HIDs and the "blue" rage.

[G-MAN]

I think everyone can pretty much agree that the "blue headlight" craze and the ensuing flooding of the market with halogen bulbs with blue glass started back when the first HID headlights started showing up on premium Euro cars here in the US. The thing I always wondered about was why people always called these lights blue. The temp of OEM HIDs is and has always been 4300K, which is almost pure white. But even I would notice that under some conditions the some OEM HIDs would appear blue at times, and others wouldn't. Then I noticed it was only HIDs in projector lights that had this peculiar blue look at times. Reflector HIDs didn't.

The answer, it seems, comes from the metal shutter that is fixed inside the projector unit to create the sharp horizontal cutoff, which when the lights are aimed properly keeps the intense light below the eye level of oncoming cars. Right at the edge of the cuttoff is a line if blue light. See the photo below. This is caused by refraction of light around the shutter. If the lights are aimed too high so that this cutoff is right in your line of vision, or if due to road conditions this cutoff passes through you line of vision, you are going to see nothing but blue light coming from the headlight.

Since reflector headlights don't use a metal shutter to block light directly at the bulb, you don't get this phenomenon with reflector HIDs.

 

[G-MAN]

Here's another shot that really shows the blue line at the cuttoff:

 

[doctorr]

The observation is correct - for OEMs, but the aftermarket sells 6000k, 8000k, even 12 and 14 "kk" capsules.

They are blue, blue-er, purplish, darn purple and "almost black-light"!!

Hopefully the higher ones are only used for auto shows.
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doc

 

[G-MAN]

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The observation is correct - for OEMs, but the aftermarket sells 6000k, 8000k, even 12 and 14 "kk" capsules.

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I know. I was talking about when HIDs first started coming in Euro cars. It was the little bit of blue at the cutoff that ignited the "blue headlight" craze, which has now infected aftermarket HID bulbs with temps over 6000K to get a totally blue light.

 

[televascular]

Don't forget, the higher Kelvin rating you go, the less light output you get. There are tons of clueless people who get the bluest, highest Kelvin bulbs they can find because it looks "bling", but their headlights illuminate half the distance that OEM bulbs do.

Be smart. Don't get 6800K bulbs. HIDs were "bling" several years ago when very few luxury cars had them, but not anymore.

 

[moribundman]

Don't confuse the blue color fringing with the high color temperature of HID lights. The blue outline at the edge of the projected beam is due to the lack of correction for chromatic aberration in projector lights' optical system. Unlike with photographic quality lenses, color fringing is deemed acceptable in headlights. Reflector lights do not cause color fringing and that's why you see fringing only with projector lights, regardless of whether they are HID or halogen lights.

Halogen lights have a color temperature of about 2,800 to 3,400 degree Kelvin -- which is warm ("yellow") light.

Average daylight has a colder ("white") color temperature at about 5,000 Kelvin.

HID lights may produce warm (4,200 K), but still colder light than halogen, or they can produce light that is colder (6,000 or even 8,000 K, which appears blue) than average daylight.

Keep in mind that the human brain color balances off-white light. We do not see halogen lighting as "yellow" unless we have another light source for comparison.

 

[G-MAN]

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Don't confuse the blue color fringing with the high color temperature of HID lights. The blue outline at the edge of the projected beam is due to the lack of correction for chromatic aberration in projector lights' optical system. Unlike with photographic quality lenses, color fringing is deemed acceptable in headlights.

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Judging by the talk on the two main HID forums, it's not just acceptable, it's highly desirable. It's fascinating reading about the various ways some of these guys modify the cutoff shield inside the projector in order to maximize the color band at the cutoff line.

 

[moribundman]

They want to project a rainbow?

http://en.wikipedia.org/wiki/Chromatic_aberration

 

[G-MAN]

The blue band at the cutoff isn't really caused by the lens, it comes from light bending around the metal shield used behind the lens to form the cutoff. The intenisty of the blue can be increased by bending the shield closer to the HID light source. This also sharpens the cutoff.

 

[moribundman]

By "lens" I mean the complete "optical system" with all its elements and not just the actual frontal glass element.

Chromatic aberration, in this case lateral chromatic aberration, is caused within the optical system, of which the shield is a part, just like a diaphragm in a photographic lens is part of the optical system. Color fringing occurs because the optical system's focal lenght is slightly different for red and blue light. The position of the diaphram (in a photographic lens) or the cutoff shield (in a projector light) determines the amount of chromatic focus error (if you want to call it that). By moving the shield longitudinally, the amount of focus error is adjustable within a limited range, because moving the shield affects the focal lenght. Lateral chromatic aberration can be hidden by unfocusing slightly.

 

[BMWR1150GS]

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The blue band at the cutoff isn't really caused by the lens, it comes from light bending around the metal shield used behind the lens to form the cutoff. The intenisty of the blue can be increased by bending the shield closer to the HID light source. This also sharpens the cutoff.

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G-Man is correct. The blue fringe is due to the light spilling over the mask inside the projector assembly. As the light spills over the mask, it is bent (similar to a prism) resulting in the blue fringe you see. Manufacturers attempt to eliminate some of the blue component in the HID by formulating the metallic salts inserted into the arc chamber during manufacture. Iodide and other halide salts are used as well as mercury. Xenon and other gasses are used to help the lamp strike. Shortly after strike, the salts are excited into a high-energy state whereby they give off light, similar to the fluorescent you have in your garage.

When you turn on the switch, HIDs require ~20,000 volts to successfully strike and begin providing light. As pressure builds inside the arc chamber, the arc changes color somewhat. Once fully excited the gas mix provides full light output with about 80-90 volts. The ballast provides the regulation. Don’t handle the wiring

at strike or you may be reminded of the first time you grabbed a spark plug wire………Ouch.

 

[BMWR1150GS]

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Halogen lights have a color temperature of about 2,800 to 3,400 degree Kelvin -- which is warm ("yellow") light.

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Not for long. Incandescents in this range usually darken and are reduced to < 1800 K rather quickly (10s of hours use) due to tungsten boiling off the filament and depositing on the inside of the lamp envelope. Also, high K. incandescents give off excess heat with the potenetial to damage, melt or start a fire in plastic headlamp assemblies. HIDs run relatively cool since there are no electrons boiling off a hot fliament.