After getting numerous threads locked and kicked out of numerous Facebook groups for preaching the gospel, I remain undaunted and am here to compile a comprehensive repository of ALL information regarding headlights, bulbs, and the associated technology. It is my hope that this will ultimately serve as the end-all, be-all of lighting threads for, specifically, the Fiesta ST, but the principles and knowledge contained herein are applicable to all vehicles. I am neither a physicist nor a professional, but I'm a decent learner and the information below is compiled from years of research on my end. Much of this is copied, pasted, borrowed, and plagiarized from sources listed below. I take no credit for anything other than compiling it into this format. I am not a lighting engineer, I am just a pedant.
Headlight beams are not simple spotlights or floodlights. Even the simple-looking ones are INCREDIBLY complex, made by people much smarter than I am, with intricate requirements for a variety of minimum and maximum intensities at a long list of angular points within the beam, as well as above, below, and around it. The whole optical system (light source, reflector/s, lens/es, bulb shield) works as a system to produce beams that meet rigorous engineering and legal requirements. Each headlamp is designed, engineered, tested, and certified/approved to give acceptable safety performance with one specific kind of light source -- a halogen bulb, an HID bulb, or an array of LEDs. Putting a different kind of light source into the headlamp, other than the kind it was designed for, ruins the headlamp's safety performance. The wrong amounts of light, in the wrong colors, of the wrong types, are sent in the wrong directions. Period. Some areas have not enough light, other areas have too much light, and the result is very unsafe, each and every time. And this is true even if you think you can see OK with your HID- or LED-modded headlamps. Even if you manage to luck out and not get ticketed, and even if you are OK with risking your own injury, death, and property damage, your modified headlamps create an elevated risk of injury, death, and property damage for everyone you share the roads with, and that's not acceptable. This is not an "it's my car, so who cares," thing, this is something that puts other motorists at risk.
In the following wall of text, I will break down the TYPES of automotive lighting and WHY these matter to you as a consumer and a driver.
Lighting Type A: Halogens
Halogen bulbs are by far the most common type of headlight in the world and have been for pretty much forever. A Halogen bulb is cheap and effective. A halogen bulb has a cylinder-shaped filament. The space immediately surrounding the cylinder of light is completely dark, and so the sharpest contrast between bright and dark is along the edges of the cylinder of light. The ends of the filament cylinder fade in a diffuse pattern. They can be mounted into a reflector housing or a projector housing. Some bulbs work better in some kinds, but in terms of the way the light output works, a halogen is a halogen.
Recently there have been a lot of "developments" in halogen technology, some good, some bad.
Manufacturers have in many cases figured out ways to cram more filaments or higher-wattage-equivalent filaments or xenon gas into conventional halogen bulbs. In some cases, there is an infrared relay used to diffuse light. This latter thing is essentially voodoo to me and since HIR bulbs aren't useful in the Fiesta ST we will disregard it. But there ARE good, white, bright halogen bulbs out there.
There are also bad ones: In order to combat the proliferation of shitty, dim, yellow halogen bulbs (such as those that come stock on the USDM Fiesta ST) a number of companies have come out with "cool blue" halogens. They're sometimes falsely advertised as "Xenon bulbs" or "HID bulbs" or "HID Look!!!" - the blue glass is claimed to somehow coax the bulb to perform better. It's crap. In fact, these bulbs reduce headlamp performance while increasing dangerous glare.
"What did you say? More glare from less light?"
That's right. The output spectrum of halogen bulbs includes a great deal of red, orange, yellow and green light - which is why they look kinda yellow - but very little blue or violet light. Blue bulbs have colored glass that allows only the blue light through the filter. Because very little blue light is produced by a halogen bulb 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, which isn't much anyway — that ever reaches the road.
