Adaptive Optics

[Pete Hanlin]

The article "Technology claims to make 'super' human vision possible" (in the 8/7/00 issue of "AOA News" pg. 9), the subject of "Adaptive Optics" is addressed. To sum, it appears Dr. David Williams, of the University of Rochester has developed a system that corrects up to 65 distinct aberrations in the cornea (as opposed to two- those being sphere and cylinder). I think I've seen reference to such "wave front" technology before on the OptiBoard, but this is the first time I've seen an article in print.

According to the article, Adaptive Optics allows the human eye to exceed it's normal performance (the actual acuity mentioned is 20/10, but it is noted that the real improvements are in contrast perception in low light conditions- where some people were able to improve their visual performance by a factor of six).

Pretty exciting stuff. Does anyone have any knowledge pertaining to ophthalmic applications of the project (which the article says is now being funded by B&L)?

Pete

[Darryl Meister]

I think that there are a few practical limitations to how far they can improve the visual acuity of the eye, even when adaptive optics are applied to correct the wavefront aberration of the eye. For instance, the resolution of even an "optically perfect" eye would still be limited by diffraction (i.e., Rayleigh's criterion) and the size and spacing of the receptor cones (i.e., Hartridge's criterion). Further, many of us can already see 20/15 under ideal conditions, without perfectly corrected eyes. The improvements in contrast sensitivity in low light conditions may be a bit misleading, too. 6 cents may be 6 times more than a penny, but it still won't buy you anything. Still, I'm sure that it's a very fascinating field with a great deal of potential.

Best regards,
Darryl

[This message has been edited by Darryl Meister (edited 08-11-2000).]

[Pete Hanlin]

The facet of this research which strikes me as particularly fascinating and productive concerns consideration of corneal aberrations other than sphere and cylinder. Having seen the readings of a corneal topographer, it is quite obvious that the ametropic eye rarely has orderly curvatures evenly spaced 90 degrees apart.

As to the degree of improvement in correction over traditional spectacle lenses and/or contact lenses, you may be (and usually are :) ) correct, however. The article mentioned that people who have trialed Adaptive Optics devices have reacted with a "wow" kinda sensation. Of course, this could be the effect of trying something "new." However, they compared it to the first time you try on regular eyewear...

For now, I'm pretty happy to pick out the 20/15 line (and usually some of the 20/10 line). My only regret is that I can't take my glasses off and get blurry vision anymore (sometimes the world looks better uncorrected ;) ).

Pete

[Darryl Meister]

Hi Pete,

Yes, the cornea is very "not spherical" in shape! ;) The cornea is relatively spherical in the central region, but then flattens in curvature until it joins the sclera. I believe that vision scientists refer to "sphere" and "cylinder" errors as "first-order" aberrations. Additional aberrations -- such as coma, spherical, and oblique astigmatism -- are referred to as "third-order" aberrations. (The "order" refers to the level of exactness used in mathematically analyzing these aberrations.) There are also higher-order aberrations, which are probably not as significant. The aspheric flattening of the cornea and the pupil help minimize some of these aberrations. If there are any OptiBoarders from the realm of vision science out there reading this, feel free to add your insight (and clarify any of our thoughts, if either of us are going astray here).

By the way, if you are really interested in visual optics, I highly recommend a book written by Alan Tunnacliffe. It is published by the Association of British Dispensing Opticians and is called "Introduction to Visual Optics." It is very comprehensive, and covers everything from colorimetry to optical illusions to spectacle lenses.

Best regards,
Darryl

[Jeff Trail]

Darryl,

Any idea's where you can find that book? :) I have tried to run it down at a few web sights I use for finding books and no luck.
I even found one sight that had the B&L coachmans manual!! LOL .. pretty interesting book especially for a lens rookie to get. Skipping the part where they talk about blocking a lens with pitch of course
Any insight on a way to locate the one you listed would be appreciated .. I know my wife would just "love" for me to order, in her words, an over priced book to add to your piles of optical "junk".
You should have seen her doing back flips when I found another copy of Tylliers book on selective base curves (some one "borrowed" my other one)

Jeff " got a room and computer FULL of books, formula's and even models :) " Trail

[Shwing]

Me too, please. I searched my favorite sites , and no friends.

Gasp! Does this mean I get to actually go to my local used bookguy and wander thru the store??

[Darryl Meister]

The ABDO's e-mail address used to be

general@abdo.demon.uk

However, I'm not sure if it still is. I'll post the their telephone number once I'm back in the office on Monday. Be sure to ask them for their catalogue -- they have several good books on ophthalmic optics.

Best regards,
Darryl

[Darryl Meister]

Okay, here is the info...

The Association of British Dispensing Opticians
6 Hurlingham Business Park,
Sulivan Road
London, England SW63DU

Phone number (dial it exactly like this):
011-44-171-736-0088

Best regards,
Darryl

[Bev Heishman]

Allot of people do not understand that the normal apex of the cornea which we normally base contact lens fitting with the keratometer is only 3mm. Topography and Placido Disc's show us so much more. The book Darryl is suggesting is great. I suggest reading it.

[RP]

Folks, the adaptive optics story is pretty well on the money.

For those who think low contrast acuity isn't important, try driving at night with a slight fog on your glasses. Improvements there are probably more important than an extra line on the eye chart. Low-light conditions also increase aberrations, because the pupil enlarges and the higher order aberrations often make greater contributions to the periphery. Night vision problems for laser surgery patients might be reduced.

Many of you may be 20/15 or 20/10 on a good day, but many people can't get there - yet they are in their preferred sphere/cyl. Wavefront aberration correction has the potential to deal with this.
Imagine if sphere was the only thing currently corrected, then someone 'discovered' cylinder - there would be much discussion on whether it helped, and it wouldn't help everyone. This is just the next few 'orders' of sphere and cyl, if you like.

"Wavefront spectacles" are highly unlikely because the wavefront is optimized on one axis only. That makes it useful for laser surgery or contact lens considerations (not RGPs, as they move too much), but the payoff in spectacles would be harder to appreciate.

By the way, the theoretical limit of visual acuity as typically measured is still being debated, but is presumed to be in the range of 20/6-20/9, depending on cone spacing in the fovea. 20/10 is probably doable on most people.

Hope this helps.
Rick

[Darryl Meister]

Hi Rick,

Nicely stated. The angle-of-view dependency of wavefront spectacles should certainly limit their utility.

As you noted, the visual acuity is limited by the density of the retinal photoreceptors (Hartridge's criterion). It is also limited by diffraction (Rayleigh's criterion). Some stated values I've seen for both of these criteria limit the resolving power of a typical eye to roughly 1/2' (about 35'') of arc. This would correspond to a visual acuity of approximately 20/10.

Best regards,
Darryl

[chip anderson]

I used to work for an ophthalmologist who often said: "You can't get 20/20 with a 20/200 brain.

The cornea is not the only thing involved in getting good acuity. While it is true that I usually got better than 20/20 back when PMMA corneal lenses were the norm. I can't imagine how one would compensate like this in spectacles. The eye does not remain fixed and stare through a single spot. Forward lenses do not move with eye movements so you are shot down on a lot of miracles.