# Optical Forums > Ophthalmic Optics >  Student Question about safety of glass and CR-39

## bk1224

I'm an apprentice and also taking classes to get my degree in Optics.  My professor asked this question: why is glass safer than cr-39.  He said that every optician believes the opposite of this but it is a fallacy and he wants us to answer this question.  Anyone know why?  I'm heading on the track that it is either because glass is chemical treated or glass going through the ball drop test and cr-39 not.

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## Uncle Fester

Is this a trick question? 

Maybe- "Safer" in what way? Would be my retort.

If it's due to impact resistance you got me from everything I've learned.

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## bk1224

Its not a trick question.  The teacher even said that when people ask opticians about this they all will say cr-39 but the answer is glass.  I'm stuck at really digging hard and thinking about this.

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## MikeAurelius

Yep, I can answer that for you.

All you have to do to prove it is break a CR-39 lens and a glass lens. 

The CR-39 will ALWAYS have very sharp pointy shards. A properly tempered glass lens will break into small chunks. A chemically strengthened or case hardened glass lens is far less likely to break than a CR-39 lens is (talking mainly plano and minus power lenses -- plus power lenses are almost certainly more impact resistant regardless of the material). It is a characteristic of the strengthening/hardening process.

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## DragonLensmanWV

I agree that CR-39 will always turn into pointy shards, but I've seen plenty of glass lenses that were hit just hard enough to crack all the way through, and at the point of impact on the rear surface of the lens the glass flakes off into very sharp tiny fragments, not like the tempered glass of auto glass which does break into cubes, more or less. These fragments are hard to remove from an eye because they're nearly invisible.
But overall, I don't see a lot of any lens broken, and zero Trivex lenses broken.

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## vintagetie

How can the only safety issue relating to lens material be impact? Even if it was, how can impact be accurately correlated to eye injury without recognition of statistical errors relating to variables like lens size,  type of impact, type of hardening, index of glass, etc.? Long term uv and ir damage is still pretty hotly debated in some circles, let alone weight affected tissue deterioration of the sinuses being a barely studied consequence of eyeglass wear. I think the topic is one in which the verdict is still waiting to come in, just my double penny's worth.

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## MikeAurelius

> How can the only safety issue relating to lens material be impact?


That wasn't the question. 

You do bring up some good issues, but the weight on sinuses??? LMAO, the weight is on the bridge of the nose, not considered part of the sinuses. What about the millions of people who wore nothing but glass prior the the mid-1970's? Or have you conveniently forgotten about those folk?

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## Fezz

Safer in a chemical splash?

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## uncut

Environmental safety?  Biodegradability, wastewater quality.

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## Yeap

interesting question here.. will keep tracking this.. 

if you comparing the impact resistant CR-39 should be better. while chemical resistant, environmental friendly  that would be glass.. i think is good if we know it said to be safer in which way..

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## Darryl Meister

I think Mike and I had a rather lengthy debate about this very subject in another thread recently.

I think you'll find that many people feel that plastic is safer, including myself. Regardless of the shape of the fragments if the lens breaks, hard resin is less likely to break than chemically-tempered glass and much less likely to break than a heat-tempered glass. Also, a deep scratch will significantly compromise the impact resistance of a tempered glass lens, but not a plastic lens.

Further, because hard resin is more ductile and has less mass than glass, I suspect that it would absorb more impact energy while being struck, thereby producing less kinetic energy when any fragments hit the eye and its adnexa. That's certainly why they use steel bearings in bombs instead of rubber balls, anyway.

Best regards,
Darryl

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## Barry Santini

The key concept, I believe, is not about the actual lens breaking.  Its quantifying the potential for additional contribution to eye or facial injury *after* it breaks.
Think about 
it.

Barry

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## Darryl Meister

But that really wouldn't be indicative of overall safety. Imagine a lens material that breaks into extremely sharp shards, but that has infinitely more impact resistance than another material that breaks into blunt pieces. You will still suffer eye damage more frequently with the lens that breaks more readily. And plastic fragments certainly aren't as sharp as something like untempered glass fragments (though glass lenses are obviously routinely tempered now).

