 |
 | Home | Contact Us | FAQ | Search & Site Map | Link to Us |
 | Sign In | Join | Other 45 Sites in Network |
Medical Forum / General / Vision / July 2005Spherical aberration
I've done a little boning up on this subject and on the larger area of wave front optics. I certainly am not an expert in this area. Also, wave front optics is still a young area scientifically, and there remains considerable variation in opinions among the experts. Having said that, I have a few observations. 1. If all aberrations were eliminated, the healthy young human vision system is capable of 20/8 vision, which is actually a modest theoretical improvement over the 20/10 commonly found in that group. 2. Wave front LASIK is useful for some patients (mostly those pre and post op myopes with large pupils) in the reduction (not elimination) of higher order aberrations. It does not routinely result in 20/8 vision, because it is not perfect. 3. The "super human" vision envisioned by some is completely spun unscientific hype. 4. The relationship between high order aberrations and vision has not been adequately studied and, due to the highly subjective nature of the visual effects produced by the former on the latter, have not been well quantified. Indeed, there is not yet even a standard system for specifying those visual effects. w.stacy, o.d. William Stacy <wstacy@obase.net> wrote in news:UGsGe.2421$kk6.1476 @newssvr13.news.prodigy.com: A lot of the science is coming out of Rochester, and I've sat on some committees for students who are working in this area. > 4. The relationship between high order aberrations and vision has not > been adequately studied and, due to the highly subjective nature of the > visual effects produced by the former on the latter, have not been well > quantified. You don't necessarily need to quantify the effects to know they're disturbing, like the trefoil induced by corneal sutures from certain procedures. > Indeed, there is not yet even a standard system for > specifying those visual effects. > > w.stacy, o.d. I can see your point, but I'm not sure its important. I would think that the high-order aberrations can be removed during a preop exam using adaptive optics techniques like moving mirror arrays (used here to optically image living photoreceptors in vivo), and you can ask the patient the "which is better" type questions. I'm not sure that the higher order corrections will serve your run of the mill patient much better that current treatments, but for those patients with severe high order problems, this technology will likely be a vast improvement.  SignatureScott Reverse name to reply The only practical use I can see for this stuff is in fixing corneal damage with a wave guided laser. There is a company that claims they have invented wave front corrected spectacles (I'll believe it when I see it) and another that has similar contacts (theoretically possible, but why bother, when regular optics RGPs "cover up" most aberrations anyway due to that dandy little tear film lens. w.stacy, o.d. > I can see your point, but I'm not sure its important. I would think that > the high-order aberrations can be removed during a preop exam using > adaptive optics techniques like moving mirror arrays (used here to > optically image living photoreceptors in vivo), and you can ask the > patient the "which is better" type questions. > I'm not sure that the higher order corrections will serve your run of the > mill patient much better that current treatments, but for those patients > with severe high order problems, this technology will likely be a vast > improvement. >1. If all aberrations were eliminated, the healthy young human vision >system is capable of 20/8 vision, which is actually a modest theoretical >improvement over the 20/10 commonly found in that group. That is what the industry thought, but that is not the reality. The reality is that the lack of optical aberrations does not necessarily improve the quality of human vision. This is because of the neurological process of vision. Doctors at the university in Rochester created an adaptive optics device that could be adjusted to eliminate all wavefront error using corrective lenses and adjustable mirrors. The problem is that when a patient had "perfect" optics, they complained of poor vision quality. Reintroduce the aberrations and the patients are able to see again. The bottom line is that some aberrations are "good"; not because they improve optics, but because throughout a lifetime the brain learns to use those aberrations to its advantage. Change them, and the vision system becomes confused. This is sometimes called aberration adaptation. >2. Wave front LASIK is useful for some patients (mostly those pre and >post op myopes with large