Identifying lens materials

[WAUGH]

Could anyone offer an alternative way to identifying lens materials
other than gently tapping the lens to differentiate the material?


Is there a lamp or light that will show a hue of a certain material once held under this lamp or light.


Or can anyone offer another viable solution?




I should say this is to assist new employees and not an season optician.

[Uilleann]

One would think that perhaps a smart soul would come up with a viable means of measuring Abbe value and/or specific gravity, and then check it against a list of known materials. We're able to measure these properties with high accuracy as we're developing lenses, so what is keeping that same tech out of the dispensary apart from initial development cost?

[LENNY]

This should be another function of auto lensometer!

[optical24/7]

From an article I wrote for a state newsletter. All info on this was from Darryl Meister.....

Determining Index of Refraction

How do we determine the index of refraction of an unknown lens material? Polycarbonate is easy to tell, we all use the “pling”test, verifying the distinct sound they make dropped on a table top (they sound like a poker chip).

But how do we tell CR39 from a mid-index, or1.60 from 1.67? There’s a neat mathematical trick to get a close approximation.The formula is..

N = .53 X ( TP/CP ) + 1

N = Index Approximation
TP = True Power
CP = Clock Power

True power is what you read in your lensometer. Clock power is your power calculation using a standard lens clock. Let’s go over using a lens clock quickly. To get a “clock power” take your standard lens clock and read the front surface curvature. Next clock the back curve ( for this calculation use the flattest back curve if the lens has cylinder). For a minus lens, subtract the front curve from the back curve to get the clock power. ( For plus lenses subtract the back from the front for the clock power).

Some examples pulled from my stock lens inventory;

Lens #1 Clocks +2.50 front/-5.75 back. -5.75 -+2.50 = -3.25. Lens reads
-3.75 in the lensometer so;
.53 X ( 3.75/3.25 ) + 1 =1.611 This lens is in fact a 1.60

Lens #2 Clocks +1.25front/-7.75 back. -7.75 - +1.25 = -6.50. Lens reads
-8.25 in the lensometer so;
.53 X ( 8.25/6.50 ) + 1 =1.672 This lens is in fact a 1.67

Keep in mind this is an approximation formula. Aspheric lenses can affect your answers because of changing curvature. But this can be a useful formula when trying to determine the index of an unknown lens.

[Java99]

My auto lensmeter does give ABBE value. Well, it would if I'd set the thing up to do it. Right now it says everything has an abbe of 60.

[Chris Ryser]

Why................make it complicated if you can tell the difference between a CR39 and a Poly by dropping them on a hard surface and the sound makes the difference. Just keep the ears clean.

[optical24/7]

There's a few more materials out there than just CR and poly...