the whole reason people do this is because when HID lighting first came out, they appeared distinctively more ‘blue’ in color compared to traditional incandescent halogen bulbs. So light bulb manufacturers made all those stupid ‘hid style’ incandescent bulbs with the blue coating on them to make them appear blue and idiots flocked to them thinking they were getting something brighter or at least they thought people would think their cars were fancy. Thing is, the “hid style” bulbs have always been way more actual BLUE than genuine DOT style HID capsules are. But tuner dorks loved them and here we are now with people even making ultra-blue HID capsules that push to the bluer end of the spectrum on purpose.
But rest easy – the science behind automotive lighting is. . . multi-factored. There is a lot more than just ‘brightness’ involved here. DOT prescribes a maximum flux rating for automotive headlights. Flux is the intensity of the light at a specific point in the beam pattern. Lumens are another topic that involve the total amount of light energy emitting from something. Focusing all the lumens a bulb/capsule generates into a useful beam pattern that also doesn’t exceed DOT flux limits makes use of all kinds of magic shaping of the projector/reflector housings of a headlight. With HID capsules, since they produce nearly twice as many lumens as an incandescent bulb, a lot of extra lumen output is directed away from the center of the beam – to the margins. This way, there is a brighter overall pattern with terrific lighting toward the shoulders of the road but not too overly bright down the road at oncoming drivers.
When people retrofit HID capsules into a housing made for an incandescent bulb, that housing’s reflector/projector isn’t optimized for this duty and instead you end up with WAY too much light down the center of the beam. HID retrofits *are* illegal but also very difficult to enforce since it’s hard to tell them from just a poorly aimed light. Which leads me to the next issue.
Aiming. It matters a lot. Modern lights have a pretty dramatic beam cutoff built in. If that cutoff appears to you, the driver, as being above the road in the distance, your lights are aimed too high and every time you go over a gentle rise or hit a bump, you’re effectively flashing oncoming drivers with full bright light. Likewise, too low and it makes the road in front of your car look bright, but you can’t see anything to the distance. Get your lights aimed right.
Then we have Color Rendition Index (CRI). CRI is a rating of how well your lights allow you to see various colors. If you’ve ever noticed the orangish colored sodium vapor street lights make everything look almost like an orange version of black-n-white, that’s because the bulbs, although extremely bright, have an extremely poor CRI. It comes down to how narrow of a spectrum of light they produce. Incandescent/halogen light has a perfect CRI of 95-100 because it generates a full spectrum of all visible wavelengths so things of any color is visible by it. This matters because things like animals which are usually various shades of brown, tend to be invisible under poor CRI lighting. Real HID lights have CRI ratings of 60-80 which is okayish, but not great. So it’s a good thing they are so bright – the extra light is needed to actually see things. (Now that cars often have LED instead of HID, it’s important to know that the CRI for LEDs can be all over the fucking place).
And finally, I hinted at this earlier, but overly bright foreground lighting (like from having your lights aimed too low or from driving with foglights on) is VERY detrimental. The pupils of your eye obviously adjust to available light… But they don’t only adjust at the small point you’re looking at – they adjust based on the entire field of view your eye is exposed to. So when you have lights that aim too low or run your fog lights and have the bit of road right in front of your car brightly illuminated, it fools your eyes into thinking they need to restrict the pupils more. That means that you now can see LESS at distance. That’s right – with JUST your headlights on, you can see better at distance than if you also turned on your fog lights.
All this is to say that when you see people driving with foglights on when the conditions are normal, or when you see overly blue headlights, you can chuckle to yourself knowing they are suffering a worse driving experience than they could. Oh, and as a bonus, ‘blue’ bulb headlights also burn out faster so they have to buy bulbs more often.