Why Are Some Headlights Blue-High Intensity Discharge Lamps
高强度气体放电（High-intensity discharge，简称HID）灯泡包括了下列这类类型的电灯：水银灯、金属卤化灯（也称作HQI）、高压钠灯、低压钠灯、高压水银灯，与较少见的短弧氙气灯。这类灯泡类型的发光元件是一颗置于耐高温灯管（弧光管）内安定的电弧放电器，并有超越 3 W/cm² 的发光能力。
HID 灯一般应用在大面积地区且需要高质量的光线时，或针对能源效率、光源密度等特殊需要时。这类地方包含体育馆、大面积的公共地区、仓库、电影院、户外活动地区、道路、停车点或巷道。近期 HID 灯如金属卤化灯，常被用于零售店和住宅环境。HID 灯更达成到室内栽培上，尤其是一些需要高质量强光的植物，如蔬菜和花卉。它们也可用于室内水族馆，重建近似于热带区域的强光。
有的 HID 灯像是水银灯会发出很多的紫外线，因此需要扩散器（diffusers）来阻挡紫外线的辐射。近几年来常常发生几个案件，由于缺陷的扩散器，导致大家遭到紧急晒伤和电弧眼。目前均以规格来监控灯泡品质，不然灯泡会由于外层灯管破裂就立刻烧掉。
近期 HID 灯常被用于汽机车的头灯。这项应用在汽机车业界仍有各种不一样的建议，主要在于 HID 灯有时会致使叫人刺眼的强光。业界一般用自动转向头灯（Automatic Self-leveling System）来减少这个问题，但这种的配备在大多车种都是昂贵的选配规格。然而，很多业者偏好用这种光线，由于他们比一般头灯能发出较清澈、较明亮、较自然的光线。
Have you ever been driving along a country road at night, and all of a sudden a car comes around the bend and shines their high beams right into your eyes? Ugh, they’re so bright!
High Intensity Discharge Lamps
You also notice that their lights look a bit bluer than you’re used to. That’s because they might be. Regular headlights are made up of white light, which is a combination of all colors of the rainbow.
But these blue lights, while still containing a mixture of all colors, contain more blue wavelengths, making them appear more blue to our eyes.
Many luxury cars have these types of lights, called High Intensity Discharge lamps, because the headlights seem brighter under most conditions.
What Happens In Fog?
They actually make visibility worse.
Fog is made up of suspended water particles in the air, like a cloud settled down on the earth. But blue light scatters more effectively off these water particles than regular headlights do.
This is because the shorter the wavelengths of light, the better it scatters off of air and water particles.
Blue Light
So blue light, which has very short wavelengths, is much more likely to scatter than the longer wavelengths, like red light.
This means that when blue light hits the water particles, it’s more likely bounce off, going back into your eyes, and reducing your ability to see through the fog.
It also increases the glare for everyone, since blue light scatters better inside our eyes as well. In the regular headlights, there are still a fair amount of smaller wavelengths, so it scatters too. But since it also has more of the longer wavelengths, more light gets through the fog. So you can still see–sort of.