Blue and violet are the shortest wavelength/highest frequency colors of visible light, and, as such, they scatter the most readily. They glare more because they scatter more and because short-wavelength light is difficult for our eyes to focus on. So, not only is there LESS light, but the light you are producing is not at the visible end of the spectrum. This is also important, and brings us to our next point:
Quick Detour: Signal Image
"Signal Images" is what an observer sees when looking at a headlight, in contrast to the "beam pattern," which is the light viewed from behind the headlamp facing forward, as by the driver of a vehicle. In order for headlamp light to be used by the driver, the light must travel forward from the headlamp to an object, bounce off the object and return to the driver's eyes. As light travels through the atmosphere, it spreads and diffuses according to the Inverse Square Law: The intensity drops as 1⁄(distance)2. Consider a reference point. We'll use a round number and say it's ten feet away from your eyes. An object at this distance will be lit to a certain level (let's call it the reference level) by your car's headlamps. An object at twice this distance (20 feet) will be lit not to 1⁄2 the reference level, but to 1⁄4 the level—that is, (1⁄2)2. An object located 3 times the reference distance away (30 feet) will be lit to (1⁄3)2 or 1⁄9 the reference level. An object located 10 times as far away (100 feet) will be lit to (1⁄10)2 or 1⁄100 the reference level. You get the idea. And then, you have to double that, because it has to travel BACK to your eyes for you to see it. However, if you're standing outside of your car looking at the light, or driving towards someone in traffic, it will be TWICE as bright to them.
This is one reason why your HID/LED kit or blue glass bulb appears brighter to you. It is putting more short-wavelength light into your eyes when you look at the lights. It is not, in actuality, any brighter - it is producing less light at the visible end of the spectrum and putting it in more directions, most of them close to the vehicle, which is why some HID and LED "kits" look acceptable when close to a flat wall.
The second reason - and a much simpler reason - that your HID/LED kit or blue glass bulb appears brighter is that, in short, our eyes are very sensitive to blue light, which does not cause your pupil to constrict in the same fashion as yellow light. As such, your eyes perceive that there is more light coming from a cooler blue light than a more yellow one, even if the light output is higher from the "warmer" and more yellow light, even when the lumen output of the blue bulb is lower.
Lighting Type B: HIDs
High-intensity discharge lamps (HID) produce light with an electric arc rather than a glowing filament. This is, rather than a straight filament, a crescent-shaped light source. It's crescent-shaped because as it passes through the space between the two electrodes, its heat causes it to try to rise. The space immediately surrounding the crescent of light glows in layers - The closer to the crescent of light, the brighter the glow. The ends of the arc crescent are the brightest points, and immediately beyond these points is completely dark, so the sharpest contrast between bright and dark is at the ends of the crescent of light. The high intensity of the arc comes from metallic salts that are vaporized within the arc chamber. This produces a LOT of light - and HEAT! Because of the increased amounts of light available from HID burners relative to halogen bulbs, HID headlamps producing a given beam pattern can be made smaller than halogen headlamps producing a comparable beam pattern - higher light output.
That said, in contrast to halogens, available in a wide variety of sizes, HIDs are made in two primary sizes and two primary configurations - D1 and D2. D1S and D2S are designed for use in HID projectors, D1R and D2R are designed for use in HID reflectors (which are rarely used anymore). These are the ONLY formats in which an HID is manufactured by a reputable supplier. All aftermarket "HID Kits" are made in third-world countries using either an HID-like design or are re-based D1 or D2 ballasts. D1 and D2 HIDs will not physically fit in a halogen housing. To make matters worse, the HID generates light lengthwise, front to back. However, most halogen bulbs generates light lengthwise, side to side, so even if you could shoehorn one of these into a halogen housing, the light would scatter in the wrong directions.
Lighting Type C: LEDs
An LED uses a semiconductor that lights up when an electrical current is run through it. The LED consists of a chip of semiconducting material doped with impurities to create a circuit ("p or n junction"). As in other diodes, current flows easily from the p-side, or anode, to the n-side, or cathode, but not in the reverse direction. Charge-carriers—electrons and holes—flow into the junction from electrodes with different voltages. When an electron meets a hole, it falls into a lower energy level and releases energy in the form of a photon.
LEDs are entirely directional. In general, a flat-surface uncoated LED semiconductor chip emits light only perpendicular to the semiconductor's surface, and a few degrees to the side, in a cone shape cleverly referred to as the "light cone." To have a wide angle of light, you need a bunch of LEDs arranged in a wide angle.
You might be noticing that each type of light source produces a different type of light - each type uses different materials to put lights in different directions (or all directions). So how do you turn this into a headlight?
Optics!
The Important Stuff: Optics
The Optics Make the Lamp. I cannot possibly stress this enough - the optics make the lamp.