pupils) in the reduction (not elimination) of >higher order aberrations. It does not routinely result in 20/8 vision, >because it is not perfect. You are talking about ablation on biological tissue, not incision on plastic. There is the whole flap variable with LASIK, and the exact wound response of an individual's cornea cannot be precisely predicted - only generally predicted. As a simple example, a 10% difference in corneal hydration can cause as much as a diopter of over/undercorrection. Wavefront refractive surgery does not reliably and predictably reduce higher order aberrations. Wavefront tends to increase HOA less than conventional ablation. On the whole, wavefront-guided ablations provide vision outcomes superior to conventional ablation, but are not perfect and never will be perfect. >3. The "super human" vision envisioned by some is completely spun >unscientific hype. The hype is that doctors can predictably provide "supervision". The reality is that it does occur on occasion but not predictably. >4. The relationship between high order aberrations and vision has not >been adequately studied and, due to the highly subjective nature of the >visual effects produced by the former on the latter, have not been well >quantified. Indeed, there is not yet even a standard system for >specifying those visual effects. I'm not exactly sure how you mean this statement. Wavefront is not a new technology. It has been around since the 1600s. Its application to human vision is new and that does need a tremendous amount of study, however not knowing everything about something does not mean you cannot use what you know. We don't know what electricity is, but that does not mean we need to sit in the dark. The standard system for specifying visual effects is somewhat self contained. Wavefront aberration mapping is the standard system for evaluation HOA changes. Glenn Hagele Executive Director USAEyes.org "Consider and Choose With Confidence" Email to glenn dot hagele at usaeyes dot org http://www.USAEyes.org http://www.ComplicatedEyes.org I am not a doctor. On 7/29/05 10:32 AM, in article huoke15t6d7omejmbcscsfgde6v6e967u0@4ax.com, >> 1. If all aberrations were eliminated, the healthy young human vision >> system is capable of 20/8 vision, which is actually a modest theoretical [quoted text clipped - 16 lines] > system becomes confused. This is sometimes called aberration > adaptation. I find that very interesting. Were the experimenters able to remove off-axis aberrations such as coma and true astigmatism? I certainly would not expect that to have been done. In that case, the on-axis correction causes off-axis vision deterioration. Bill > I find that very interesting. Were the experimenters able to remove off-axis > aberrations such as coma and true astigmatism? I certainly would not expect > that to have been done. In that case, the on-axis correction causes off-axis > vision deterioration. It is being done clinically now. Not perfectly, as Glenn pointed out, but is being done. Any off axis (peripheral) deterioration may or may not be noticed by the patient as mentioned in my last post, and is probably insignificant. What works for human vision certainly won't work for cameras, because the granularity of film/digital pickups is uniform, while the human retina has a huge variation. You can easily visualize this variation by looking at the "x" below and trying to read anything on else on your monitor while doing so... x w.stacy, o.d. > I find that very interesting. Were the experimenters able to remove > off-axis aberrations such as coma and true astigmatism? I certainly > would not expect that to have been done. In that case, the on-axis > correction causes off-axis vision deterioration. > > Bill http://www.journalofvision.org/4/4/4/ They remove ALL aberrations. They use the same adaptive optics sytems that the telescope guys use to correct for irregularities in the atmosphere. When turned around the other way (i.e., for an observer to view the retina), which is what the system they use was actually designed for, it lets them correct for any irregularities along the light path and view photoreceptors at the single-cell level. SignatureScott Reverse name to reply > I'm not exactly sure how you mean this statement. Wavefront is not a > new technology. It has been around since the 1600s. ok so maybe my terminology was a bit loose. Most people understand the current discussion to be about the new wave front LASER applications, esp. as they relate to vision, which is quite recent. On 7/29/05 12:11 PM, in article rbvGe.1842$iM7.64@newssvr21.news.prodigy.com, "William Stacy" <wstacy@obase.net> wrote: >> I'm not exactly sure how you mean this statement. Wavefront is not a >> new technology. It has been