Atoric Lenses

[eyesguy]

Hi:

In general, if a single vision Rx already has an aspheric front (cx), can its optical performance, i.e., astigmatism, lateral color, etc. be improved by any "significant" amount by using an aspheric or atoric back? Asked another way, as opposed to a sphere, is the lens performance gain worth the cost and complexity of aspheric backs? Or, is it more a cosmetic issue? I tend to think the latter.

Regards,

eyesguy

[David Wilson]

Hi Eyesguy,
A lens with an aspheric front surface is primarily designed to reduce obliqe astigmatism, though it will also reduce the effects of some other aberrations, including transverse chromatic aberration. The asphericity, however, can only be correct for one back surface. So if the person has a reasonable amount of astigmatism the asphericity will be a compromize. This is where atoroidal back surfaces come in. They can produce the ideal combination for both principal meridians. Some of the new "as worn'" progressive lenses use such surfaces.

As for front and back aspherics, most manufacturers prefer front surface aspherics, oblate ellipsoids for plus lenses and prolate ellipsoids for minus lenses. Other conicoids, such as hyperboloids, can also be used. Indeed, Mo Jalie holds a patent for a series of aspheric lenses with hyperboloidal surfaces. The minus lenses, however, are back surface hyperboloids which is not too popular with the manufacturers (despite the optical advantages).
Regards
David

[eyesguy]

Hi David:

Thank you for your reply. I recently completed an aspheric design for one of my clients and it was necessary to compromise the design in order to obtain acceptable performance in both the sphere and cylinder meridions. So your analysis makes perfect sense.

Thanks again,

eyesguy