Is it worthwhile at all to swap the fogs for those yellow ones?
10-30-2006, 04:20 AM
#12
AudiWorld Expert
The 4.2 (2000-2004) and the post-facelift other models (2002-2004) have much better fogs...
I noticed that with the independent fog mod. The 2001 2.7T (pre-facelift) had barely any use at all, the domed lens cast a pretty bad beam on the ground. However, the 4.2 (which shares the same fog setup as the face-lifted A6) has a much better beam pattern, and actually casts a good light source to the ground just in front of the car. I use those when my wife and I swap parking places in the driveway at night, and I never have to turn on the headlights.
I have not yet tested these out in actual fog, but I would imagine that they would be sufficient for me to drive (slowly) in dense fog.
I have not yet tested these out in actual fog, but I would imagine that they would be sufficient for me to drive (slowly) in dense fog.
10-30-2006, 05:16 AM
#14
AudiWorld Expert
Hmmm..I am not sure about that for the 4.2...
in my pre-facelift 2001 2.7T (with the domed lense) a new lense was not available seperately, only comes alttached to an entire fog assembly.
Not sure about the face-lift versionof the fog (which we ahve on the 4.2). I'll have to guess no for now, but I'll look into that one later.
Hijack back...you coming up on the 15th???
Not sure about the face-lift versionof the fog (which we ahve on the 4.2). I'll have to guess no for now, but I'll look into that one later.
Hijack back...you coming up on the 15th???
10-30-2006, 05:30 AM
#15
AudiWorld Super User
Join Date: Dec 2003
Posts: 4,246
Likes: 0
Received 0 Likes
on
0 Posts
I think it's possible to order the lens (front half of the lamp) w/o ordering the entire assembly.
I saw it as an option from one of the online sights. Sadly I don't remeber which site it was under. Some one posted it last week.
12-04-2006, 11:42 AM
#19
AudiWorld Super User
Remembered this post...added some info I found....What is Selective-Yellow Light?
What is Selective-Yellow Light?
It's what happens when you subtract blue from the output of a lamp producing white (all colors) light. In 1936, the French for tactical reasons wanted a way to identify the registration nationality of vehicles at night. However, they did not want to reduce roadway safety, and wanted in fact to improve it if possible. So, they figured to remove the blue from the output spectrum of their vehicles' front lamps. White light with the blue component subtracted is known as "selective yellow" light. It is a pure yellow color with little or no orange component--hence the French yellow headlamps. Yellow lamps were subjectively ranked as better in poor weather and lower in glare than white ones, and this matches my own experimental experience with fog lamps that produce yellow light. But is the effect real or just an illusion?
One problem with this conclusion as drawn from the French experience with selective-yellow headlamps in France is that when the question was being considered, the lamps that were being compared with white lamps reduced the absolute intensity of the beam by about 12 percent. This fact may have had a part in reducing the glare. Because the requirement for yellow light no longer exists (though such light is optional in many countries) we probably will never know the vagaries of the answer to this question.
So, what explains the persistent subjective preference amongst experienced poor-weather drivers for yellow fog lamps, despite decades of white fog lamp prevalence? Selective-yellow light can improve a driver's ability to see in fog or rain or snow, but not because it 'penetrates fog better' or 'reflects less off droplets' as is commonly thought. That effect is known as Rayleigh Scattering, and is why the sky appears blue. However, it occurs only when the droplet size is equal or smaller than the wavelength of the light, which is certainly not the case with ordinary fog, rain or snow. Roadway Fog droplets are several orders of magnitude larger than visible light wavelengths--no Rayleigh Scattering.
So, why do yellow fog lamps work better? It's because of the way the human eye interacts with different colors of light. Blue and violet are very difficult for the human optical system to process correctly. They are the shortest visible wavelengths and tend to focus in front of our eyes' retinae, rather than upon it. To demonstrate this to yourself, find a dark blue store front 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.) Blue also is a very difficult color of light to look at if it is at all intense...it stimulates the reaction we call "glare". So, culling the blue out of the spectrum lightens the optical workload and reduces glare. For more in-depth discussion of the matter, see Bullough & Rea's excellent study on the topic.
