What material is the Prevencia sample?

**Dr. Bill Stacy** · 08-30-2015, 10:22 AM

I got a Prevencia sample from Essilor but am not sure what the base lens material is. It came in a little black microfiber bag and is 70 mm, and I'm pretty sure it's not poly. It's the lens I'm sending Chris for spectrophotometry so it would be nice to know. I'm hoping that someone else got the same sample and had the wits to ask what the base material is. I'm going to Vision Expo in 2 weeks so maybe someone from Essilor will be there to answer.

Sure wish I had an easy way to measure index of ref.

**newguyaroundhere** · 08-31-2015, 10:29 AM

I have the sample in my office. Its a polycarbonate lens.

**Tallboy** · 08-31-2015, 11:43 AM

Yeah I'm pretty sure mine is their Airwear Poly lens, which is Poly albeit very very well produced poly that even smells different when you edge it.

**newguyaroundhere** · 08-31-2015, 01:09 PM

I physically removed the lens and did the old drop test to see what it was. Plus if that was plastic it surely would have chipped from me removing and re-inserting it into the test frame

**Dr. Bill Stacy** · 08-31-2015, 01:18 PM

Still looking for a more elegant way to check index of refraction...

**newguyaroundhere** · 08-31-2015, 01:53 PM

Sorry Doc, wish I had a more "elegant" way of checking it but all I have is that sample and my two hands. Maybe contacting the lab that gave you the sample to ensure what the exact material is?

**Dr. Bill Stacy** · 08-31-2015, 02:24 PM

It was an Essilor rep. I didn't think it was poly because my method of checking for poly is to drop the lens on a ceramic surface and listen for the tinny "ting" sound. Works better on an edged lens than on a 70 mm blank. It's on it's way to Chris in Quebec for spectralysis.

But really, the reason I want a refractometer for lenses is very often I am confronted with patient's old lenses of unknown origin and it would be nice to know what it is.

**optical24/7** · 08-31-2015, 02:37 PM

It doesn't work with a Plano lens, but with powered lenses you could use the lens approximation formula:

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

N = Index approximation

TP = True power

CP = Clocked power

**Dr. Bill Stacy** · 08-31-2015, 02:40 PM

Thanks, I'll give that a shot and if it's close enough, I'll incorporate it into my software!

**Dr. Bill Stacy** · 08-31-2015, 07:26 PM

So I just tried it on a +6.25 sphere that clocks +7.0 on the front and -2.75 on the back

that equates to a CP of +4.25

and the actual Rx is +6.25 which agrees with the lensometer, so according the the formula:

n=.53x(6.25/4.25)+1=1.78 and 1.74 was ordered, so it's pretty close. Certainly closer than any other available index.

Just to be sure, I measured a known poly lens -6 true, 5.75 clocked, n=1.55 also pretty close. Tried an aspheric which was a little off, but then I don't much care for aspherics anyway...

Good formula. My search is over.

Many thanks.