Scatter in restored headlights

[pasadena_commut]

The headlights on my 2003 Honda Civic Hybrid hazed over and the history of the various restoration steps is here:

https://www.autopia.org/forums/auto...storing-3-sylvania-headlight-restoration.html

Lately they have been looking a little yellow, and I thought it was probably the silicone oil plus dirt, rather than the plastic itself. Scrubbed them thoroughly with a bit of detergent and a lot of water and they were clear again. For a change I just waxed them instead of applying oil. The wax gets into any tiny scratches and keeps them from scattering light. After that the headlights were about as pristine as possible. By eye they were just clear, from any angle the structures inside were very sharp. There was a slight pattern visible from the Sylvania coat. Got in the car and turned on the headlights aimed up against the garage door. Again, sharp. Good as new, right?

Not quite.

Then I did something I have never done before. With the headlights on I got out of the car and looked at the headlights. There was a remarkable amount of scatter, so that they were quite bright about 30 degrees off the main axis. Subjectively they sort of looked like they were glowing at those angles. My wife`s Accord was sitting right next to it, and that garage queen (the car, not the wife) has very clear headlights. No off angle scatter there. The headlights on the garage door were roughly as bright for both cars. Light sensitivity is logarithmic, so while to my eye that scatter looked pretty bright, in reality it was might have been only 1/100th of the beam intensity.

Anyway, have any of you ever examined the optics of a headlight in use, vs. the subjective "this looks clear now" test? I didn`t want to blind myself by looking too closely but I couldn`t really tell what was causing the scattering. It could be residual damage in the outer part of the plastic, or something about the Sylvania coat/plastic interface, or it could even be a thin coating of deposited gunk on the inside of the headlight, analogous the stuff that forms on the inside of the cabin glass. That last one would be a pain since the gunk isn`t very water soluble and I usually remove it from glass with dilute ammonia and newspaper. Not an option inside a headlight!

 

[Accumulator]

Just FWIW, I commend you on paying attention to such stuff and *caring*. Very different from how a lot of people approach their headlights.

 

[Stokdgs]

Pasadena_commut --
Read all of our posts from 2016 to present..
Totally relate to what you are wanting to know about, and have done to this point..
You want the good quality headlight performance you had, as close as possible to how it was 15 years ago..
I would want that also..

Thank you for all that hard work and experimentation you accomplished and then took the time to post here..

I remember when I was child, my Dad had a machine on wheels in the Shop that was used to align headlights, and it gave you a small picture of the beam pattern.
This was of course way back before a lot of headlights were self-adjusting as the ones are on my `09 Grand Cherokee, for example..

If you are seeing beam scatter, then that is because the surface is not clear of defects, and the light cannot reflect - straight - back..
Your eyes are seeing that, in my humble opinion..

I believe, as it is in the paint correction world some of us live in, that as long as there are tiny scratches, imperfections, etc., in paintwork, it will never reflect light, straight back as clearly and the light will also be scattered all around making it very uncomfortable to look at in the sun..

So, I think you are seeing that "light scatter" because the plastic is not perfectly clear (for whatever reason) and the light from behind is still true, but as it hits the imperfections, the light travels in those directions..
Dan F

 

[pasadena_commut]

If you are seeing beam scatter, then that is because the surface is not clear of defects, and the light cannot reflect - straight - back..

Pedantic correction: this is a refraction issue, not a reflection one.

The odd thing is that light from the sun, through the plastic, to the internal structures, including the one housing the bulb, back to my eye results in a clear image when I look at it ~30 degrees off the beam axis. Conversely, the bright light from the bulb produces notable scatter out of the beam seen from the same vantage point. (Moving in closer to the axis than that is too bright to see anything.). But the path from the bulb holder to my eye is the same. Probably my eye could not detect a 1/100th uniform increase in intensity in the first case. The scatter is probably there, just very hard to see.

The outer surface of the headlight is now quite smooth. The wax will have filled in any fine scratches in the plastic lens and polishing will have produced a smooth outer surface. There will still be a slight change in refractive index between the plastic lens (~!.58) and the wax (Carnuuba ~1.47) but that is tiny compared to the 1.58 to 1.0 transition at the plastic/air interface. Perhaps the remaining .09 is enough to account for the scatter? It is also possible that polycarbonate accumulates defects internally over time which scatter some light. Some of that could be from mechanical issues (pressure changes, heat flexing, bugs and rocks bouncing off),. I suppose that cosmic rays and other forms of radiation might also produce optical defects over 15 years.

In any case, what I think I need to do is to take a long exposure picture of the beam pattern for the Civic and the "control" Accord. (It isn`t a perfect control of course because the internal structure of the lights is very different.) There should be diffuse halo for the Civic, and much less of that for the Accord. Perhaps that way it will be possible to at least roughly measure the amount of scatter. A hand held light meter would be a better way to go about this, but I don`t have one. (And have not seen one in decades, since back when film cameras had no electronics in them.)

 

[Accumulator]

Pedantic correction: this is a refraction issue, not a reflection one...

Heh heh, you sound *just* like Yours Truly, right down to use of "pedantic"

Perhaps that way it will be possible to at least roughly measure the amount of scatter. A hand held light meter would be a better way to go about this, but I don`t have one. (And have not seen one in decades, since back when film cameras had no electronics in them.)

Good thinking, wonder if it`d work...it`d have to be a pretty significant diff to affect the output level, rather than the nature of that output, that much. I used handheld meters for a long, long time even after in-camera ones were common, probably oughta have a better idea about this than I do

 

[pasadena_commut]

Yesterday in full sunlight I inspected the headlight lenses with the highest power magnifying glass we have. There were quite a number of defects visible. These may have been cracks in the Sylvania kit`s coating or small cracks in the surface of the polycarbonate lens right below it, or both. Dust or other contaminant in the Sylvania coating or wax layer? Cannot rule that out. They seemed to have mostly a horizontal orientation but all directions were observed. They did not look like impact damage, in no case did I see concentric circles of them, or star shaped clusters. They were very tiny but must have been scattering quite a bit of light or they wouldn`t have been visible. My guess is that whatever these are they account for most of the headlight beam scattering which was observed.

 

[Accumulator]

Ah, sounds like a good inspection! The horizontal nature of the majority of the scratches sorta says "caused by something somebody did".

What`s the plan now? New ones, or live with `em?

 

[pasadena_commut]

The horizontal nature of the majority of the scratches sorta says "caused by something somebody did".

Well, maybe. They are not orderly like a swirl pattern or brush marks. There are just a lot of little defects which are mostly oriented in that direction, but not entirely so.

What`s the plan now? New ones, or live with `em?

Good question. RockAuto has replacements for only $36 per side, so they would be cheap enough to replace. On the other hand, the car isn`t worth much, the CVT is slightly off (even with multiple fluid changes) and might blow at any minute, and the headlights even as they are still throw a reasonable amount of light. So I think for now I will just wait a bit. If something crucial and expensive blows in the near future the car will be replaced. Karma being what it is, this strategy will probably guarantee that nothing of importance fails before the headlights fog over again, and conversely, if I were to replace the headlights the CVT would probably explode the next day.

 

[Accumulator]

Heh heh, yeah...that CVT might be a bit worrisome. Just don`t wait until something bad happens that could`ve been avoided had your lights just been a *little bit* better