So,What's the best method of getting selective-yellow light? Until the mid 1990s, headlamps in France were required to produce yellow light. This was accomplished in one of several ways: With a headlamp lens made out of yellow glass, with a yellow glass balloon in front of the bulb either as part of the bulb or as part of the lamp unit, or, more recently, with a yellow-pass dichroic filter coating on a lamp's lens, reflector, condensor or on halogen bulbs themselves.
The blue-appearing lenses in many Asian-made fog lamps ("ion crystal", "gold irridium", and other nonsensical marketing names) are coated with a multilayer dichroic interference coating which passes selective-yellow light "on axis", which means "straight ahead". Unfortunately, these coatings tend to glow blue when viewed off-axis, which has caused problems with people getting pulled over for illegal "blue" lights 'cause the cop sees blue as he drives by and pulls a quick U-turn.
Many lamps involving dichroic filter coatings on the reflector or lens tend to create "blue haze" above the beam cutoff or, in the case of a driving or SAE headlamp beam, scattered throughout the beam. That's because of the irridescence of these coatings, which causes or aggravates secondary-reflection problems where none would exist absent the coating. With the mirrorlike dichroic coating reflecting images of the glowing filament, light gets where it doesn't belong. Cadmium glass was used to make the old French-market Selective Yellow bulbs; now that Cadmium's been more or less banned from auto parts for environmental reasons, the best remaining options are dichroic bulbs or non-dichroic filters applied to one of the optical elements (lens or reflector).
Dichroic selective-yellow bulbs are available in some of the common fog lamp bulb formats (H1, H3, 9006...). That is certainly the easiest method of getting selective yellow light from a lamp with colorless optics. For those willing to go to the time and effort, applying a coating to an optical element is a more permanent, somewhat better option that relieves the vehicle owner of the need to find and get special bulbs. The best coating for field conversion of lamps from white to selective yellow is POR-15's Colorchrome, in the "Morning Sun" color. Results of conversion can be seen here.
Of course, when we talk about light color in an automotive context, we need to address the question of legality. Under US Federal Motor Vehicle Safety Standard number 108 and Canadian Motor Vehicle Standards 108 and 108.1, headlamps as originally installed on motor vehicles (and as installed by anyone other than the vehicle owner) must produce white light.
Let's stop there for a moment. What is "White"? FMVSS108 contains a reference to an SAE standard that defines "white" light in terms of wavelengths. But it's not just one set color. The standard includes a wavelength aggregate RANGE that is considered "white". That's why arc-discharge headlamps, with their decidedly bluish cast, still are considered "white". It's why "blue ion" or "crystal blue" bulbs with blue-pass dichroic filters sold to poseurs who want to try to pretend they have arc-discharge lamps are NOT considered "white". But more relevant to this discussion, the light can tend towards a yellow tint to a certain degree and still qualify as acceptable "white" light. Osram, Narva, Philips and other established European bulbmakers have been offering partial-tint selective-yellow bulbs for some time now.
It's what happens when you subtract blue from the output of a lamp producing white (all colors) light. In 1936, the French for tactical reasons wanted a way to identify the registration nationality of vehicles at night. However, they did not want to reduce roadway safety, and wanted in fact to improve it if possible. So, they figured to remove the blue from the output spectrum of their vehicles' front lamps. White light with the blue component subtracted is known as "selective yellow" light. It is a pure yellow color with little or no orange component--hence the French yellow headlamps. Yellow lamps were subjectively ranked as better in poor weather and lower in glare than white ones, and this matches my own experimental experience with fog lamps that produce yellow light. But is the effect real or just an illusion?
One problem with this conclusion as drawn from the French experience with selective-yellow headlamps in France is that when the question was being considered, the lamps that were being compared with white lamps reduced the absolute intensity of the beam by about 12 percent. This fact may have had a part in reducing the glare. Because the requirement for yellow light no longer exists (though such light is optional in many countries) we probably will never know the vagaries of the answer to this question.
