I recently bought a lot of national optronic equipments which including the 2g, 7e, tracer, blocker, and Visual Lab Pro. I have a lot of high minus in my area. I have a hard time to figure out how to do a biconcave lenses with the new system. Help!
Any advice on surfacing biconcave lenses w/ my National Optronic equipments.
I recently bought a lot of national optronic equipments which including the 2g, 7e, tracer, blocker, and Visual Lab Pro. I have a lot of high minus in my area. I have a hard time to figure out how to do a biconcave lenses with the new system. Help!
Well, that's a tricky one. First you'd have to figure out what minus front curve you want to use to match up with the rear curve to get your power. Try using say, a minus 3 front base curve and type that into your software which will then give you the rear curve. Use spheres only for the front if possible. The really tricky part is figuring your center thickness. Surface your front curve first and measure the CT of the remaining blank and only take off enough to leave you your desired center thickness.
DragonlensmanWV N.A.O.L.
"There is nothing patriotic about hating your government or pretending you can hate your government but love your country."
Thank you for your reply, I did tried minus 3.50 front curve. The problem that I have is cutting the back curve. The rx is -13.50-2.00x95. It wouldn't cut the back side. I tried to change the center thickness and the edge thickness, but the blade wouldn't touch the back side. What did I do wrong?
Is it appearing to cut a curve? If so, try reducing your center thickness in 1 mm increments until it starts to hit the lens, then you can take more off until you reach your desired thickness. What material are you using?
DragonlensmanWV N.A.O.L.
"There is nothing patriotic about hating your government or pretending you can hate your government but love your country."
What surface block are you using? It's possible (though admittedly unlikely) that the block is sitting "in" the front curve, in which case the lens would not stick out far enough for the generator blade to touch the lens.
Does it not touch the lens at all through the entire cycle?
I always found it easier (when possible) to simply take a low base curve lens, measure the back curve of the blank, flip it so that becomes the front, and then surface what used to be the front of the lens. You don't have as many choices as to front base curves, but you're only cutting the lens once.
There are rules. Knowing those are easy. There are exceptions to the rules. Knowing those are easy. Knowing when to use them is slightly less easy. There are exceptions to the exceptions. Knowing those is a little more tricky, and know when to use those is even more so. Our industry is FULL of all of the above.
Willowracer,Originally Posted by willowracer
I can't help with the manufacturing problem, but I can say that from a wearer's perspective, a bi-concave design has very poor optics off-axis.
Science is a way of trying not to fool yourself. - Richard P. Feynman
Experience is the hardest teacher. She gives the test before the lesson.
If it's what they're used to and like it..... personally I think the Rx is too low to need a bi-concave.
There are rules. Knowing those are easy. There are exceptions to the rules. Knowing those are easy. Knowing when to use them is slightly less easy. There are exceptions to the exceptions. Knowing those is a little more tricky, and know when to use those is even more so. Our industry is FULL of all of the above.
Yeah, that's perfect for 1.70.
That's a good idea about finding a blank and using the back side for the front. Most of the ones I look at have a few defects (usually coating drips) that would make them a bit iffy to use. And I only have a metal sag gauge, so it would dimple the lens, but I suppose a lens clock would be pretty close to the actual curve of the lens. Kinda hard to get it to thousandths with a clock, though.
DragonlensmanWV N.A.O.L.
"There is nothing patriotic about hating your government or pretending you can hate your government but love your country."
And that's why lenses aren't the only things going digital.....;)
Usually, though not always, lens technical data will also include the back curve of the blank. And some vendors are definitely better than others.
There are rules. Knowing those are easy. There are exceptions to the rules. Knowing those are easy. Knowing when to use them is slightly less easy. There are exceptions to the exceptions. Knowing those is a little more tricky, and know when to use those is even more so. Our industry is FULL of all of the above.
Thank you for all the tips, I will try them out tomorrow. As of the material, I am using Younger Easy Lite blank ~1.56 index. I am also using the coburn 50mm and .50base block. Should I just scrapped the idea of using biconcave for this case and go for the 1.7 index? Do they make 1.74 semi?
Yes they do make the 1.74, BUT it also has a very low ABBE value. Your customer will appreciate the 1.70, since it has a significantly better ABBE. And the difference in thickness is not noticeable.
DragonlensmanWV N.A.O.L.
"There is nothing patriotic about hating your government or pretending you can hate your government but love your country."
As always, I really appreciate all your help. First, I took the advice from WFruit and recheck the block. It does sit in the front curve. I recalculated the edge thickness and it started to cut. The first cut was at 5.0 ct and it grinded only the center 1/3 of the lense. As I lowered the CT by 1.0 increment like Dragon recommended, it only thinned out the rest of the 2/3 on the lens leaving the 1/3 center untouched. Now I have a thin lens w/ outer 2/3 being flat and the center 1/3 curve. What did I do wrong? Dragon, where do you get the 1.7 semi-finish lens?
You can get 1.70 SFSV from Hoya, Vision Ease, or Polycore (1.71).
And you didn't do anything wrong surfacing it. You just didn't use a blank that was thick enough to give you a full backside bowl on the lens.
The abbe on a 1.74 is the same as that of a 1.67. At that power they will notice the abberation from the low abbe value, but in my view, even that would be better than a 1.56 bi-concave....
Or we could compromise and go with a 1.60.
There are rules. Knowing those are easy. There are exceptions to the rules. Knowing those are easy. Knowing when to use them is slightly less easy. There are exceptions to the exceptions. Knowing those is a little more tricky, and know when to use those is even more so. Our industry is FULL of all of the above.
WFruit, the plank started out 17mm thick. In regarding to the 1.70 sfsv, which of those three companies would best fit me? I have a small wholesale lab, Thanks.
Well, that depends on whether or not you have a dip coater. Both Hoya and Vision Ease require their lenses to be dip coated. However, both of them will also process the lenses for you if you send them the Rx. Personally, I like Vision Ease a little bit more, but you can't go wrong either way.
Polycore's do come front side coated so you'd only have to coat the back using any method you choose. I must say however, that I am NOT a fan of Polycore, although it has little to do with the quality of their lenses.
And I'm familiar with Younger's Easy Lite lenses, we use them here as well. 17mm thickness does not change the fact that, for what you are trying to do, you need a thicker blank. My next question would be what is the frame size and patient's PD. It could be that even though you don't have a full bowl on the back, it will cut out anyway.
There are rules. Knowing those are easy. There are exceptions to the rules. Knowing those are easy. Knowing when to use them is slightly less easy. There are exceptions to the exceptions. Knowing those is a little more tricky, and know when to use those is even more so. Our industry is FULL of all of the above.
Thanks again for all your helpful tips. I have been so busy at the lab, so I used my corbin 108ct to cut the second surface. It seems to be OK so far. Now another problem I have is the fine and polish of the poly lenses. I just posted a new threat for that, I hope you guys can give some more advice.
Put the patient in contacts and give him glasses just to find his way around the house before he puts them in.
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