How to tell the difference ?

[npk]

I have been frequently face with this problem.

New patients come in to fix their spec & they ask me if their lenses belong to the polycarbonate or just another normal CR-39 Hi Index.

Is there an easy way to figure that out ?
Since the lenses is already edge into the frame ?:(

[chip anderson]

There's a CR-39 high index?
Poly has a slightly blue-gray tint. Each plastic has a different "ring" when dropped on the edge on a table top. Each has a different texture and smell when being ground.
And of course each has different reactions when being subjected to certian chemicals.

Chip

[FVCCHRIS]

Can't wait to subject the lenses of the next patient who asks to all of those tests.:D

Hey, they asked right??

[scriptfiller]

If the poly does not have A/R, it has a "rainbow" surface appearance. This is bi-fringence, differing refractive indicies of the coating and the substrate cause this.

[Chris Ryser]

There's a CR-39 high index?

And of course each has different reactions when being subjected to certian chemicals.

Chip

CR39 is the closest plastic to the old glass lenses and there would be no high index or it would not be CR39.

CR 39 is not affected by most chemicals, unlike all the other lens materials.

[drk]

Polycarb is usually satin finished with ridges. Yes, the surface looks like an oil slick. (Is that birefringence? I thought that was a refractive property, not a reflective property?)

High index is frosty or edge treated, but it will look thin for the Rx.

CR 39 is frosty or edge treated, but will look thick.

[Chris Ryser]

High index is frosty or edge treated, but it will look thin for the Rx.

CR 39 is frosty or edge treated, but will look thick.

drk...........what means frosty ? In optical terms a frosted lens is opaque, which means non transparent.

Edges are only treated if one wants to do it.............and a thin CR39 does not look thick. :D

[drk]

Doh! I meant the edges.

I thought he meant high power lenses.

[HarryChiling]

Bifringence is a good initial indicator. If there is no bifringence then it's: CR-39 or glass in which case the weight will give it away.

If there is bifringence then it could be poly or hi-n. So take it out of the frame and drop it onto a counter top, poly will have a more twangy sound to it when dropped. Also if your country is like the US (with lazy opticians) you might notice when you remove the frame that the pin bevel has some swarf left on the lens somewhere, if it's stringy then it's poly as hi-n tends to grind to powder. You could also remove the lenses from the frame and pin bevel it to see what the swarf coming off of it is.

If it's spherical (non-aspheric) you could also read the power in a lensometer and take clock readings to reverse the index of the lens using the lensmakers equation (all readings clock and lensometer must be on the same meridian). Use the lens clock to find the front curve and the back curve, then convert these to radius measures with the following formula:

R = (0.53) / D_clock

Do this for the front R_front and back R_back, then punch those numbers into the lensmakers equation with the power reading from your lensometer:

Index = {Power / [(1/R_front) + (1/R_back)]} + 1

[DragonLensmanWV]

Order a plano stock lens of each material , label them carefully, keep them in the proper package, drop them one by one on the countertop. You will be able in no time to discern the different sounds of the varying materials. There are some easily audible differences in sounds.

[Crazy-bout-Optics]

To know if it is poly, pick it up from the edges and let it drop on the counter from like 1/2 an inch high. If it sounds like a poker chip, its poly.

[cocoisland58]

You will develop an ear for it.


Hi Index = plink

Poly = plank

CR39 = plunk.


Years ago used to click my teeth against the lens to see if it was glass. Yuck.

[npk]

Wow, there quite a few ways to find out then, but if is certain chemicals that can be used to find out. What are those chemicals ?

Is it you have to apply to the edge of the lens, would't it leave a mark after that ?

[chip anderson]

NPK:

The chemicals are more or less given in jest. They destroy the material. If it's destroyed it's X, if not it's either Y or Z so you try another destructive chemical to see which it is or is not.
Or I suppose if you wanted to go to a lot more trouble than it's worth you could run a specific gravity test. All materials have different specific gravity.
Be a lot easier to take a bottle or a six-pack over to the local wholesale lab at closing and get a lesson.

Chip

[Johns]

Smack it with a hammer. If it breaks, it's not poly.

An easier way to tell is to touch the edge lightly on the handstone...if it "powders" or "dusts" it's not poly. Poly will burn or melt on the wheel and will curl. Other materials won't.

[Witness]

If the lens is sized in properly and not flexed to heck (like DBL has been doing to me!!!:angry:) you should be able to tap it with a fingernail and 'hear' the difference. Glass will feel like you're tapping a steel blank and be HEAVY, trivex is almost too 'gooey' to make sound, but will be poly light. Poly sounds 'tinny' and feels a little flexy. The trouble is always between the hi index lines. Spectralite polishes like crap, if you can see the edge you can usually tell, but 1.60/1.67/1.70/1.74 is next to impossible unless you're cutting it. (If you ARE cutting it, Hoya 1.70 or Essilor 1.74 rates a 10 on the stinky scale, 1.67 rates a 7 or so, and 1.6 is about a lowly 4.)

[cityconnection]

I dont know if its just me, but poly and cr39 have a shiny finish when polished where as high index is not as shiny. Poly even when feathered is shiny in that area. Am I mistaken?

[Chris Ryser]

Wow, there quite a few ways to find out then, but if is certain chemicals that can be used to find out. What are those chemicals ?

Is it you have to apply to the edge of the lens, would't it leave a mark after that ?

Plain Methanol will do the trick................next day the lens will all crazed, thats Poly.

Solvent or ReagentCoated Polycarbonate
Ethanol L
Propanol L
Acetone S
MEK S
PetrolLDiluteAmmonia M
Dilute Caustic Soda S
Concentrated Caustic Soda S
Dilute Organic Acid L
Dilute Inorganic Acid L

S - Short term resistance- drops / spills
M - Medium Term resistance- up to 8 hours
L - Long term resistance- no attack

[Redzy]

cocoisland58... using your teeth? Are you sure you weren't testing gold instead of lenses? ;)