Ever notice those little bumps on headlights? I’ve seen tons of cars with them. They’re like glass rounded spikes protruding about 2cm out. What are they?
those “bumps” are used as locators for a machine to aim the headlight beams on your car…those are found on “older” style headlights, not on the newer and halogen headlights…
2 WAGs here:
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They’re to help the light diffuse more, like the lenses in a lighthouse.
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They’re to deflect pebbles and things. Maybe bumpy headlights last longer than smooth glass.
Aim the headlights?
I’m talking about new cars too. From last year.
Yes, to make sure that they are aimed at the road, and not at oncoming traffic, or the barrow ditch (holy moly, is that a word?)
The actually have handheld versions. It is a piece of plastic with a few holes to set over the bumps. There is a bubble like in a level that you line up with the ranges to make sure your headlight are aimed at the correct place.
Barrow Ditch? I always thought it was Bar Ditch. Hell I’m not sure either.
I thought the bumps were sprue holes, that is, formed from where the glass was injected into the mold for the headlights?
Perhaps I am thinking of something different though.
Its possible they started out as injection points. All the ones I have seen are very exact, and these days I’d imagine they could do it with only one instead of three(let alone grind them off if they wern’t supposed to be there). I’ve never seen one on a non adjustable headlight, meaning the $350 dollar molded plastic pieces that cover the whole front of the car don’t have them, but for cars that use an adjustable sealed headlight, or that used a small light in an adjustable glass case, do have them.
What’s all this talk about “aiming headlights”? I didn’t think that was done anymore. 
They are for shredding the sponge when you wash the front of your vehicle.
The OP described them as being 2cm, almost an inch long, but they’re more like 2-3mm long.
Ok, maybe not 2cm… but 1cm for sure.
Here’s my understanding of the reason, which may be wrong:
They are indeed for aiming the headlights, but that takes a little more explanation. Modern headlight units, contoured into the body for aerodynamics, are actually adjustable left/right and up/down using screws at the sides and (usually) top. The exterior transparent part is a single, fixed piece including the reflector dish behind it, but there is a replaceable bulb inserted into the back of the dish.
So what, you ask? Well, these things still need to be aimed so they point straight ahead and a little downward. To avoid a tedious step at the factory where every car would have to be parked in front of a target while somebody adjusted screws, like you could see done at your neighborhood garage or body shop, the headlight units are made with 3 bumps. There is a alignment fixture used on the assembly line which contacts the 3 bumps (which are closer to 2mm than 2cm tall, btw) and locates them in the appropriate position for that car model, and automatically tightens the adjusting screws. With manufacturing tolerances in the headlight unit being held tightly enough, that provides adequate alignment right on the assembly line. If you don’t fiddle with the screws, or have a fender bender that would throw the alignment off, you don’t ever have to worry about it again, even after changing the bulbs.
The 3 bumps (no more, no less; that’s exactly the right number) are also found on '70’s-'80’s cars with sealed-beam headlights (the kind that look flat and rectangular, and have the filament and stuff integrated into a single piece) for the same reason. The difference is that the screws adjust a separate metal cup which hold the sealed beam. On older, round-headlight cars, every new bulb requires both aligning and focusing.
I’m sure that’s more than you wanted to know. It may be more than I know, too.
Not on my cars. Hmm, I’ve made mistakes before, better check ::runs outside:: Yep, they’re probably not even half a cm. Which car did you see where they protruded 1 cm?