So, what explains the persistent subjective preference amongst experienced poor-weather drivers for yellow fog lamps, despite decades of white fog lamp prevalence? Selective-yellow light can improve a driver's ability to see in fog or rain or snow, but not because it 'penetrates fog better' or 'reflects less off droplets' as is commonly thought. That effect is known as Rayleigh Scattering, and is why the sky appears blue. However, it occurs only when the droplet size is equal or smaller than the wavelength of the light, which is certainly not the case with ordinary fog, rain or snow. Roadway Fog droplets are several orders of magnitude larger than visible light wavelengths--no Rayleigh Scattering.
So, why do yellow fog lamps work better? It's because of the way the human eye interacts with different colors of light. Blue and violet are very difficult for the human optical system to process correctly. They are the shortest visible wavelengths and tend to focus in front of our eyes' retinae, rather than upon it. To demonstrate this to yourself, find a dark blue store front 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.) Blue also is a very difficult color of light to look at if it is at all intense...it stimulates the reaction we call "glare". So, culling the blue out of the spectrum lightens the optical workload and reduces glare. For more in-depth discussion of the matter, see Bullough & Rea's excellent study on the topic.
So,What's the best method of getting selective-yellow light? Until the mid 1990s, headlamps in France were required to produce yellow light. This was accomplished in one of several ways: With a headlamp lens made out of yellow glass, with a yellow glass balloon in front of the bulb either as part of the bulb or as part of the lamp unit, or, more recently, with a yellow-pass dichroic filter coating on a lamp's lens, reflector, condensor or on halogen bulbs themselves.
The blue-appearing lenses in many Asian-made fog lamps ("ion crystal", "gold irridium", and other nonsensical marketing names) are coated with a multilayer dichroic interference coating which passes selective-yellow light "on axis", which means "straight ahead". Unfortunately, these coatings tend to glow blue when viewed off-axis, which has caused problems with people getting pulled over for illegal "blue" lights 'cause the cop sees blue as he drives by and pulls a quick U-turn.
Many lamps involving dichroic filter coatings on the reflector or lens tend to create "blue haze" above the beam cutoff or, in the case of a driving or SAE headlamp beam, scattered throughout the beam. That's because of the irridescence of these coatings, which causes or aggravates secondary-reflection problems where none would exist absent the coating. With the mirrorlike dichroic coating reflecting images of the glowing filament, light gets where it doesn't belong. Cadmium glass was used to make the old French-market Selective Yellow bulbs; now that Cadmium's been more or less banned from auto parts for environmental reasons, the best remaining options are dichroic bulbs or non-dichroic filters applied to one of the optical elements (lens or reflector).
Dichroic selective-yellow bulbs are available in some of the common fog lamp bulb formats (H1, H3, 9006...). That is certainly the easiest method of getting selective yellow light from a lamp with colorless optics. For those willing to go to the time and effort, applying a coating to an optical element is a more permanent, somewhat better option that relieves the vehicle owner of the need to find and get special bulbs. The best coating for field conversion of lamps from white to selective yellow is POR-15's Colorchrome, in the "Morning Sun" color. Results of conversion can be seen here.
Of course, when we talk about light color in an automotive context, we need to address the question of legality. Under US Federal Motor Vehicle Safety Standard number 108 and Canadian Motor Vehicle Standards 108 and 108.1, headlamps as originally installed on motor vehicles (and as installed by anyone other than the vehicle owner) must produce white light.
Let's stop there for a moment. What is "White"? FMVSS108 contains a reference to an SAE standard that defines "white" light in terms of wavelengths. But it's not just one set color. The standard includes a wavelength aggregate RANGE that is considered "white". That's why arc-discharge headlamps, with their decidedly bluish cast, still are considered "white". It's why "blue ion" or "crystal blue" bulbs with blue-pass dichroic filters sold to poseurs who want to try to pretend they have arc-discharge lamps are NOT considered "white". But more relevant to this discussion, the light can tend towards a yellow tint to a certain degree and still qualify as acceptable "white" light. Osram, Narva, Philips and other established European bulbmakers have been offering partial-tint selective-yellow bulbs for some time now.
