Accuracy I need accuracy..
I am testing a new generator for accuracy & need to verify lenses as presise as possible .I am presently using a Humphrey 360 with 380 software, but I sometimes question it . I need to be accurate within .01D .
Any sugestions ?
The best way to check it is go pull a couple of lens from the trial lens set :-) .. I usually do this atleast once a month in all my lensometers.. I zero out the lensometer for me .. then check it with a +2 sphere and then a -2 sphere....
Same would apply for the Humphry ... always better to check with a crown glass lens then a cr39...
Jeff "prefer the Riechert myself when it comes to digital" Trail
B&L mod 70
You will never get accuracy to 0.01D on a manual dial lensometer. The LA360 should have a setting to round (unround?) down to the 0.01D level. Some clinic models do not have this feature however - best they can round to is 0.12(5)D. Use a Younger test lens set. It contains a +/-6.00D, a +/-12.00D, axis alignment, and a thickness check.
In reference to Jeff's test, using a +/-2.00D lens - this will not tell you if the lensometer is out of calibration via the lens stop (which screws in-out. For accurate testing you must use lenses in the +/-10.00D range.
As a last resort you can reverse formulate a lens to find the power. For this you would need an accurate sag gauge and a good thickness gauge. Assuming you already know the true index of material.
JR,
He is using a Humphry's not a regular lensometer :) .. here's the problem I saw with some of the "techs" I deal with.. when they used a fairly high powered lens they were not positioning the blanks correctly and as you know the higher the power the more things happen when you "tilt" it or get it off center a ways (which they were doing) .. with my digital as well as theirs I found checking it with that lower powered lens works better..and since mine is self calibrating I have yet run into a problem anyway (knock on wood)
BTW I should have thought of just using a sag gauge and calipers to check base curve and power.. I don't know why you would want a generator that has a tolerence of less then .01^.. what about eliptical error? or tool error ? ... I would be more then happy with one thats in a .10th any day...shoot to be that exact you better have a very precise set of tools and your pads BETTER be all exactly the same grit and thickness to make sure you stay with in a .01^ ..
Oh well nice to see someone striving for perfection.. I think it's expecting a little "to much" to have it that close though..
Jeff " isn't optics great? " Trail
If anyone out there has an extra 1.00, 1.50, 2.00, or 2.50 or all, in a prism ring for an Amercian Optical lensometer please let me know!
AO made a digital lensometer years ago that could read in .01 diopters or .25 diopters. It was awesome to switch it to the .01 setting in checking if a patient had their contact lenses reversed.
If you need .01 dio. accuracy what are you making? Telescopes, microscopes, or binoculars?
Chip
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