Putting a bulb, ballast, or LED array in something with optics it's not designed for is like wearing someone else's glasses - they will physically fit on your face but you won't be able to see as well.
Here's Why:
There are two main types of automotive optic: a reflector system and a projector system. Older cars had parabolic reflectors and fluted glass lenses. Glass lenses direct light very well but it's very hard to change the direction of the light a great deal with fluting.
Reflectors, or more specifically multi-reflectors- are parabolic reflectors with many different surfaces that are specifically shaped to put light from a particular source into a particular area, creating a specific beam.
We call this output the beam pattern, it's what you see sitting in your car behind your headlight (rather than staring into it directly).
Multi-reflectors can have relatively sharp cutoff and defined beam, but they are actually less efficient at projecting light across distance than a projector system. This is because the small edges between areas of reflectors scatter light in unusable directions, and more energy is wasted. This lost light decreases the efficiency of the lamp. Some of the light energy is absorbed by the reflector as heat, and some is scattered. because of this, the lamp is usually larger to make up for the inefficiency.
The other type is a projector, which uses a parabolic reflector to focus light into a glass lens. The lens is usually flat on one side and convex on that other. This type of lens takes a beam of light and spreads it over a wider area as the distance increases. This requires the lamp to have a focal distance from light source to lens that depends solely on the lens shape and magnification index. a very, very small change in the shape of the lens will cause a different focal length. Similarly, a minute change in lens positioning will throw the focus of that focal length off.
Because optical glass and lens forming is expensive, many optic companies will make the same lens for halogen and HID based projector systems. They then change the cutoff shield, focal length, and reflector bowl to shape the light coming into the lens differently. The desired output is a function of the vectors of light coming into the lens, which is to say that the light is designed around the source of the light - halogen, HID, or LED.
An example of this can be found here in the parts diagrams for the 2008 Subaru Impreza. The Impreza and Outback trims had halogen headlights, the WRX and STI trim had HID headlights. Externally they looked the same, but you can see differences in the bulb, ballast, leveling motors, reflectors, and the shape of the reflector bulb, even though the lenses themselves are the same:
Halogen:
HID:
To my knowledge there are no cars that have options for Halogen, HID, and LED lights, but if someone tracks such a thing down I will be happy to research further.
LED optics are another ballpark altogether due to the way that LEDs emit light. LED headlamps, from the factory, are matrixed into smaller projector units that can be individually moved and controlled, as shown below:
The system dims light that would shine directly onto oncoming and preceding vehicles, but continues to cast its full light on the zones between and beside them. This works because the LED high beams are split into numerous individual light-emitting diodes with individual controls and sensors. LEDs in headlights are arranged in a matrix and adapt fully electronically to the surroundings in milliseconds. They are activated and deactivated or dimmed individually by a control unit. One notable exception to this is the current-gen Toyota Corolla, which uses a single LED matrix in a single projector designed for that matrix, rather than a series of them as found on Audi, Acura, etc. The light is a sealed, self-contained unit with its own custom circuit panel, sensors, ballasts, etc. It is not retrofittable without significant modification to the lens, housing, etc - which is to say, significantly more work than is involved in an HID projector retrofit. There are other single-LED units on the market but that's the only one I can think of off the top of my head.
The functional importance of lighting optics - whichever their design - is paramount. It puts the light where it needs to be, and keeps it from going where it shouldn't. It does this with great specificity and intense amounts of design and engineering, and must be done with a deep understanding of physical science, optics, and, perhaps most importantly, human vision. When you change anything in the lighting system, the optics determine the change in output that systematic change will affect. Sharper, brighter, or wider are NOT universally better characteristics of a lighting system. A problem in the optics directly affects the output of the lamp system and these effects are as wide ranging as there are possibilities to change the system. With optics, there's hardly any change that can make things better unless the optics are upgraded to focus on the change, speaking literally and figuratively.
Real-world Tests: What Does this Mean for Me?
At this point the dangers and problems of HID kits in halogen housings are well-documented. I have linked a number of the HID tests below, but less has been done with LED "kits." Sac02 on the Candlepower forums bothered to run a few tests. He purchased H11 bulbs, H9 bulbs, high-output H11 bulbs, and an LED kit with an H11 base. The H11 LED kit is identical to the ones sold under different names such as XenonDepot and Auxbeam. He put them in the halogen projector housings on his Mazda, leveled them about ten feet away from a flat white sheet, and recorded his results with a lumen meter as well as with a good-quality camera. He then ran similar tests while driving.