12-04-2006, 11:43 AM
#20
AudiWorld Super User
What are Fog Lamps Really For?
What are Fog Lamps Really For?
A good fog lamp produces a wide, bar-shaped beam of light with a sharp horizontal cutoff (dark above, bright below) at the top of the beam, and minimal upward light above the cutoff. Almost all factory-installed or dealer-optional fog lamps, and a great many aftermarket units, are essentially useless for any purpose, especially for extremely demanding poor-weather driving. Many of them are too small to produce enough light to make a difference, produce beam patterns too narrow to help, lack a sufficiently-sharp cutoff, and throw too much glare light into the eyes of other drivers, no matter how they're aimed.
Good (and legal) fog lamps produce white or Selective Yellow light, and use tungsten-halogen bulbs. Xenon or HID bulbs are inherently unsuitable for use in fog lamps, and blue or other-colored lights are also the wrong choice.
The fog lamps' job is to show you the edges of the road, the lane markings, and the immediate foreground. When used in combination with the headlamps, good fog lamps weight the overall beam pattern towards the foreground so that even though there may be a relatively high level of upward stray light from the headlamps causing glareback from the fog or falling rain or snow, there will be more foreground light than usual without a corresponding increase in upward stray light, giving back some of the vision you lose to precipitation.
When used without headlamps in conditions of extremely poor visibility due to snow, fog or heavy rain, good fog lamps light the foreground and the road edges only, so you can see your way safely at reduced speeds.
In clear conditions, more foreground light is not a good thing, it's a bad thing. Some foreground light is necessary so you can use your peripheral vision to see where you are relative to the road edges, the lane markings and that pothole 10 feet in front of your left wheels. But foreground light is far less safety-critical than light cast well down the road into the distance, because at any significant speed (much above 30 mph), what's in the foreground is too close for you to do much about. If you increase the foreground light, your pupils react to the bright, wide pool of light by constricting, which in turn substantially reduces your distance vision--especially since there's no increase in down-the-road distance light to go along with the increased foreground light. It's insidious, because high levels of foreground light give the illusion, the subjective impression, of comfort and security and "good lighting".
US-DOT headlamps have historically tended to provide relatively low, arguably inadequate levels of light in the foreground and to the sides. Many US DOT headlamps have what seems to be a "black hole" in front of the car, with essentially the entire beam concentrated in a narrow band or ball of light thrown into the distance. With headlamps like these, a decent argument can be made for the use of fog lamps to fill the "black hole", that is, to add-back the missing foreground and lateral-spread light when driving at moderate speeds on dark and/or twisty roads. Of course, lamps to rectify inadequate foreground light must be thoughtfully and carefully selected, correctly aimed and properly used. Otherwise, they're useless at best and dangerous at worst.
In some places, the law prohibits the use of fog lamps without the low beam headlamps also being on. Whether or not this is the case where you drive, it's vital to realize that fog lamp beams, by definition, have a much shorter reach than headlamp beams. If you drive in conditions foul enough to call for the use of fog lamps without headlamps, it's essential to have good fog lamps that are up to the task and are properly aimed, and it's imperative that you slow down because even with high-performance fog lamps, you can't see as far with fog lamps and in poor weather as you can with headlamps and in clear weather.
If the road is wet or slick with ice, but there's no falling precipitation, fog lamps should be used with discretion. Their extra downward light can help compensate for the tendency of water to "soak up" the light on the road from your headlamps. But, this extra downward light hitting a road surface shiny with water or ice will also create high levels of reflected glare for other drivers. Since we're all "other drivers" to everybody else on the road, it's well to think of roadway safety as a cooperative effort. In most driving situations, fog lamps are neither useful nor necessary, but more people use their fog lamps when the prevailing conditions don't call for their use, than use them when the conditions do call for their use. Nobody thinks your car is cool because it has fog lamps, and glare is dangerous, so do yourself and everyone a favor: choose them carefully, aim them properly, use them thoughtfully, and leave them off except when they're genuinely necessary.
