This is from a sci.med.vision thread about attempting to correct higher order aberrations with lenses. Some of what I say has been mentioned in this thread, but I thought I'd add my two cents:
The HUGE problem with attempting to incorporate wavefront correcting Higher Order Aberration (HOA) correction into lenses is registration. The slightest misalignment of optics to correction causes a cascading effect and degradation of vision.
I am relatively knowledgeable of optic wavefront analysis in humans and the use of wavefront-guided corneal ablation during Lasik and similar refractive surgery with the goal of correcting Lower Order Aberration (LOA), which is sphere and cylinder, and not inducing HOA, which is commonly represented in Zernike polynomials.
I have just returned from a medical convention where iZon had a booth. I have always been suspicious of iZon's "wavefront glasses" because I understand the role of registration in HOA correction. Let me give a (believe it or not) simple example.
The Zernike polynomial Spherical Aberration (SA) can be described as the periphery of the optic having a different spherical refractive error as the center. A person can be plano in the middle of the optic and a have a 2.00 myopic ring around the edges.
Make an "O" with your right thumb and finger, put it in front of you, look through the center, and imagine that this represents the SA. Center is plano, your fingers represent a ring that is -2.00. Now make a circle with your left thumb and finger. This circle is a lens created to correct the SA. The outer ring has 2.00 diopters of correction and the center is plano. Align the two rings together. This is the theoretical correction of the HOA known as Spherical Aberration with the use of a lens. What is -2.00 in the optic of the right hand is corrected with a perfectly aligned 2.00 lens.
Now move your left hand to the left and take the two O's out of alignment. What have you done?
Where the lens (left hand) is over the plano portion of the optic (right hand), you have created 2.00 diopters of hyperopia, but only where these two planes intersect. Where the lens is over the myopic portion of the optic you have plano. Where there is no lens over the plano center of the optic you have plano. What you have created is the HOA known as Horizontal Coma, which is on the next level down the Zernike pyramid. All this by simply changing the alignment of the lens to the optics.
And that is only with SA, which is probably the least exotic of the Zernike polynomials of HOA. Each time you misalign optic to correction at a Zernike, you induce HOA for each lower level of the Zernike pyramid, cascading down through an infinite number of mathematical possibilities.
That is a long way to say that you cannot correct HOA if you cannot align the correction precisely to the aberrated optic. When you get down to the seventh level of Zernike, which is what iZon says their aberrometer measures, you are talking about alignment with a tolerance of less than a portion of microns and less than a percentage of a single degree.
Glasses cannot maintain this kind of alignment. Even contact lenses are unable to maintain this kind of alignment. Think of your toric lens fittings. Try to achieve that to within .0005 degree. And forget about what the tear film does.
I had talked with iZon's representatives before and never received an adequate answer to my probing questions about their "wavefront
glasses". At this convention I spoke at length with their representatives and this time I received comprehensive responses.
Well, at least more than I had received before.
The iZon aberrometer purportedly measures LOA (sphere and cylinder) and HOA down to the seventh Zernike level. That really is far enough as much farther and the aberrations are of proportionately less clinical significance. Anyway, they take this "wavefront" and translate it into a spherocylindrical correction which is then ground onto high quality material lenses within 1/8th of a diopter tolerance.
The representative finally acknowledged that they don't really correct HOA with their glasses, but that they use a wavefront analysis to create a spherocylindrical lens.
Okay, now that makes sense. That is also why if you check a set of iZon glasses, you will have both spherical and cylinder correction at all points on the lens. Sorry iZon, but that is not correcting HOA. That is using an autorefractor (albeit an autorefractor on steroids that is probably better than anything else on the market) and very good materials with a higher than average tolerance.
Don't get me wrong. I'm not suggesting that iZon glasses are inferior. Not at all. In fact we have received reports from patients with refractive surgery induced vision problems report that they were greatly impressed with the quality of their vision with iZon spectacles and I often suggest that a patient consider iZon and discuss them with there eye doctor. But for all that iZon glasses are, they are not correcting HOA.
I'm sure that an iZon mucky-muck would say that they have never actually said that the correct the HOA wavefront, but you have all heard the sales pitch so you draw your own conclusions. Not surprisingly my questioning raised another question that the rep could not answer. The iZon representative said that their aberrometer does not use Hartmann-Shack or Tscherning technology for their aberrometer...but could not say what they do use. That is like saying that their car does not use an internal combustion engine or an electric motor, they don't know what is under the hood, but it goes faster than everyone else's car.
I guess you can describe me as a cynic who desperately wants to believe.
BTW: Jack Holladay, MD is currently chairperson of our governing Board of Trustees. His knowledge of visual neuroadaptation is vast. If you don't believe it happens, then look at your blind spot.
Glenn Hagele
Executive Director
USAEyes (R)
Patient Advocacy Surgeon Certification
"Consider and Choose With Confidence" (TM)
Email to glenn dot hagele at usaeyes dot org
Lasik Forum
I am not a doctor.
Copyright 2007
All Rights Reserved
Originally Posted by DR.P
No less an authority?
This make no sense!
Bookmarks