Is possible the power of the lenses will change after Hard Coating...?
Power before coating is +2.11 sph, which is under the tolerance for +2.00 sph. But after Hard coating and multi-coating I got the power +2.13. This will make the power +2.25 on 0.25 step power checking, and it is out of tolerance also. So I am little bit doubted to give it to customer.
Is any body know that after coating(specially hard coat) the power can change...?
and the tolerance for the ophthalmic lens is for finished lens? or there is any tolerance for before surfacing?
It's possible that the hardcoating will change the power, though very unlikely, and the coating would have to be especially thick to do so (Is it possible that it got coated twice?). I know you're in Kuwait, but here in the US, that lens would be fine to use. We usually round to the nearest 0.125 diopter (although admitedly it is 0.005 outside of tolerance).
I would say that 0.02 diopters is within statistical variance when checking a lens multiple times.
Usually the only time I've seen power changed after hard coating is on Minus (-) lenses, where either the heat or suction on the spin coater has changed the front curve of the lens (usually the lens is too thin as well).
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, I love the "Making Fiends" Cartoon avatar! I love that series!
Did you check you reticle beforehand? You're talking about amounts of power that can't really be accurately measured- I get variations like that moving the same lens in and out of the lensometer. Did the lens cool to room temp. before measuring? Did you use the lens support? Are all the optics clean and ready? Are you using Auto-lensometer? I'm always amused by labs/lens equipment companies talking about how their new lenses are accurate to 1/100th a diopter. What for? Are we going to upgrade the standards to show accuracy down to say 1/10th a diopter? How about phoropters? Trial lenses? Etc. Not needed, not discernable, not wanted. Which is better 1,2,3,4,5,6,7,8,9 or #10? Huh? which is it, hurry up now...Originally Posted by WFruit
Chris Beard
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Given that the ANSI standards state tolarances, for example, +/- 6.50 diopters as 0.13 (thirteen hundredths), I'd say the standards have been upgraded.....
What I think most people either forget or don't realize, is that there is a line, sometimes a very fine line, between what a patient can tolerate, and what they can't (and of course that can vary wildly from patient to patient). Sometimes (probably most of the time) a patient can see just fine on a +2.15 (+2.00 prescribed, rounds to 2.13 in the lensometer) despite it technically being out of tolerance. But sometimes that little extra bit is too much and they can't see.
Of course it's not usually economically feasable to remake every lens that's 0.01 out of tolerance, but I think it's a very good think that the lab is checking and concered about whether they should use the lens or not.
@Sharpstick777 Amy Winfrey is a genius. Very, very sick person, but brilliant non the less. IF you haven't already, you should watch her web episodes, and check out all of her other work. All of which can be found here: http://makingfiends.com/
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.
Heck you can get that much variation just from wearing glasses, especially if your wearing progressives .
Just wait till the ECP's start ordering in 100's of a diopter!
I'm already getting some in 1/10 diopter. :(
Dear all
Thanks for your suggestions and comments about this matter. But still we are getting the power variations after coating..recently we noticed in 1.74 lenses the Rx is getting more 0.03 to 0.06 which is really making us in confusion. we checked the oven temperature and all other procedures, all are perfect. another main problem is sometime the lenses are getting after surfacing is very near to the tolerance. If we do the coating on that lenses the power will jumped to next step. (not only increasing the Rx after coating, some time decreasing too)
now the main question is how we will overcome this situation?
Are you actually sagging the lenses and checking them to what your database shows? I never trust the database numbers, as they are nothing more than an average of a given production run. Sags will vary by lens.
Being that you are in Kuwait, are you chilling your fining water and polish? You temps should be around 5 C.
Ya we checked the polish temperature .. now we are using new chiller to make the temp under 8C
What about checking your sags on the semi-finished lenses?
seldom we are checking the sag in SF lenses.. because for each lenses is practically not possible as you knows if the production is more..some lenses we are getting the sag which is different than what it actually should be.
That's your problem right there.
It doesn't take much difference to throw off the power of the finished lens. If the sag in your data base for a 6 base lens is 3.69 and the actual sag is 3.61, that's 0.125 difference in base curve, and there's your lens power problem in a nutshell. 1/8th diopter is equal to .08 mm sag with a 50 mm bell.
Lens databases are notorious for sagittal data not reflecting current production values.
That's most likely why you are getting +2.00 lenses that are reading +2.11. Fix the sag problem and you won't have to worry about any issues with coating adding a tiny bit of power to the lens.
I'd also check your tooling to ensure that you have a good tool and not one that is mis-marked or racked back in the wrong pocket.
Last edited by MikeAurelius; 08-03-2010 at 02:10 PM. Reason: grammer and additional thoughts
I sag various brands of lenses often and they rarely have a .02mm sag variation.
What are the increments of your laps (1/8, 1/10, 1/16?) at 1.74 it's very important.
What was the computer print-out of expected power of the +2.00? (+1.97, +2.04, +2.10?)
Computer print after surfacing is +2.11, and we have the laps 1/4 and 1/8.
but some time our surfacing technician using plastic tape(blocking tape) to adjust the tool to get 1/10 or 1/16.
almost time the power is getting good.. (but near to the tolerance). When I asked the reason for it to the technician, his answer was
We are using Rxpert for calculation.. if we use the recommended tool its not giving us perfect power. so that he is sticking the tape on the tool to get the correct power because of unavailability of 1/10 and 1/16 laps.
Computer print after surfacing is +2.11, and we have the laps 1/4 and 1/8.
but some time our surfacing technician using plastic tape(blocking tape) to adjust the tool to get 1/10 or 1/16.
almost time the power is getting good.. (but near to the tolerance). When I asked the reason for it to the technician, his answer was
We are using Rxpert for calculation.. if we use the recommended tool its not giving us perfect power. so that he is sticking the tape on the tool to get the correct power because of unavailability of 1/10 and 1/16 laps.
You should be using at least 1/8 increments on your tools.
OK...but I'd contact the software supplier to find out why the software is calculating the power so close to the cut-off point. That doesn't make any sense to me at all. It almost sounds like there is a built in 1/8th error in the calculations.
Your technician is on the right track, however, if he substituted the next tool up (example a 537 instead of a 525), he'd nail the power dead on. Merely putting tape on a tool isn't going to change the power all that much, all that's happening is he's essentially changing the pad compensation. He'd have to layer several stripes of tape to achieve what he wants, so it is far better to just move to the next tool instead.
Not on the power you list above, however I have seen thinner minus lenses ruined, due to the suction of the holder and the heat of the curing lamp. Make sure the suction is not turnrd way up. Also I found it nessessary on some lenses to piggy back them to another lens with a leap pad in the curing process.
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