The threads are here:
http://www.candlepowerforums.com/vb...wall-amp-lux-comparison-LED-H11-LL-H11-100-H9
http://www.candlepowerforums.com/vb...amic-driving-evaluation-LED-H11-LL-H11-100-H9
Here's the summary:
They're worth a read but the summary is that, in a properly designed halogen projector housing, at distances needed to drive safely, the LED kit produces 550 lumens, as compared to 5350 lumens from a high output H11. That's a MASSIVE difference, over 80% less light. The LEDs clearly show unacceptable light output at close range against a flat surface, but while driving, the vast majority of the light that they did put out was concentrated very close to the car. You don't want light close to the car, you want it far away so that you can see where you're going. These problems are due to the way LEDs emit light - a unidirectional diode vs an omnidirectional filament in a halogen bulb. There is absolutely NO technological "breakthrough" that will change this unless someone figures out a way to get LEDs to emit light in the EXACT same way that a halogen filament does, which is impossible. The problem is at the level of physics. Do not be duped by marketing.
As pointed out later in this thread, there is no lumen rating in real-world applications and this is not the best test that could be done. I would be happy to do my own if someone wanted to supply the bulbs, but I will not be giving these companies any of my own money.
Addendum October 2018
It has come to my attention that Philips, generally a reputable company, has been marketing and selling a set of LED "retrofits" for halogen fog lights. My understanding is that Philips has run a number of tests of these and that in some fog lights, they are quite good - this, again, comes down to optics. Philips is careful to specify that they are for fog lights only. My thinking here is that the bar is significantly lower for fog lights, as most fog lights on modern cars are basically cosmetic anyway. I also note from their design that most of the LED "kits" on the market are based on Philips' design, which Philips has expressly stated is not suitable for halogen headlight housings as the light output simply isn't acceptable, both in beam pattern and lumen output. You should not put these in your headlights.
Cliffs Notes
Your car comes from the factory with either halogen reflectors or halogen projector headlamps. HIDs and LEDs cannot be safely or legally used in these headlights. Your only safe, legal, option is high-efficiency, high-output halogen bulbs. The GE Nighthawk, Philips Xtreme Vision, and Osram Nightbreaker are all good options. They look nice and white and you will be able to see better. If you have the Euro projectors, the H9 bulbs can be a good option as they have similar bases and identical wiring to the stock H11s, but are designed for halogen projectors. They are slightly higher wattage and technically not street legal, but neither are, say, catless downpipes. There's a debate on this and I don't consider myself knowledgeable enough about the specific optics of the Euro projectors to make a definitive call. I'll be running standard bulbs.
Works Cited and Further Reading
About 80% of the credit for this belongs to Daniel Stern and Handsdown on the NASIOC forums, both of whom I cribbed heavily from and in a few places outright borrowed from. Should either of them stumble upon this and wish for me to take it down, please let me know.
http://www.danielsternlighting.com/tech/bulbs/Hid/conversions/conversions.html
http://www.danielsternlighting.com/tech/bulbs/blue/bad/bad.html
https://forums.nasioc.com/forums/showthread.php?t=1670587
FAQs and Frequent Responses
1. "Well, it looks good to me, and I think my kit is brighter and better."
Consult the passage about signal image.
2. "What about [x kit] made by [y company], they say that theirs is good"
This is marketing. Period. If none of the world's foremost automotive lighting companies, with huge teams of highly qualified engineers and massive budgets, have figured this out yet, and their closest effort is for fog lights only, it is highly unlikely that some no-name Chinese company selling things on Amazon has cracked the code. It's great that they say that their product is the good one, but without any sort of technical sheets or testing documents, you're just regurgitating marketing and sales pitches.
3. "It's my car, I can do what I want."
I generally agree, but as lighting is safety equipment, these choices put you - and, perhaps more importantly, other motorists - at risk in a way that, say, underglow neon and a big wing doesn't. Take care of yourself and be respectful of other drivers.
4. "My buddy said it looks fine."
Is your friend a physicist? A lighting engineer? What's that? He's not? Well then.
