Perhaps the most regulated part of a US car is its lighting system. Reams of federal text and diagrams spell out brightness, position, aim and more. Yet we encounter cars with wildly different lights as we face them in oncoming traffic. Part of the reason is that headlights have been advancing fast, so there is a wide array of technologies on the road and there will be for some time.
Regardless of era, the nature of a car's headlights is determined by five technologies:
Source: Our cars use a mix of sealed beams, halogen bulbs, HID modules, LED lights and even laser emitters as their forward light source. Each one of those generates a different color, spectrum width and intensity of light. Generally, modern cars have been moving toward brighter and bluer light as that helps illuminate objects more accurately.
Lens: The clear front part of a headlight may be smooth or have a dimpled pattern to shape how the light disperses. More cars today have a clear front, which can make a light seem more intense than that coming from an older car.
Position: While those reams of regulations spell out where a headlight must be positioned, there's still some leeway. And the difference in height between two cars facing each other is something no regulation can fully anticipate. There are simply times when you'll be facing a much taller vehicle and feel like you're being interrogated.
Reflectors: The main reflector is the shiny dish that forms the rear of most headlights. The front reflector covers the front of the bulb to bounce its light toward the rear reflector. The design and interplay of these two curved mirrors is complex, and unique between different models of cars.
Shutter: Cars with projector headlamps (the ones that look like they have a 2-inch crystal ball in them) often have a internal mechanical shutter that moves to change the cutoff between high and low beams.
Finally, there's the wild card that I suspect a lot of you are seeing: DIY retrofits. HID bulb kits can emit a much brighter, bluer light source than what the factory designed the headlamp assembly to work with. That can potentially overwhelm the reflectors and lens, allowing too much of a good thing to flood the road -- and the retinas of oncoming drivers.