Irregular seg issue.

**eyeguy21** · 08-31-2010, 10:59 AM

I’m working with a gentleman who is noticing a weird curvature on the left seg of a new set of FT35’s. The puzzling thing is that the lenses we just made are a warranty replacement pair and he didn’t have any problems with the older pair. When looking at the lenses using a strait edge (PD stick) they appear even but when you hold them up against a seg aligner the left seg sure does have a sort of curved “U” shape to it rather than being strait across like the right seg. Everything appears to be equal to the pair that he had before the replacement. Rx, Cyl, Add power, BC, thickness, materials. Etc. The Rx is pretty simple:

+1.00 +.50 X 100
+.75 +1.00 X 177 Add +2.50

At this point it’s starting to look like just a mnf defect but if anyone out there has any alternative ideas/suggestions I’d appreciate it.

**Wes** · 08-31-2010, 11:51 AM

I see this pretty often, actually. It comes from cheap manufacturing methods, poor molds, etc. Much more prevalent in 6 base curves and up. For your bifocals, you could explain it to the lab and ask for a redo, or try a lab that uses a different blank supplier.

**eyeguy21** · 08-31-2010, 12:05 PM

Originally Posted by wss2020

I see this pretty often, actually. It comes from cheap manufacturing methods, poor molds, etc. Much more prevalent in 6 base curves and up. For your bifocals, you could explain it to the lab and ask for a redo, or try a lab that uses a different blank supplier.

Ok Thanks. I suspected something along those lines but I wanted to give them the benefit of the doubt on using the cheap materials. I've seen it before but I've always been able to attribute it to something. Would you say this happens more frequently on the larger segs over ft28's?

Our lab is a local Zeiss lab and we're pretty much made to order exclusively from them. That's my employers decision not mine. We have a couple different labs (that a lot of times runs much smoother) in the area but nothing that's an option for us. When I called to pick some of the brains at our lab about it they were quick to offer a courtesy remake but they were trying to suggest it was the cyl power being heavier on the left side. Sometimes I'll take the bait just because I like to be able to trust our lab guys and I definitely don't have all the answers, but in this case it's just a remake.

**MikeAurelius** · 08-31-2010, 12:14 PM

You are going to see it on higher add powers, not necessarily higher base curves. It can come from taking the lens out of the mold before the casting has fully cured. It can also occur if not enough release agent has been applied to the mold. In any case, sloppy manufacturing technique. Send 'em back for a redo.

**eyeguy21** · 08-31-2010, 12:30 PM

Originally Posted by MikeAurelius

You are going to see it on higher add powers, not necessarily higher base curves. It can come from taking the lens out of the mold before the casting has fully cured. It can also occur if not enough release agent has been applied to the mold. In any case, sloppy manufacturing technique. Send 'em back for a redo.

Roger that. They're going back.

**Wes** · 08-31-2010, 12:50 PM

Mike I've never seen this on a pl or 2 base, no matter the strength of the add. Of course that doesn't mean it's not possible. You're right about the higher adds, though, it is much more prevalent on them. I stick to my guns about the steeper blanks. Maybe it's the difficulty of putting a straight line on a curved surface? Really, I suspect it's a combination of both because I don't recall the phenomenon on anything other than 6+ bc and high adds.
Glad you brought it up. I'll be watching a little more closely in the lab now looking for trends.

**MikeAurelius** · 08-31-2010, 02:32 PM

It's going to depend on how the mold is made. If it is a metal mold, the segment is machine ground and polished, and there is very little if any curve to the line, no matter the base curve.

On glass molds, it really depends on the quality of the mold maker. If the segment upper is fused too hot or spends too much time in the fusing furnace, the line will curve almost every time. Glass molds are cheaper to make but last shorter time periods than metal molds.

**DragonLensmanWV** · 08-31-2010, 06:01 PM

You take any Exec, FT35, FT40 or FT45 and put them in an AIT Speede blocker and they will all have a curve to them visible in the reticle. Doesn't mean they are actually curved because if you take a PD stick and lay it on the line it will be straight.

Now,speaking of irregularities, one place I worked at got some glass lenses from State Optical of Maryland. You had to really look to find a matching pair. The FT 25s could be as small as 23 mm or as big as 27mm. I guess for them a 2 mm tolerance was tight enough.

I can hear Mike now.

**MikeAurelius** · 08-31-2010, 06:18 PM

ummm...........

You do know, don't you, that SOM purchased only 2nds & thirds, right? God that brings up memories!!! We used to sell "economy" lenses to them all the time, single vision bifocal grind-downs with thicknesses marked. Made a lot of money off them way back in the day.

That was when the single lens process was but a pipe dream...majors pitch blocked on large aluminum carriers, countersinks ground and polished...that's what started my lower back problems, leaning over the fine grinders moving the carriers around. Then after the bifocals were fused and generated down, they were all "proportioned" by hand on a diamond lap. Did you ever run across blanks that were prismatic vertically through the seg? That was because they fused badly and had to be reshaped by stopping the lens from spinning and force grinding the lens in one direction or another. Man, those were the days!!! Thanks for bringing back those memories!!