Here's what a lot of this comes down to. A lot of people want to change my mind and argue with the science on this. Show me a tech sheet. Show me an independent third-party laboratory demonstrating that your kit has acceptable light output, cutoff, and light color. Until then, as I said above, you're just taking the marketing claims of some no-name fly-by-night company at face value, claims that are only made so you'll buy their products. It's not research, you're just copying and pasting a sales pitch. Don't do that.
Headlight beams are not simple spotlights or floodlights. Even the simple-looking ones are INCREDIBLY complex, made by people much smarter than I am, with intricate requirements for a variety of minimum and maximum intensities at a long list of angular points within the beam, as well as above, below, and around it. The whole optical system (light source, reflector/s, lens/es, bulb shield) works as a system to produce beams that meet rigorous engineering and legal requirements. Each headlamp is designed, engineered, tested, and certified/approved to give acceptable safety performance with one specific kind of light source -- a halogen bulb, an HID bulb, or an array of LEDs. Putting a different kind of light source into the headlamp, other than the kind it was designed for, ruins the headlamp's safety performance. The wrong amounts of light, in the wrong colors, of the wrong types, are sent in the wrong directions. Period. Some areas have not enough light, other areas have too much light, and the result is very unsafe, each and every time. And this is true even if you think you can see OK with your HID- or LED-modded headlamps. Even if you manage to luck out and not get ticketed, and even if you are OK with risking your own injury, death, and property damage, your modified headlamps create an elevated risk of injury, death, and property damage for everyone you share the roads with, and that's not acceptable. This is not an "it's my car, so who cares," thing, this is something that puts other motorists at risk.
In the following wall of text, I will break down the TYPES of automotive lighting and WHY these matter to you as a consumer and a driver.
Lighting Type A: Halogens
Halogen bulbs are by far the most common type of headlight in the world and have been for pretty much forever. A Halogen bulb is cheap and effective. A halogen bulb has a cylinder-shaped filament. The space immediately surrounding the cylinder of light is completely dark, and so the sharpest contrast between bright and dark is along the edges of the cylinder of light. The ends of the filament cylinder fade in a diffuse pattern. They can be mounted into a reflector housing or a projector housing. Some bulbs work better in some kinds, but in terms of the way the light output works, a halogen is a halogen.
Recently there have been a lot of "developments" in halogen technology, some good, some bad.
Manufacturers have in many cases figured out ways to cram more filaments or higher-wattage-equivalent filaments or xenon gas into conventional halogen bulbs. In some cases, there is an infrared relay used to diffuse light. This latter thing is essentially voodoo to me and since HIR bulbs aren't useful in the Fiesta ST we will disregard it. But there ARE good, white, bright halogen bulbs out there.
There are also bad ones: In order to combat the proliferation of shitty, dim, yellow halogen bulbs (such as those that come stock on the USDM Fiesta ST) a number of companies have come out with "cool blue" halogens. They're sometimes falsely advertised as "Xenon bulbs" or "HID bulbs" or "HID Look!!!" - the blue glass is claimed to somehow coax the bulb to perform better. It's crap. In fact, these bulbs reduce headlamp performance while increasing dangerous glare.
"What did you say? More glare from less light?"
That's right. The output spectrum of halogen bulbs includes a great deal of red, orange, yellow and green light - which is why they look kinda yellow - but very little blue or violet light. Blue bulbs have colored glass that allows only the blue light through the filter. Because very little blue light is produced by a halogen bulb 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, which isn't much anyway — that ever reaches the road.
Blue and violet are the shortest wavelength/highest frequency colors of visible light, and, as such, they scatter the most readily. They glare more because they scatter more and because short-wavelength light is difficult for our eyes to focus on. So, not only is there LESS light, but the light you are producing is not at the visible end of the spectrum. This is also important, and brings us to our next point:
Quick Detour: Signal Image
"Signal Images" is what an observer sees when looking at a headlight, in contrast to the "beam pattern," which is the light viewed from behind the headlamp facing forward, as by the driver of a vehicle. In order for headlamp light to be used by the driver, the light must travel forward from the headlamp to an object, bounce off the object and return to the driver's eyes. As light travels through the atmosphere, it spreads and diffuses according to the Inverse Square Law: The intensity drops as 1⁄(distance)2. Consider a reference point. We'll use a round number and say it's ten feet away from your eyes. An object at this distance will be lit to a certain level (let's call it the reference level) by your car's headlamps. An object at twice this distance (20 feet) will be lit not to 1⁄2 the reference level, but to 1⁄4 the level—that is, (1⁄2)2. An object located 3 times the reference distance away (30 feet) will be lit to (1⁄3)2 or 1⁄9 the reference level. An object located 10 times as far away (100 feet) will be lit to (1⁄10)2 or 1⁄100 the reference level. You get the idea. And then, you have to double that, because it has to travel BACK to your eyes for you to see it. However, if you're standing outside of your car looking at the light, or driving towards someone in traffic, it will be TWICE as bright to them.
