21
$\begingroup$

Why does the glass on my car's headlights have such a strange shape/texture? Are there physics-based explanations or insights for these features?

Wouldn't it be better if it was just transparent and even?

enter image description here

(car model: BMW E30 1981-1985)

enter image description here

$\endgroup$
5
  • 1
    $\begingroup$ Relevant meta-post: Are engineering questions appropriate for this site? Seems debatable in this instance, but maybe Motor Vehicle Maintenance & Repair would be a good place to ask anyway/also. $\endgroup$ Jul 20, 2014 at 22:27
  • 5
    $\begingroup$ @NickStauner Right, but I am asking for an insight/explanation/answer from physics point of view, I did not write that down - but if I post the question here, it should be obvious that I am not looking for math or engineering or technology or chemistry or economics point of view. $\endgroup$
    – VividD
    Jul 20, 2014 at 22:32
  • 2
    $\begingroup$ It should be, but people often aren't aware of the site's policies on scope limitations or alternative sites...so it's good you wrote that just now :) $\endgroup$ Jul 20, 2014 at 22:35
  • 2
    $\begingroup$ See en.wikipedia.org/wiki/Headlamp#Optical_systems for details on headlamp design including the lenses. $\endgroup$
    – user6972
    Jul 21, 2014 at 1:52
  • $\begingroup$ Where have you been for the last 15 years of automotive development? We have projector headlights for a while now and laser headlights are on their way into the mass market. In other words: it would be better when engineered right. $\endgroup$
    – the-wabbit
    Aug 31, 2015 at 22:27

3 Answers 3

22
$\begingroup$

The type of headlight lenses shown in the images, with the rows of fringes, are Fresnel lenses. This is essentially a lens with a series of fringes that act as prisms, each at a slightly different angle but with the same focal length. That has the effect of reflecting the non-directional light from the bulb in a particular direction.

I'd think the purpose of using a Fresnel lens is both to concentrate/shape the beam in the direction the car is traveling more than a conventional lens of the same size could, and to direct the high and low beams.

If you tour an old lighthouse, the lenses used are also large Fresnel lenses. Especially before electric lamps were developed, these were needed to concentrate the light from, say, an oil lamp so that ships could see it at a distance. A conventional lens would have been too large and heavy.

$\endgroup$
3
  • 6
    $\begingroup$ There are extensive legal requirements for the lighting pattern produced by headlamps (the US version runs to about 40 pages), and it's easier to produce that pattern using Fresnel lensing elements than either classical lenses or reflectors. $\endgroup$
    – Mark
    Jul 21, 2014 at 8:51
  • $\begingroup$ This and the accepted answer contradict each other! Who's right? Here you claim the lines are fresnel lens like, to CONCENTRATE the light, while in the accepted answer it's claimed they are to DIVERGE the beam to not make a lousy small very bright spot! $\endgroup$ Apr 26, 2019 at 1:29
  • 1
    $\begingroup$ They don't contradict each other IMO, rather they address different aspects. A too-concentrated beam is not of much use, but neither is a diffuse light source, What is needed is a shaped beam that provides the optimal area of forward illumination of the road for a given light source. $\endgroup$
    – paisanco
    May 5, 2019 at 2:31
14
$\begingroup$

There are two parts to those lights:

  • the reflector, which gathers the bulb's output and creates as focused a beam as possible
  • the lens, which modifies that beam as desired.

A focused beam makes a lousy headlight. You only see a small patch way in front. The extra ridges in the pictured lenses are vertical, which means they will spread the beam horizontally, thus producing the curb-to curb beam we want without also blinding oncoming drivers.

Modern headlights moved most of the optical mechanics to the reflector. This way the designers can integrate the lens with the body. The math is a LOT more complex, but so is the design software. When those headlights were designed they used a pencil, a slide rule, and a lot of intuition.

$\endgroup$
3
$\begingroup$

This is not a full explanation, its only an addition to the previous answers. most vehicles use this light pattern:

enter image description here

As you said, a clear front shield without patterns would be better, but they require inner reflector patterns or a projector (light bulb is inside the round magnifying glass or lens). See image below:

enter image description here

The low beam has a projector and high beam uses inner reflector pattern.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.