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Headlight Glass
https://rypn.org/forums/viewtopic.php?f=1&t=38825		 Page 1 of 1
Author:  John E. McNamara [ Wed Dec 02, 2015 10:46 pm ]
Post subject:  Headlight Glass
Other than using patterned glass (like Fresnel lenses) in headlights, are there any benefits to using plain glass that is curved as opposed to plain glass that is flat?

Thanks!
Author:  NYCRRson [ Wed Dec 02, 2015 11:37 pm ]
Post subject:  Re: Headlight Glass
John, if the curved glass in front of a headlight bulb is "parallel" (i.e. the thickness of the glass is uniform) it forms a window. It will not "focus" a light beam any better than a flat window.

It might be easier to keep clean, or look nicer, but optically it has no effect on the projection of the light leaving the light bulb.

If the thickness changes as you move away from the central axis of the light beam then it acts as a lens and may concentrate the light beam into a smaller angle around the central axis.

The "segmented" lenses used in lighthouses, etc. are Fresnel lenses which simply make for a more compact (less thick) version of an optical lens.

Cheers, KevinK
Author:  sc 'doc' lewis [ Thu Dec 03, 2015 10:51 am ]
Post subject:  Re: Headlight Glass
I would think that the use of curved glass has to do with reflection off of the surface from an external source (e.g. the rising or setting sun). The angle of incidence equals the angle of reflection. It would seem the reflected light would be more intense off of the flat surface than if the surface was curved, thereby somewhat diffusing (?) the incident light.

Another thought is that any air/glass (and glass/air) interface is going to allow a portion to transmit through the glass and a percentage will reflect back. This happens at both interfaces and is the concept that is combated with the precision 'anti-reflection' coatings on high-end lenses (camera, spectacle, telescope, binocular, etc.). Is the glass curved to take advantage of these 'internal' reflections in combination with the parabolic mirror of the headlight to increase the percentage (intensify) the amount of headlight output?

Random thoughts...

sc 'doc' lewis
Author:  David Johnston [ Thu Dec 03, 2015 12:27 pm ]
Post subject:  Re: Headlight Glass
Before Pyrex came along there were problems with hot headlight glass covers cracking in cold weather. One thing that was done was cutting the glass into strips to give it some room to adjust to the temperature changes. The curved cover glass may be an effort the allow for adjusting to thermal stress without cracking the glass. it also moves the glass a little farther away from the hot bulb.
Author:  rcw7585 [ Thu Dec 03, 2015 4:05 pm ]
Post subject:  Re: Headlight Glass
David Johnston wrote:
Before Pyrex came along there were problems with hot headlight glass covers cracking in cold weather. One thing that was done was cutting the glass into strips to give it some room to adjust to the temperature changes. The curved cover glass may be an effort the allow for adjusting to thermal stress without cracking the glass. it also moves the glass a little farther away from the hot bulb.


This is the correct answer. Curved glass will bow more readily rather than crack, whether from mechanical or thermally-induced forces within the glass. The curved glass does not act as a lens focusing the beam. The beam is focused (or diffused slightly) and aimed by moving the light source... where the filament is located near the focus of the parabolic reflector.
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