This is one reason why your HID/LED kit or blue glass bulb appears brighter to you. It is putting more short-wavelength light into your eyes when you look at the lights. It is not, in actuality, any brighter - it is producing less light at the visible end of the spectrum and putting it in more directions, most of them close to the vehicle, which is why some HID and LED "kits" look acceptable when close to a flat wall.
The second reason - and a much simpler reason - that your HID/LED kit or blue glass bulb appears brighter is that, in short, our eyes are very sensitive to blue light, which does not cause your pupil to constrict in the same fashion as yellow light. As such, your eyes perceive that there is more light coming from a cooler blue light than a more yellow one, even if the light output is higher from the "warmer" and more yellow light, even when the lumen output of the blue bulb is lower.
Lighting Type B: HIDs
High-intensity discharge lamps (HID) produce light with an electric arc rather than a glowing filament. This is, rather than a straight filament, a crescent-shaped light source. It's crescent-shaped because as it passes through the space between the two electrodes, its heat causes it to try to rise. The space immediately surrounding the crescent of light glows in layers - The closer to the crescent of light, the brighter the glow. The ends of the arc crescent are the brightest points, and immediately beyond these points is completely dark, so the sharpest contrast between bright and dark is at the ends of the crescent of light. The high intensity of the arc comes from metallic salts that are vaporized within the arc chamber. This produces a LOT of light - and HEAT! Because of the increased amounts of light available from HID burners relative to halogen bulbs, HID headlamps producing a given beam pattern can be made smaller than halogen headlamps producing a comparable beam pattern - higher light output.
That said, in contrast to halogens, available in a wide variety of sizes, HIDs are made in two primary sizes and two primary configurations - D1 and D2. D1S and D2S are designed for use in HID projectors, D1R and D2R are designed for use in HID reflectors (which are rarely used anymore). These are the ONLY formats in which an HID is manufactured by a reputable supplier. All aftermarket "HID Kits" are made in third-world countries using either an HID-like design or are re-based D1 or D2 ballasts. D1 and D2 HIDs will not physically fit in a halogen housing. To make matters worse, the HID generates light lengthwise, front to back. However, most halogen bulbs generates light lengthwise, side to side, so even if you could shoehorn one of these into a halogen housing, the light would scatter in the wrong directions.
Lighting Type C: LEDs
An LED uses a semiconductor that lights up when an electrical current is run through it. The LED consists of a chip of semiconducting material doped with impurities to create a circuit ("p or n junction"). As in other diodes, current flows easily from the p-side, or anode, to the n-side, or cathode, but not in the reverse direction. Charge-carriers—electrons and holes—flow into the junction from electrodes with different voltages. When an electron meets a hole, it falls into a lower energy level and releases energy in the form of a photon.
LEDs are entirely directional. In general, a flat-surface uncoated LED semiconductor chip emits light only perpendicular to the semiconductor's surface, and a few degrees to the side, in a cone shape cleverly referred to as the "light cone." To have a wide angle of light, you need a bunch of LEDs arranged in a wide angle.
You might be noticing that each type of light source produces a different type of light - each type uses different materials to put lights in different directions (or all directions). So how do you turn this into a headlight?
Optics!
The Important Stuff: Optics
The Optics Make the Lamp. I cannot possibly stress this enough - the optics make the lamp.
Putting a bulb, ballast, or LED array in something with optics it's not designed for is like wearing someone else's glasses - they will physically fit on your face but you won't be able to see as well.
Here's Why:
There are two main types of automotive optic: a reflector system and a projector system. Older cars had parabolic reflectors and fluted glass lenses. Glass lenses direct light very well but it's very hard to change the direction of the light a great deal with fluting.