A good fog lamp produces a wide, bar-shaped beam of light with a sharp horizontal cutoff (dark above, bright below) at the top of the beam, and minimal upward light above the cutoff. Almost all factory-installed or dealer-optional fog lamps, and a great many aftermarket units, are essentially useless for any purpose, especially for extremely demanding poor-weather driving. Many of them are too small to produce enough light to make a difference, produce beam patterns too narrow to help, lack a sufficiently-sharp cutoff, and throw too much glare light into the eyes of other drivers, no matter how they're aimed.
Good (and legal) fog lamps produce white or Selective Yellow light, and use tungsten-halogen bulbs. Xenon or HID bulbs are inherently unsuitable for use in fog lamps, and blue or other-colored lights are also the wrong choice.
The fog lamps' job is to show you the edges of the road, the lane markings, and the immediate foreground. When used in combination with the headlamps, good fog lamps weight the overall beam pattern towards the foreground so that even though there may be a relatively high level of upward stray light from the headlamps causing glareback from the fog or falling rain or snow, there will be more foreground light than usual without a corresponding increase in upward stray light, giving back some of the vision you lose to precipitation.
When used without headlamps in conditions of extremely poor visibility due to snow, fog or heavy rain, good fog lamps light the foreground and the road edges only, so you can see your way safely at reduced speeds.
In clear conditions, more foreground light is not a good thing, it's a bad thing. Some foreground light is necessary so you can use your peripheral vision to see where you are relative to the road edges, the lane markings and that pothole 10 feet in front of your left wheels. But foreground light is far less safety-critical than light cast well down the road into the distance, because at any significant speed (much above 30 mph), what's in the foreground is too close for you to do much about. If you increase the foreground light, your pupils react to the bright, wide pool of light by constricting, which in turn substantially reduces your distance vision--especially since there's no increase in down-the-road distance light to go along with the increased foreground light. It's insidious, because high levels of foreground light give the illusion, the subjective impression, of comfort and security and "good lighting".
US-DOT headlamps have historically tended to provide relatively low, arguably inadequate levels of light in the foreground and to the sides. Many US DOT headlamps have what seems to be a "black hole" in front of the car, with essentially the entire beam concentrated in a narrow band or ball of light thrown into the distance. With headlamps like these, a decent argument can be made for the use of fog lamps to fill the "black hole", that is, to add-back the missing foreground and lateral-spread light when driving at moderate speeds on dark and/or twisty roads. Of course, lamps to rectify inadequate foreground light must be thoughtfully and carefully selected, correctly aimed and properly used. Otherwise, they're useless at best and dangerous at worst.
In some places, the law prohibits the use of fog lamps without the low beam headlamps also being on. Whether or not this is the case where you drive, it's vital to realize that fog lamp beams, by definition, have a much shorter reach than headlamp beams. If you drive in conditions foul enough to call for the use of fog lamps without headlamps, it's essential to have good fog lamps that are up to the task and are properly aimed, and it's imperative that you slow down because even with high-performance fog lamps, you can't see as far with fog lamps and in poor weather as you can with headlamps and in clear weather.
If the road is wet or slick with ice, but there's no falling precipitation, fog lamps should be used with discretion. Their extra downward light can help compensate for the tendency of water to "soak up" the light on the road from your headlamps. But, this extra downward light hitting a road surface shiny with water or ice will also create high levels of reflected glare for other drivers. Since we're all "other drivers" to everybody else on the road, it's well to think of roadway safety as a cooperative effort. In most driving situations, fog lamps are neither useful nor necessary, but more people use their fog lamps when the prevailing conditions don't call for their use, than use them when the conditions do call for their use. Nobody thinks your car is cool because it has fog lamps, and glare is dangerous, so do yourself and everyone a favor: choose them carefully, aim them properly, use them thoughtfully, and leave them off except when they're genuinely necessary.