Automotive HID headlight systems are rather complex. Some of the engineering considerations;
2- Lamp color
3- DOT approval
5- Headlight optics
Currently, the Chinese make most of the aftermarket HID kits we install on our motor vehicles. Of course, these installations for the most part, are not DOT compliant and some may even be dangerous.
As for quality, I have visited some of the glass shops in China and seen first-hand how short arc lamps, HPS, MH and hollow cathode lamps are made. Many of these facilities are dirt floor shops with little in the way of controls that would insure a consistently performing lamp. During manufacture, the quartz lamp kernel is filled with halide salts and noble gasses that determine the lamps wattage as well as lamp color (K or Kelvin). Then the lamp is tested to make sure it functions. Further performance testing is often non-existent.
As for the ballasts, it is likely easier to make a reliable ballast than a good lamp. However, some of the Chinese soldering and assembly is pretty shoddy.
Remember, the Chinese culture is different than that of the West…there is little incentive to do it right unless the Chinese based manufacturing is under the control of a Westerner.
Next, the HID lamp has a different light emitting pattern from the hot filament incandescent bulb. If the OE optics are not designed for HID, there will be performance issues such as, poor light pattern, limited long distance throw, light back scatter, offensive glare for oncoming drivers and eye fatigue.
One of the most important considerations for HID conversion is the headlamp optics. Without proper optics, HID light goes places where it is either useless or unwanted. If you look carefully at an automobile OE equipped with HIDs, you’ll see a bulbous lens and a mask or cutoff. These features focus the light properly, reduce offensive oncoming glare and discomfort as well as put the HID light where it belongs. Water vapor, dust, snow and rain all effect the way the light behaves and light from HIDs even more so, hence the need for projector optics and a cut-off mask to control back scatter.
Remember also, the light we see when driving down a country road at night is reflected light. What you see started with light at your headlight and bounced off (was reflected from) the object of interest. The round trip for the light was 2X the actual distance between the observer and the object of interest. Also, light intensity diminishes by the square of the distance so, you need all the light at the optimum wavelength and color mixture in order to see well into the night.
Next, lamp color plays an important role in performance. The human eye has peak sensitivity in the 500nm wavelength range. However most light we observe is a mixture of wavelengths. Light at 4,500K is equal to 666nm (red) at 6,000K the peak wavelength is 500nm (bluish green) and at 7,500K the peak wavelength is 400nm (deep blue) approaching UV which is not well processed by the human eye.
Sunlight, streetlights and HID light is a mixture of colors. So, when you order a HID at 5000K, it’s like ordering Chinese food…Kung Pow Chicken from one carry out place is likely different than the same dish from another joint. It depends what they put in it to flavor it…same with HIDs…a little Na salt here, a little K salt there and some hexameta moon dust and you’ve got a different flavor (color) light.
Oh, and various materials transmit light wavelengths with varying efficiency. Plastics and glass possess various light transmission performance.
So, generally, you’re screwed when you place an order for HID light kits unless the seller is responsible and your happen to choose a well made kit. I would order HIDs from a supplier here in the US with a published warranty policy who have been in the business for awhile. And stick with the kits under 5000K lamp color otherwise, your HID performance will suffer and so will your eyes.
The OE HIDs found on automobiles are at 4300K, optimum for night time driving. Also, this color typically provides about 3200 lumens vs. ~2000 lumen for the stock incandescent variety. That’s a big difference.