Reflectors, or more specifically multi-reflectors- are parabolic reflectors with many different surfaces that are specifically shaped to put light from a particular source into a particular area, creating a specific beam.
We call this output the beam pattern, it's what you see sitting in your car behind your headlight (rather than staring into it directly).
Multi-reflectors can have relatively sharp cutoff and defined beam, but they are actually less efficient at projecting light across distance than a projector system. This is because the small edges between areas of reflectors scatter light in unusable directions, and more energy is wasted. This lost light decreases the efficiency of the lamp. Some of the light energy is absorbed by the reflector as heat, and some is scattered. because of this, the lamp is usually larger to make up for the inefficiency.
The other type is a projector, which uses a parabolic reflector to focus light into a glass lens. The lens is usually flat on one side and convex on that other. This type of lens takes a beam of light and spreads it over a wider area as the distance increases. This requires the lamp to have a focal distance from light source to lens that depends solely on the lens shape and magnification index. a very, very small change in the shape of the lens will cause a different focal length. Similarly, a minute change in lens positioning will throw the focus of that focal length off.
Because optical glass and lens forming is expensive, many optic companies will make the same lens for halogen and HID based projector systems. They then change the cutoff shield, focal length, and reflector bowl to shape the light coming into the lens differently. The desired output is a function of the vectors of light coming into the lens, which is to say that the light is designed around the source of the light - halogen, HID, or LED.
An example of this can be found here in the parts diagrams for the 2008 Subaru Impreza. The Impreza and Outback trims had halogen headlights, the WRX and STI trim had HID headlights. Externally they looked the same, but you can see differences in the bulb, ballast, leveling motors, reflectors, and the shape of the reflector bulb, even though the lenses themselves are the same:
Halogen:
HID:
To my knowledge there are no cars that have options for Halogen, HID, and LED lights, but if someone tracks such a thing down I will be happy to research further.
LED optics are another ballpark altogether due to the way that LEDs emit light. LED headlamps, from the factory, are matrixed into smaller projector units that can be individually moved and controlled, as shown below:
The system dims light that would shine directly onto oncoming and preceding vehicles, but continues to cast its full light on the zones between and beside them. This works because the LED high beams are split into numerous individual light-emitting diodes with individual controls and sensors. LEDs in headlights are arranged in a matrix and adapt fully electronically to the surroundings in milliseconds. They are activated and deactivated or dimmed individually by a control unit. One notable exception to this is the current-gen Toyota Corolla, which uses a single LED matrix in a single projector designed for that matrix, rather than a series of them as found on Audi, Acura, etc. The light is a sealed, self-contained unit with its own custom circuit panel, sensors, ballasts, etc. It is not retrofittable without significant modification to the lens, housing, etc - which is to say, significantly more work than is involved in an HID projector retrofit. There are other single-LED units on the market but that's the only one I can think of off the top of my head.
The functional importance of lighting optics - whichever their design - is paramount. It puts the light where it needs to be, and keeps it from going where it shouldn't. It does this with great specificity and intense amounts of design and engineering, and must be done with a deep understanding of physical science, optics, and, perhaps most importantly, human vision. When you change anything in the lighting system, the optics determine the change in output that systematic change will affect. Sharper, brighter, or wider are NOT universally better characteristics of a lighting system. A problem in the optics directly affects the output of the lamp system and these effects are as wide ranging as there are possibilities to change the system. With optics, there's hardly any change that can make things better unless the optics are upgraded to focus on the change, speaking literally and figuratively.
Real-world Tests: What Does this Mean for Me?
At this point the dangers and problems of HID kits in halogen housings are well-documented. I have linked a number of the HID tests below, but less has been done with LED "kits." Sac02 on the Candlepower forums bothered to run a few tests. He purchased H11 bulbs, H9 bulbs, high-output H11 bulbs, and an LED kit with an H11 base. The H11 LED kit is identical to the ones sold under different names such as XenonDepot and Auxbeam. He put them in the halogen projector housings on his Mazda, leveled them about ten feet away from a flat white sheet, and recorded his results with a lumen meter as well as with a good-quality camera. He then ran similar tests while driving.
