Does a light with a UV component allow you to see things at a greater distance when there is fog or rain? There are a lot of cars with those blue-white headlights and I’ve been wondering whether they actually allow you to see further because there is a UV component to the light or are they simply brighter?
Thanks
Testy
Your eyes can’t see UV so that wouldn’t help.
The blue-ish lights you are seeing are HID (high intensity discharge) lights. They work a bit like fluorescent lamps or mercury vapor lamps. Because they are a metal halide lamp containing xenon, you may also see them called “xenon lamps”. They cost a bit more, but they produce more light for less electricity. You can also make a smaller HID lamp with essentially the same beam pattern as you can an equivalent halogen lamp, or if you want the same size as a halogen you can make a better beam pattern.
So, smaller, better performance, more economical to operate, but they cost more up front. The cost difference up front is probably quite a bit larger than the savings you’ll see from reduced electricity usage though.
If the ones you see seem to make it so you can see better, then chances are it’s designed to be the same size as a halogen but with a better/stronger beam pattern.
Eng Comp Geek
It seems that it is nothing more than a more intense light. They do seem to be better than regular incandescent bulbs but I wasn’t sure whether it was the spectrum or the intensity since they seem to be tilted toward the blue end of the spectrum.
Thanks again
testy
Filament lamps are black body emitters, the hotter the filament the more tilted toward blue the overall spectrum is. Hence colour temperature actually has a physical meaning. Discharge lamps emit a messy spectrum made up of the various line emissions characteristic of the excited gas. The colour temperature rating is a fudge based upon human perception of the colour and then fitting that part of the spectrum to a black body and calling that the temperature. It doesn’t really have any physical meaning. Although they look very blue, there is no specific reason to expect that HID lamps have much UV content (some do, some have lots, but the blueness we see isn’t a guide). Safety regulations will limit the amount of UV allowed for most lamps.
The human eye is a mixed bag with UV. The retina is sensitive to UV, but the lens absorbs UV, so we don’t see it. The retina is broadly sensitive - so people without UV absorbing lenses (i.e. after cataract surgery) can see in the near UV, but it has no specific colour, and generally makes colours look washed out. But essentially by definition UV is invisible. If we could see it, it wouldn’t be ultra. There is the vague value that a florescent material visible in the headlights would light up brightly if there was much UV in the light.
Francis Vaughan
Thanks for this. I remember hearing at one time that the HID lamps emitted UV and were thus able to penetrate fog/snow/rain better than ordinary white light but this was obviously some kind of marketing ploy. The UV may indeed penetrate adverse weather better than ordinary white light. If I can’t see it though, who cares?
Thanks again
Testy