Furthermore, remember that the fragments of the lens are often the least of your worries. If the lens shatters at all, you then have to contend with the actual projectile that broke the lens in the first place, which is probably even more likely to cause eye damage than the lens fragments, given the impact energy necessary to break modern lenses. This is exactly why we wear eye protection in the first place.

Best regards,
Darryl

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## rdcoach5

> But that really wouldn't be indicative of overall safety. Imagine a lens material that breaks into extremely sharp shards, but that has infinitely more impact resistance than another material that breaks into blunt pieces. You will still suffer eye damage more frequently with the lens that breaks more readily. And plastic fragments certainly aren't as sharp as something like untempered glass fragments (though glass lenses are obviously routinely tempered now).
> 
> Furthermore, remember that the fragments of the lens are often the least of your worries. If the lens shatters at all, you then have to contend with the actual projectile that broke the lens in the first place, which is probably even more likely to cause eye damage than the lens fragments, given the impact energy necessary to break modern lenses. This is exactly why we wear eye protection in the first place.
> 
> Best regards,
> Darryl


Also, if we have any surgeons reading this, how hard is it to dig tiny, almost invisible glass fragments out of the eye?

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## MikeAurelius

> Also, if we have any surgeons reading this, how hard is it to dig tiny, almost invisible glass fragments out of the eye?


But consider his last paragraph...that's the key here.

Yes, glass WILL shatter into small shards. I don't deny it. Yes, it will be difficult to get them out, however, if the doctor has access to an x-ray machine, an x-ray of the eye will show glass fragments, but it won't show plastic fragments.

But let's set that aside for the moment. What are the actual statistics on lens breakage with pieces impacting and damaging the eye? What were they in 1960, 1970, 1980, 1990, 2000 and 2010. By going back 50 years when there was nothing but glass on the market and comparing it to today, you should be able to see a trend IF THERE IS ONE. What is the actual incidence of eye injury in cases per 10,000 or 100,000? I suspect it is very small (excluding the Z87 market).

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## KellyR

Here's a shot in the dark. CR-39 will scratch up horribly in a gritty, dusty work environment. Glass won't. Maybe the instructor is leaning towards the fact that you cannot see hazards in the workplace through scratched lenses, therefore making you more likely to have a workplace accident. I also agree with the gentleman that has posted above me. CR-39, when impacted, will leave very sharp lens shards that can cause very serious facial injury. Glass, especially tempered glass, will shatter into tiny, not-so-sharp pieces, kinda like a car windshield.

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## DragonLensmanWV

Well, a car windshield is not tempered, it's laminated with a vinyl middle. The other windows in a car are tempered.

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## Barry Santini

> But that really wouldn't be indicative of overall safety. Imagine a lens material that breaks into extremely sharp shards, but that has infinitely more impact resistance than another material that breaks into blunt pieces. You will still suffer eye damage more frequently with the lens that breaks more readily. And plastic fragments certainly aren't as sharp as something like untempered glass fragments (though glass lenses are obviously routinely tempered now).
> 
> Furthermore, remember that the fragments of the lens are often the least of your worries. If the lens shatters at all, you then have to contend with the actual projectile that broke the lens in the first place, which is probably even more likely to cause eye damage than the lens fragments, given the impact energy necessary to break modern lenses. This is exactly why we wear eye protection in the first place.
> 
> Best regards,
> Darryl


 
Thank you, Darryl, for putting the above rationale out, and to begin revising and remaking the FDA impact resistance standard from the antiquated drop ball to something more comprehensive and descriptive of the associated factors affecting eye or face injury.

Looking forward to it...

Barry

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## MikeAurelius

> Well, a car windshield is not tempered, it's laminated with a vinyl middle. The other windows in a car are tempered.


The glass elements are also tempered. If you look at them through polarized lenses from a distance, you will see a cross hatch pattern.

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## DragonLensmanWV

> The glass elements are also tempered. If you look at them through polarized lenses from a distance, you will see a cross hatch pattern.



Not so sure about that.