The threads are here:
http://www.candlepowerforums.com/vb...wall-amp-lux-comparison-LED-H11-LL-H11-100-H9
http://www.candlepowerforums.com/vb...amic-driving-evaluation-LED-H11-LL-H11-100-H9
Here's the summary:
They're worth a read but the summary is that, in a properly designed halogen projector housing, at distances needed to drive safely, the LED kit produces 550 lumens, as compared to 5350 lumens from a high output H11. That's a MASSIVE difference, over 80% less light. The LEDs clearly show unacceptable light output at close range against a flat surface, but while driving, the vast majority of the light that they did put out was concentrated very close to the car. You don't want light close to the car, you want it far away so that you can see where you're going. These problems are due to the way LEDs emit light - a unidirectional diode vs an omnidirectional filament in a halogen bulb. There is absolutely NO technological "breakthrough" that will change this unless someone figures out a way to get LEDs to emit light in the EXACT same way that a halogen filament does, which is impossible. The problem is at the level of physics. Do not be duped by marketing.
As pointed out later in this thread, there is no lumen rating in real-world applications and this is not the best test that could be done. I would be happy to do my own if someone wanted to supply the bulbs, but I will not be giving these companies any of my own money.
Addendum October 2018
It has come to my attention that Philips, generally a reputable company, has been marketing and selling a set of LED "retrofits" for halogen fog lights. My understanding is that Philips has run a number of tests of these and that in some fog lights, they are quite good - this, again, comes down to optics. Philips is careful to specify that they are for fog lights only. My thinking here is that the bar is significantly lower for fog lights, as most fog lights on modern cars are basically cosmetic anyway. I also note from their design that most of the LED "kits" on the market are based on Philips' design, which Philips has expressly stated is not suitable for halogen headlight housings as the light output simply isn't acceptable, both in beam pattern and lumen output. You should not put these in your headlights.
Cliffs Notes
Your car comes from the factory with either halogen reflectors or halogen projector headlamps. HIDs and LEDs cannot be safely or legally used in these headlights. Your only safe, legal, option is high-efficiency, high-output halogen bulbs. The GE Nighthawk, Philips Xtreme Vision, and Osram Nightbreaker are all good options. They look nice and white and you will be able to see better. If you have the Euro projectors, the H9 bulbs can be a good option as they have similar bases and identical wiring to the stock H11s, but are designed for halogen projectors. They are slightly higher wattage and technically not street legal, but neither are, say, catless downpipes. There's a debate on this and I don't consider myself knowledgeable enough about the specific optics of the Euro projectors to make a definitive call. I'll be running standard bulbs.
Works Cited and Further Reading
About 80% of the credit for this belongs to Daniel Stern and Handsdown on the NASIOC forums, both of whom I cribbed heavily from and in a few places outright borrowed from. Should either of them stumble upon this and wish for me to take it down, please let me know.
http://www.danielsternlighting.com/tech/bulbs/Hid/conversions/conversions.html
http://www.danielsternlighting.com/tech/bulbs/blue/bad/bad.html
https://forums.nasioc.com/forums/showthread.php?t=1670587
FAQs and Frequent Responses
1. "Well, it looks good to me, and I think my kit is brighter and better."
Consult the passage about signal image.
2. "What about [x kit] made by [y company], they say that theirs is good"
This is marketing. Period. If none of the world's foremost automotive lighting companies, with huge teams of highly qualified engineers and massive budgets, have figured this out yet, and their closest effort is for fog lights only, it is highly unlikely that some no-name Chinese company selling things on Amazon has cracked the code. It's great that they say that their product is the good one, but without any sort of technical sheets or testing documents, you're just regurgitating marketing and sales pitches.
3. "It's my car, I can do what I want."
I generally agree, but as lighting is safety equipment, these choices put you - and, perhaps more importantly, other motorists - at risk in a way that, say, underglow neon and a big wing doesn't. Take care of yourself and be respectful of other drivers.
4. "My buddy said it looks fine."
Is your friend a physicist? A lighting engineer? What's that? He's not? Well then.
Here's what a lot of this comes down to. A lot of people want to change my mind and argue with the science on this. Show me a tech sheet. Show me an independent third-party laboratory demonstrating that your kit has acceptable light output, cutoff, and light color. Until then, as I said above, you're just taking the marketing claims of some no-name fly-by-night company at face value, claims that are only made so you'll buy their products. It's not research, you're just copying and pasting a sales pitch. Don't do that.
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