Do not install an HID in the stock TW200 light housing. It's dangerous for oncoming vetches and is illegal. a 35W HID is going to be 2-3x the brightness of the stock bulb. HID light behaves differently and needs special optics in order to be safe for road use.
Photons are photons, and all photons follow the same laws of physics. HID photons behave no differently than photons from any other source. DOT limits the wattage of headlight bulbs, not the lumens output (how bright they are) or intensity (how bright they are perceived). That is, unless some rules have been recently changed.
It isn't how bright a light is that annoys oncoming drivers, but the 1) color, expressed as "temperature" in degrees Kelvin (*K), 2) beam pattern, 3) how the light is aimed, and 4) how high the light is off the ground. The following is about lights intended to be used when a vehicle is in forward motion, to light up the terrain in front of the vehicle. Cornering and back-up lights have their own set of guidelines.
1) COLOR
The original HID headlights were about 6000 to 7000*K and had a distinctly blue tint. Blue increases perceived intensity--the eyes see things as "brighter" than the really are. That is an illusion, because the increased intensity causes the pupils of the eye to constrict, reducing the amount of light reaching the retina. Worse, the blue color causes the pupils of oncoming drivers to constrict, reducing their ability to see as well. Techically, the use of blue lights (and red lights visible from in front of the vehicle) is restricted to authorized emergency vehicles, so high *K can get you a ticket, or even arrested for imperonating an emergency vehicle. Blue light will not reflect well off orange to a true red--objects those colors will appear grey, black, or, rarely, not at all.
Yellow light, about 3000*K, increases contrast in low light conditions but does not cause as severe a pupil constriction as blue light. Since yellow light is actually a combination of red and green light, it reflects off all colors, but poorly off true blues. Yellow light also does not reflect so much off airborne particles (tends to have less range of illumination because its wavelength has less energy) so does not create as much glare in foggy or smoky conditions as blue or white light.
For a light to illuminate and object, light must leave the lamp, reflect off the object, and travel to the eye of the beholder. A light with a low output of a particular wavelength will not make visible an object that only reflects the missing wavelength. White light is made up of all many wavelengths of light (green + red makes white in stage lighting). Broad spectrum white provides the best visibility overall in most circumstances. Broad spectrum white has a Kelvin temperature of about 4300*K. What you want most of the time is white light, about 4300*K. True white light reflects off all colors.
2) BEAM PATTERN
Beam pattern will be discussed as it illuminates a flat vertical surface in front of a vehicle. Patterns listed by range at which they will illuminate the same object.
Flood lights have a very wide pattern horizontally and vertically.
Fog lights have a very wide beam pattern with a flat top.
Low beams have for years had a wide pattern with a flat or arced top, and the intensity of light should fade from little at the bottom to much at the top. Newer low beams are flat on top on the left side, then angle up about 30* from horizontal on the passenger side to illuminate signs on the side of the road (and blind drivers in roads to the right and the mirrors of drivers in front of you on the right).
Auxilary headlights have a medium pattern, either round or rectangular. These are rare in the U. S. of A.
High beams have a narrow vertically oblong or rectangular pattern, with a significant amount of light patterned upward to illuminate overhead signs.
Driving beams have a very narrow pattern vertically and horizontally. Sometimes called spot lights.
Pencil beams have an even narrower pattern. Sometimes called spot lights.
"Spurious illumination" is light that escapes the intended pattern. Cheap lights tend to have a lot. Expesive lights tend to have less.
3) AIMING
In most cases, fog lights, low beams, and auxilary headlights used with low beams will not blind oncoming drivers IF they are properly aimed (top of pattern 1 inch below top of lamp for every 18 inches the center of the lamp is above the ground at 25 feet). These are the only three types of lights that should be used with oncoming traffic.
Flood lights, high beams, and driving lights should be aimed so that the center of their patterns is the same height as the center of the lamp. Pencil beams are so narrow that they are used in batteries of 6 or more, and are individually adjusted so that the edges of their patterns are parallel and touching. That is, assuming a 10* beam, the center two will each aim 10* on opposite sides of parallel, the next 2 will aim 20* to the left, for the driver side light, 20* to the right for the passenger side light, the next pair will aim 30* to the right and left. The result is a far-reaching 60* wide X 10* tall composite pattern. Sometimes vehicles are equipped with 2 or more rows of pencil beams, with the rows aimed 10* apart vertically to provide a taller pattern of illumination. None of these types of lights should be used when there is oncoming traffic.
