Hi, you posted examples of where the light housing was redesigned with the emphasis being on focusing the light output rather than reflecting the light as traditional.
Except it wasn't redesigned. The XJ8 uses the exact same reflector housing whether the light source was HID or halogen. So did the Explorer.
If projectors are not needed for HIDs, then explain why the experts (including one you yourself quoted) explained the science behind how HIDs ideally work and why projectors are needed?
They aren't needed to work, but a projector is better - but that applies to halogen lighting as well. Halogen lighting works better with a projector too.
If reflector based housing were acceptable for HIDs, then why is it that automobile manufacturers currently use projectors? Why didn't they stay with the reflector based unit? Why did dot say that reflector based housing units cannot be used with HID systems unless the housing is modified for specific use with HIDs such as the X308 series of Jaguar XJ8s, the Ford Explorer, and the Lincoln Mark VIII like you mentioned?
Again, the housing wasn't modified on the XJ8
- what they did was change the mounts so the auto-leveling requirement was met and change the light source. That's how the Explorer worked as well. The Mark VIII predated the autoleveling requirement so it didn't even have that. All three passed DOT testing and were approved for road usage.
There was also the fully legal, approved by DOT Sylvania Xenarc retrofit kits:
I don't see any projector there, do you? Remember, this was a fully legal, approved by DOT and sold by a major lighting company kit.
As for why the auto makers used projectors, initially it was for not just better light performance but lower hood lines, which lead to better aero and better fuel economy. Halogens got projectors on many higher end cars, too - in fact, halogen projectors were used long before automotive HID was even a possibility. This is a BMW E32, produced long before automotive HIDs were available. Note the low-beam projectors.
The advent of Euro pedestrian safety regulations meant the end of low hood lines so halogen cars went back to mostly reflector only. However, the advent of the autoleveling requirement for cars plus the market demand for steerable headlights has led to manufacturers going with projectors for their HIDs. It's much easier and cheaper to buy one of the commodity two-axis motorized projectors and incorporate it into a new headlight design that bolts up to the halogen lights' static mounts than it is to design the car chassis to accommodate the static halogen light unit then engineer motorized mounts to get HID reflector units that can move. (Yes, I asked a contact at Jaguar the same question years ago.)
Of course, using these off the shelf projector units runs into other problems. Like the fact that some new cars using them as part of the design now require you to take the bumper and much of the front end of the car off to change bulbs. It costs two hours of dealer labor to change bulbs (HID or not) in a 2010+ Chevy Malibu, for example. It can take the same amount of time or more change the HID bulbs in a Cadillac SRX or CTS:
Further, I've never said that HID swaps will work in all or even most reflector units. I would never put them in my truck or my XJ6's current headlights because their reflectors are more conventionally shaped. The only reason I thought it might work in the 919's is because the reflector is pretty strangely/poorly shaped for conventional halogen lighting and it looked to me like they had been thinking about HID lighting when they designed it - it has a pretty typical profile for a dual use reflector.
In the long run, it probably won't matter much as everyone moves to LED array headlights which (as others have done here at WT) come with a new headlight unit anyway. It's where I plan to go in the near future.