How to measure aspheric lens power ?

[grandotitat]

I have a fundus lens with double aspheric surfaces (a common 20D lens for indirect binocular ophthalmoscopy). I'm wondering if you can measure the power of this kind of lenses (78D, 90D etc) in a regular lensmeter (manual or auto) or this kind of surfaces requiere a different equipment? I have tried in a couple of lensmeters but one of them reads +18 diopters and the other +16.75. The one reading 18 is the popular NJC-4 chinese model and the one reading 16.75 is a Topcon LM-P4 which might requiere measue adjustment but still both of them are way off the supposed 20 diopters of the lens. Or are these type of lenses just named "20D" but their real power is less?

Thank you for the inputs!

[Lensman11]

You should be able to read this in a standard lensometer. Many Lensometers are inaccurate in the higher power lenses. Take a 20 drop trial lens and see what that reads in the lensometer.

[ChrisBowers]

Lensometers are made to measure power on the concave side lenses. Your double aspheric might be changing the vertex depth expected by the machine as well as a small aspheric design measured in the 7 mm reticule. Again, if this is a yellow lens you might be experiencing false focal points. If I may ask, why do you want to measure this. What effect are you trying to prove or disapprove of? Lastly, I think the manufacturer should validate the lens and its measurement process.

Chris

[dima]

Usually Auto Lens meter have a range of 25-30 dpt max

[grandotitat]

Lensometers are made to measure power on the concave side lenses. Your double aspheric might be changing the vertex depth expected by the machine as well as a small aspheric design measured in the 7 mm reticule. Again, if this is a yellow lens you might be experiencing false focal points. If I may ask, why do you want to measure this. What effect are you trying to prove or disapprove of? Lastly, I think the manufacturer should validate the lens and its measurement process.

Chris

Hi Chris. I was asking because i work on tech service for lensmeters within other devices for optometry/ophthalmology. I was requested to calibrate and adjust a lensmeter, which i did using a gaugemaster as a pattern. Then the customer comes back telling me that measurments were off and i found they were using a 20D double aspheric fundus lens a verification pattern. So their logic is that if it is a 20D lens, then a lensmeter should read 20 diopters. I wasn't able to confirm of disprove their logic but the point is that some lensmeters will read the 20d but others won't, so it makes sense your comment on false focal points.

Thank you everyone for the inputs!