Note that lamps with dual filaments, such as the factory headlights on TWs, should be aimed on low beam. Supposedly, high beam will automatically be perfect that way. I've seen discount lamps of the sealed beam type and replacement bulbs for composite headlights that weren't even close. Most came from China, Taiwan, or some other slave labor country.
4) MOUNTING HEIGHT
Even a light aimed down can blind others if the light is higher than the others' eyes. Legal height of any type of forward-facing lamp intended to illuminate the path of a vehicle while it is in motion is a minimum of 18 inches and a maximum of 54 inches, measured at the center of the lamp. LEOs can have a field day with jacked up 4x4s if they really wanted to, or even those touring types who mount lights to lower fork legs and on the bottom of their crash bars. There have been a few rumblings of lowering these limits as many vehicles come with OEM fog lights already lower than the law allows, and reducing the maximum would get those bright lights below the eye levels of more drivers.
Before we discuss the lack of DOT approved HID conversions, it is necessary to understand how a lamp creates a certain beam. First, conventional incandescent lamps (everything cuurently used except LED and HID) convert electrical energy to light energy by heating a piece of wire so hot it glows. This little piece of wire is called a filament, and gives of light (photons) in all directions from each point on its surface. Some of these photons proceed directly to the translucent part of the lamp housing, which, for simplicity's sake, we will call the lens. Some of these photons proceedss to the opaque part of the lamp housing, which we shall call the reflector, bounce off, and continue to another part of the reflector or the lens. By controlling the reflection of photons off the reflector and the reflection of photons off and refraction of photons through the lens, the paths of most of the photons end up being roughly parallel or with a limited angle of variability, creating a beam. It is the entirely measurable and predictable patterns of reflection and refraction that determine the beam and spurious transmission characteristics of any lamp. It matters not what the source of the photons is--they all reflect and refract by exactly the same principles inside the lamp.
Now that the facts concerning lighting are laid out, let's look at why most HID conversions lack DOT approval. HIDs do not have a filament. The bulb generates an arc, just like an electric welder or an arc lamp. As long as the location and size of the arc in an HID bulb is exactly the same as the size and location of the incandescent bulb it replaces, it will project exactly the same beam. The reason most HID conversions do not have DOT approval is because the arc is not exactly the same size and location as the filament in the bulb it replaces. Hence, the lamp no longer patterns well. Even if the arc in the conversion bulb is perfect, it triples or better the intensity of the spurious light transmission, which can easily go from annoying to blinding.
When you consider replacing a dual filament bulb, such as a TW's, that cannot be done with a dual arc HID for a couple reasons. First, when an HID is firt turned on it takes a couple seconds after the light is turned on for the transformer to charge the capacitors enough to strike the arc. Changing a dual arc HID from low to high beam, or high to low beam, would result in a couple seconds of darkness before the arc struck. Not good. Second, the arc locations are so close together that it would be highly likely that the positive side of the low beam would arc to the negative side of the high beam, and vis-versa, and the resulting arc would be cockeyed in the bulb, ruining the beam pattern. The solution is to only have HID on the low beam, and a regular filament on the high beam, such that both stay on on high beam. A second solution is to only have one arc, but use shutters to reflect the light from low beam position to high beam position, which requires a complex system of servo and linkage. A third solution is to move the arc itself drom one position to the other with a simple servo. A fourth solution is to tip the entire lamp up and down, with a secondary lens to kick a little light upward to light up signs.
The latter three solutions are called bi-xenon because they are HID on both beams. The first and third of these three solutions suffer from durability problems because of mechanical complexity. The mechanics of moving the arc on a servo arm are about as simple as an electric door lock--elegant in its simplicity. Such conversions can and often do pattern exactly like the incandescent bulbs they replace. Philips, Bosch, and Osram bulbs are available in this type. Notice that these are name brands. They are expensive. You will not get a good conversion for $79.95. You'll do well to get a pair of quality bulbs for $79.95. be aware that some unscrupulous sellers are calling the HID+incandescent bulbs bi-xenon when they are not.
When choosing a conversion kit be sure you have space to mount the transformers, which are about the size of a pack of cigaretes.
Be sure you buy a kit with waterproof transformers.
Be sure to add a switch to turn the headlight completely off when starting--HIDs hate low voltage--dramatically shortens bulb and transformer life.