All windshields have three layers. You have a glass   in the inside and a another glass in the outside. Both of these glass  sandwich a strong vinyl called PVB (poly Vinyl butry), in language terms  it's a strong lamination.  When impact accrues in the outer glass, the  lamination is intact to hold the break together, resulting in a crack.  Tempered glass, when made is in a state of compression. It's made in  very hot temperatures then it's rapidly cooled this is why when it  breaks it's in pieces, which is called dicing.  Tempered glass can only  be found in your side and back glass. The purpose for temped glass is  for safety evacuation in case of a emergency where the doors won't open  and the only way out is thur the window, temper glass makes it easier to  break in such cases. Now these days more and more luxury cars are  coming out with laminated side window, but only the two front side  window will be laminated for instance the Mercedes Benz S500.  So the  bottom line is YES the front windshield is laminated by law according to  FMVSS (Federal Motor vehicle Safety Standard). I hope i resolve your  argument
*Source(s):*

         PPG-CERT MASTER AUTO GLASS TECHNICIAN

MEMBER OF NGA (NATIONAL GLASS ASSCIATION)

INTAKE EDITOR FOR NAGS (NATIONAL AUTO GLASS SPECIFICATION)

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## MikeAurelius

The inner layer of glass is tempered, but the outer layer is not. At least that's what my ABRA guy told me. I've seen the crosses on the glass (not on all cars though...). It could also be that I'm looking at an older windshield, so who knows LOL.

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## DragonLensmanWV

> The inner layer of glass is tempered, but the outer layer is not. At least that's what my ABRA guy told me. I've seen the crosses on the glass (not on all cars though...). It could also be that I'm looking at an older windshield, so who knows LOL.



You could be seeing interference patters from the stressed vinyl inner layer. I see none in my windshield through my polarized glasses. The window tinting on the sides, however...Rainbow City!

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## Darryl Meister

> Thank you, Darryl, for putting the above rationale out, and to begin revising and remaking the FDA impact resistance standard from the antiquated drop ball to something more comprehensive and descriptive of the associated factors affecting eye or face injury


Actually, that is another good point to make: The FDA has never really cared about the lens fragments, just whether the lens breaks or not.

Best regards,
Darryl

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## Uncle Fester

> I'm an apprentice and also taking classes to get my degree in Optics.  My professor asked this question: why is glass safer than cr-39.  He said that every optician believes the opposite of this but it is a fallacy and he wants us to answer this question.  Anyone know why?  I'm heading on the track that it is either because glass is chemical treated or glass going through the ball drop test and cr-39 not.


bk 1224  Please don't leave us with an unanswered question. :Confused:

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## grumpybear

What is the projectile?  What is the size of the projectile?  Is there a projectile?  Is he just torturing you?

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## harry a saake

having started in the safety glass business with B@L, many years ago and stayed with it for 10 years, i believe i can make some comments here.

When glass lenses are PROPERLY heat or chemically treated, to make them more impact resistant, the major factor that takes place, is that if and when the lens breaks into many tiny pieces, there are no sharp edges, they are essentially rounded off.

We used to break one and take the pieces and rub the pieces between our hands, and you would not get cut

So what would you rather have , a lot of pieces that wont cut or a few less that probably will.

Also keep in mind that especially with minus glass lenses, (due to griffith flaws) they may only be good for one impact

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## k12311997

> I'm an apprentice and also taking classes to get my degree in Optics. My professor asked this question: why is glass safer than cr-39. He said that every optician believes the opposite of this but it is a fallacy and he wants us to answer this question. Anyone know why? I'm heading on the track that it is either because glass is chemical treated or glass going through the ball drop test and cr-39 not.





> bk 1224 Please don't leave us with an unanswered question.


still waiting for your prof's answer?

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## Aarlan

I will look for the actual reference, but I distinctly recall data that showed that chem tempered GLASS lenses were LESS likely to break with impact with a slower larger object (softball, airbag deployment, etc), however the advantage switched over th CR39 when analyzing smaller, higher velocity objects (BB fired from air gun, grinding debris, etc).

AA

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