around since the 1600s. > > ok so maybe my terminology was a bit loose. Most people understand the > current discussion to be about the new wave front LASER applications, > esp. as they relate to vision, which is quite recent. I think that wavefront as used for LASIK is just recent application of interferometric measurement. The deviation of the surface from that required for ideal imaging, in this case the corneal surface, is measured in wavelengths. Subsequently, that tells you how much material to remove to compensate for the structural optical error. In other words, using the measurement technique, it is now possible to find out how much of the cornea to remove at any point to correct refractive error. The laser lets you remove it. That is, it is now possible to figure a living cornea to compensate for error. Bill On 7/29/05 9:20 AM, in article UGsGe.2421$kk6.1476@newssvr13.news.prodigy.com, "William Stacy" <wstacy@obase.net> wrote: > I've done a little boning up on this subject and on the larger area of > wave front optics. I certainly am not an expert in this area. Also, > wave front optics is still a young area scientifically, and there > remains considerable variation in opinions among the experts. The study of aberrations has a history going back over a century. Studying aberrations in terms of wavefronts is somewhat newer but still old. See the classic by Born and Wolf. Wavefront technique applied to ophthalmology is relatively recent. Optical shops outside the vision trade have long used wavefront correction to improve the performance of components. The process is called "figuring." After measurement, small errors are removed by local polishing. The Hubble telescope mirror was figured incorrectly when made. > Having said that, I have a few observations. > > 1. If all aberrations were eliminated, the healthy young human vision > system is capable of 20/8 vision, which is actually a modest theoretical > improvement over the 20/10 commonly found in that group. I find it hard to believe that any any aberration other than spherical or chromatic is important to human vision. What optometry calls "astigmatism" does not correspond to astigmatism as used in other optical fields. In a camera lens, astigmatism is an off-axis aberration. Nevertheless, optometric astigmatism contributes a cylindrical wavefront component that can be corrected with a cylindrical lens or figuring. > 2. Wave front LASIK is useful for some patients (mostly those pre and > post op myopes with large pupils) in the reduction (not elimination) of > higher order aberrations. It does not routinely result in 20/8 vision, > because it is not perfect. I am not sure what you mean by higher order aberration. From the little I know of LASIK, its primary goal is to get rid of refractive errors that are ordinarily not considered aberrations in other fields of optics. That is, the corrections remove spherical focus error and cylindrical focus error. It cannot do anything about chromatic aberration which is not a wavefront error per se. My guess is, that for most people, true aberration, especially high order aberration, is not a big problem. I suppose a conical cornea could be largely corrected using wavefront techniques. > 3. The "super human" vision envisioned by some is completely spun > unscientific hype. [quoted text clipped - 4 lines] > quantified. Indeed, there is not yet even a standard system for > specifying those visual effects. My guess is that any wavefront correction for off-axis vision is unnecessary and will cause on-axis vision to deteriorate. Pilots, for example, primarily use their central vision. Off-axis vision is used primarily to know where to look with central vision. Thus, if I were in the market for LASIK I would go for the best possible on-axis correction with a large pupil. Even so, when it gets dark enough, good central vision will disappear anyway. In that case, off-axis vision will have been degraded by the correction for central vision. > w.stacy, o.d. Bill > I find it hard to believe that any any aberration other than spherical or > chromatic is important to human vision. In normal, healthy eyes neither of those types is very important either, which is kind of where this whole discussion started. > I am not sure what you mean by higher order aberration. The wave front LASER people have created a paradigm that calls dioptric errors (defocus, regular astigmatism, and chromatic aberration) lower order aberration, since they can be expressed in ordinary diopters. They call all other aberrations higher order because they can only be expressed in terms of zernicke polynomials, complex curve fitting formulas. These include spherical aberration, coma, irregular astigmatism, field