance.

HOW DO THESE NON-WHITE-LIGHT BULBS WORK, ANYWAY? AND WHY DO THEY LOOK ALL SHINY IN THE STORE?

These bulbs use a dichroic filter coating. a dichroic coating is defined as one that REFLECTS one color and PASSES its planar opposite.

DICHROISM: the property of some crystals and solutions of absorbing one of two plane-polarized components of transmitted light more strongly than the other.

That means that the glass of a bulb coated with a blue-pass dichroic filter coating will reflect the opposite of blue--that is yellow--thereby "trapping" the yellow light inside the bulb so it can't get out and subtracting yellow from the final output spectrum. The resultant output light will be blue.

Conversely, a bulb coated with a yellow-pass dichroic coating will reflect blue, "trapping" the blue light inside the bulb so it can't get out and subtracting blue from the final output spectrum. The resultant output light will be yellow.

The irridescent appearance of dichroically-coated bulbs when they're turned off is a result of the coating reflecting the ambient light that is reflecting off of it. You see the color opposite to the one that whatever coating you're examining will pass.

ARE BLUE BULBS THE SAME AS THESE "XENON" BULBS I HAVE SEEN FOR DIRECT REPLACEMENT OF REGULAR HALOGEN BULBS?

No. It is confusing, not only because of the explosion in recent years of all sorts of new products in the field of lighting and signalling, but also because some of the terms are being used to describe more than one product. For example, in the field of lighting, a "Xenon lamp" is a gas-discharge (or High Intensity Discharge)-sourced light, just like the light source in your camera's electronic flash. And we're seeing more and more such lights (under a plethora of brand names, which makes things even more confusing) on today's cars. BUT, we also have some companies using "Xenon" in their trade names for halogen bulbs that have a higher percentage of Xenon in their fill gas mix, and that gas is under higher pressure than normal halogen bulbs. And there is nothing preventing somebody from having high-Xenon-percentage halogen bulbs made with a blue-pass (or an anything-pass, for that matter) dichroic filter coating. The resultant mishmash of terms and technologies can be quite difficult to sort out.

The high-Xenon-percentage halogen bulbs are not a gimmick and not a scam, if they're not blue. All of the new bulb designs being produced for new headlamps--such as the new H7 bulb size--use this newly-tweaked, higher-pressure gas mix, and the results have been good, with the H7 achieving a higher luminous flux (amount of available usable light) from a given wattage than was achievable with halogen bulbs that used the old gas mix under the old lower pressure.

So after a few years' experience with H7s, the manufacturers have moved to update the older traditional bulb types with the new gas mix under the new, higher pressure. This increases the luminous flux of the bulb in a ready-made form facter that can be used in existing lamp assemblies.

This appearance of high-pressure/high-Xenon-percentage halogen bulbs is very comparable to the new-for-1979 halogen sealed beam headlamps that were available to replace the old-type non-halogen sealed beam headlamps. The shape, size, fit and electrical requirements didn't change, but the luminous flux did because of the replacement of the old inert gas fill with the new active halogen gas fill. A halogen gas fill allows the filament to run at a higher temperature without failing, increasing the luminous flux of a given-wattage filament. Likewise, the new high-Xenon-percentage/high-pressure gas fill that is beginning to appear in headlamp bulbs in traditional form factors offers a brightness improvement without damaging other performance characteristics of the lamp.