distortion, trefoil, quadrafoil and others that don't even have names because they are pretty insignificant. Astigmatism and spherical aberration are odd in that the classic optical definitions of those terms are not the same as the wave front definitions. From the little I > know of LASIK, its primary goal is to get rid of refractive errors that are > ordinarily not considered aberrations in other fields of optics. That is, > the corrections remove spherical focus error and cylindrical focus error. For ordinary LASIK you are right, but for wave front guided LASIK, the "higher order" aberrations are attempted to be corrected, especially in large pupils where they do have an effect. Probably the most promising use of this new technology is in rehabilitating roughly shaped, diseased corneas. > My guess is, that for most people, true aberration, especially high order > aberration, is not a big problem. I suppose a conical cornea could be > largely corrected using wavefront techniques. Right, except keratoconus usually means abnormally thin corneas, so unfortunately is not a good candidate. > My guess is that any wavefront correction for off-axis vision is unnecessary > and will cause on-axis vision to deteriorate. Pilots, for example, primarily [quoted text clipped - 4 lines] > case, off-axis vision will have been degraded by the correction for central > vision. Actually, wave front LASIK aims at para-central rays. These are parallel with the axis, bot not coincident with them, which is why pupil size is so important. It doesn't really deal with your "off axis" rays that we call peripheral vision. Peripheral vision blur is not considered a big deal because acuity in the periphery is so poor anyway. w.stacy, o.d. William Stacy <wstacy@obase.net> wrote in news:RyvGe.2499$kk6.1778 @newssvr13.news.prodigy.com: > Right, except keratoconus usually means abnormally thin corneas, so > unfortunately is not a good candidate Believe it or not, I think I've heard that post-op keratoconus patients with corneal transplants are Lasik candidates SignatureScott Reverse name to reply Yes, some DLKP and PKP patients have had refractive surgery, but PRK or LASEK with Mitomycin C would make more sense. There is less IOP raise and more untouched tissue. Much depends upon the individual circumstances. I would not say that refractive surgery on a transplant patient due to keratoconus is common. Glenn Hagele Executive Director USAEyes.org "Consider and Choose With Confidence" Email to glenn dot hagele at usaeyes dot org http://www.USAEyes.org http://www.ComplicatedEyes.org I am not a doctor. On 7/29/05 12:36 PM, in article RyvGe.2499$kk6.1778@newssvr13.news.prodigy.com, "William Stacy" <wstacy@obase.net> wrote: >> I find it hard to believe that any any aberration other than spherical or >> chromatic is important to human vision. [quoted text clipped - 14 lines] > spherical aberration are odd in that the classic optical definitions > of those terms are not the same as the wave front definitions. Unfortunately, it appears too late to avoid another discrepancy between optometric and optical nomenclature. In my training, defocus would not be considered an aberration at all because it is correctible using spherical refractive surfaces. <snip> > Actually, wave front LASIK aims at para-central rays. These are > parallel with the axis, bot not coincident with them, which is why pupil > size is so important. It doesn't really deal with your "off axis" rays > that we call peripheral vision. Peripheral vision blur is not > considered a big deal because acuity in the periphery is so poor anyway. <glenn.hageleSTOPSPAM@USAEyes.org> had an intersting post here on that subject Bill > My guess is, that for most people, true aberration, especially high order > aberration, is not a big problem. Most people experience a big dose of spherical abb when they get their eyes dilated. It's surprising that people with large pupils don't walk in with a special set of problems. -MT On 7/29/05 6:01 PM, in article bkAGe.10956$oZ.5386@newsread2.news.atl.earthlink.net, "Mike Tyner" <mtyner@mindspring.com> wrote: >> My guess is, that for most people, true aberration, especially high order >> aberration, is not a big problem. [quoted text clipped - 6 lines] > > -MT I have wondered about that. Apparently, there is insufficient selective pressure to make aspheric refractive surfaces in real eyes. Diurnal people's vision probably has other requirements. If there is an intelligent designer, he/she forgot about aplanatic surfaces. Are there any nocturnal animals that do have aspheric optical systems? Do they also have cones for central vision? Bill > I have wondered about that. Apparently, there is insufficient selective > pressure to make aspheric refractive surfaces in real eyes. Diurnal people's > vision probably has other requirements. If there is an intelligent designer, > he/she forgot about aplanatic surfaces. Here we go again. No, the cornea is not spherical and is indeed aspheric and was definitely "designed" correctly. It is precisely the flattening in the normal peripheral cornea that corrects what would be a horrific amount of ordinary spherical aberration under low light conditions if we had spherical corneas. We don't, and as a result, have very little spherical aberration. w.stacy, o.d. On 7/29/05 8:23 PM, in article DoCGe.2721$kk6.2286@newssvr13.news.prodigy.com, "William Stacy" <wstacy@obase.net> wrote: >> I have wondered about that. Apparently, there is insufficient selective >> pressure to make aspheric refractive surfaces in real eyes. Diurnal people's [quoted text clipped - 7 lines] > conditions if we had spherical corneas. We don't, and as a result, have > very little spherical aberration. Sorry about that. I really should not have implied that the cornea was spheric. What I do ask is if the eye's optical system is aplanatic or whatever the correct term is when the image surface, like the retina, is not plane. So I will repeat my questions in a different way. Is the human eye close to being aplanatic because of its aspherity? I doubt it because, as stated in earlier posts, eye performance degrades with larger pupils. Even with perfect aplanaticity there is a loss of depth of field that might subjectively appear to be degraded acuity. Again, does the typical nocturnal animal such as an owl or cat have better aplanaticity than a human? Bill I've read that the trilobite had an aplanatic eye. DrG > Sorry about that. I really should not have implied that the cornea was > spheric. What I do ask is if the eye's optical system is aplanatic or > whatever the correct term is when the image surface, like the retina, is not > plane. I'm sure this has been investigated, but not by me, except that it would make sense, since the retina is concave, the optics would correspond to that. I will say that the region of the eye that is most concerned with focus/defocus and image quality is the macula, which can be considered, at a first approximation, to be flat. > So I will repeat my questions in a different way. Is the human eye close to > being aplanatic because of its aspherity? I doubt it because, as stated in > earlier posts, eye performance degrades with larger pupils. Even with > perfect aplanaticity there is a loss of depth of field that might > subjectively appear to be degraded acuity. I think it is, within the norms of pupil size. When you talk about huge pupils sure, you are bound to encounter the edges of the optical part of the cornea and get some degradation from the limbal area. That part was not designed to be used for vision, just to make the structural connection to the scleral. It is thicker and less transparent, so you'd expect the image to degrade. Like opening up the F stop of a camera so wide that you get lens edge/housing whatever in the mix. > Again, does the typical nocturnal animal such as an owl or cat have better > aplanaticity than a human? Yes. They have larger pupils so the optics are better in the areas they actually use on a regular basis. We humans mostly stayed in at night during our design process. w.stacy, o.d. On 7/30/05 7:18 AM, in article J_LGe.8343$_%4.4223@newssvr14.news.prodigy.com, "William Stacy" <wstacy@obase.net> wrote: > I think it is, within the norms of pupil size. When you talk about huge > pupils sure, you are bound to encounter the edges of the optical part of [quoted text clipped - 3 lines] > expect the image to degrade. Like opening up the F stop of a camera so > wide that you get lens edge/housing whatever in the mix. "Apodization" is a term that is probably not used much in optometry. The transmissivity decreases ("less transparent") toward the periphery of the pupil. It is used to avoid diffractive ring patterns near focal spots. I do not expect that eye optical quality is good enough to where apodization cab be useful. I do not see fringes around text on my computer monitor. The reduction in transmissivity must serve some other evolutionary purpose. Even in bright illumination, when pupils shrink, I never see any indication that apodization could be useful. Bill You gotta be talking huge huge pupil for that to occur. Just my opinion. DrG
|
Reply to this Message |
 Sign In
 Join
 My Latest Posts My Monitored Threads My Blog My Photo Gallery My Profile My Homepage Start New Thread Enable EMail Alerts Rate this Thread
|
©2008 Advenet LLC Privacy Policy - Terms of Use
This website includes both content owned or controlled by Advenet as well as content owned or controlled by third parties.