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On left, the patient’s left eye has no cataract and all
structures are visible. On right, retinal image from fundus camera confirms the presence of a cataract. (From Choi,
Hjelmstad, Taibl, and Sayegh, SPIE Proc. 85671Y, 2013)
Article by guest blogger Roger S. Reiss, SPIE Fellow and recipient of the 2000 SPIE President's Award. Reiss was the original Ad Hoc Chair of SPIE Optomechanical Working Group. He manages the LinkedIn Group “Photonic Engineering and Photonic Instruments.”
The human eye and its interface with the human brain fit the definition of an "instrument system." The human eye by itself is also an instrument by definition.
After the invention of the microscope and the telescope, the
human eye was the first and only detector for hundreds of years, only to be
supplemented and in most cases supplanted by an electro-optical detector of
various configurations.
The evolution of the eye has been and still is a
mystery. In National Geographic
(February 2016) an excellent article titled "Seeing the Light" has a very good explanation of the eye's development.
Having recently had cataract surgery, my interest in the eye
was stimulated. First, I wondered why "cataract surgery" is called
"cataract surgery."
In cataract surgery, no surgery is performed on the
cataracts (cataract material). A very small incision is made in the lens pocket
and the cataract material is flushed out by using the opening to introduce the
flushing substance, and the flushing substance carries out the cataract
material through that opening. The cataract material may require ultrasonic
fracturing to reduce particle size. A
man- and machine-made lens is inserted into the opening. The opening may or may not require suturing.
This procedure should more accurately be known as "lens replacement
surgery."
Why are a large number of measurements made on the eye
before the eye surgery?
Without invasion of the eyeball, a great many measurements
from outside it must be made to determine the required focal length of the
replacement lens. (Some people do need corrective glasses to achieve the correct
value.) When I asked about all these measurements (made by high-precision
lasers) other important factors were brought up, including knowledge of the
instruments by the operator, guesswork, and finally…some luck. Luckily, without glasses distance vision is
infinite after surgery, but reading glasses are a necessity. Today, there are
many options available to cataract patients, including multifocal lenses, which
may enable complete independence from glasses.
After having lens replacement surgery myself, two haunting
questions remain unanswered in my mind.
A. Where did the optical-quality fluid (vitreous) in the
original eye lens and the eyeball come from, and how did it know where it
belonged? Optical-quality liquid or gel occurs in the human eye but nowhere
else in the human body.
B. How did the Creator (or whoever or whatever, a religious
question) -- without Physics 101 or Optics 101 or Warren Smith's book on basic
optics -- determine the focal length of the eye lens (the distance from the eye
lens to the retina; some people do need corrective glasses to achieve the
correct value). The focal length of the human eye lens is a mathematical value
based on measurements and calculations or both and could not have just evolved
without some knowledge and information about basic optics.
I wish I could answer either of these two questions but I
will have to wait for someone smarter than me. Until then let’s at least change
the name of the operational procedure to reflect what is actually being
performed, so that people will understand that their cataracts are not being
operated on but that their eye lens is being replaced.
Meanwhile, beyond these everyday procedures for improving
vision, exciting advances are emerging from labs around the world, enabled by
photonics. These include smart contact lenses for monitoring and even treating
disease. Artificial retinas under development at Stanford, USC, and elsewhere
offer the promise of vision to the blind. The results might not be as clear as
what we are used to (yet), but imaging technologies and/or nanomaterials that
send visual signals to the brain are helping counter the effects of age-related
macular degeneration and other vision problems. New devices and treatments may
offer a bright future to those with previously intractable vision problems.
Excellent article, Roger. "Lens replacement surgery" it is!
ReplyDeleteHi Roger, thanks for sharing your thoughts and experiences with this. You raise a number of interesting points. Allow me to share my thoughts from a perspective of an optical engineer immersed in ophthalmology since the 80's...The focal length of the eye (ave total optical power is ~ 58D) is a perfect optimization of field of view (>110 degrees depending on orientation) depth of field and resolution. The crystalline lens remains flexible (until we reach our 40's in most cases) to allow for good near field vision and importantly, stereopsis, without which we would not have the fine motor skills that we do. All of this does argue for intelligent design, and a Designer, but I will leave the religious implications there. On the question of the vitreous, its functions are poorly understood by most optical engineers and modelers of the eye. It is a matrix of collagen bands and hyaluronic acid that forms a very consistent optical transmission "window" throughout the visible, adds UV protection, but importantly acts as a mechanical scaffold for the eye as well as a shock absorber. It is the last line of protection for the retina, which has no real mechanical strength. Many good books on the subject, Atchison and Smiths "Optics of the Human Eye" goes into a lot of detail about the optical performance of the eye.
ReplyDeleteRoger, sorry my ID was omitted, Scott Rowe posted above.
DeleteHi Roger, thanks for sharing your thoughts and experiences with this. You raise a number of interesting points. Allow me to share my thoughts from a perspective of an optical engineer immersed in ophthalmology since the 80's...The focal length of the eye (ave total optical power is ~ 58D) is a perfect optimization of field of view (>110 degrees depending on orientation) depth of field and resolution. The crystalline lens remains flexible (until we reach our 40's in most cases) to allow for good near field vision and importantly, stereopsis, without which we would not have the fine motor skills that we do. All of this does argue for intelligent design, and a Designer, but I will leave the religious implications there. On the question of the vitreous, its functions are poorly understood by most optical engineers and modelers of the eye. It is a matrix of collagen bands and hyaluronic acid that forms a very consistent optical transmission "window" throughout the visible, adds UV protection, but importantly acts as a mechanical scaffold for the eye as well as a shock absorber. It is the last line of protection for the retina, which has no real mechanical strength. Many good books on the subject, Atchison and Smiths "Optics of the Human Eye" goes into a lot of detail about the optical performance of the eye.
ReplyDeleteI think you could say the image is consistent with a cataract but it could also be consistent with uveitis or Exposure keratitis or other media opacity like PCO a growth on an ICL or intraocular lenses lens or glistenings in intraocular lenses
ReplyDeleteInteresting blog.
ReplyDeleteRemember that people live a lot longer now than they once did. Presbyopia may be a fairly recent development, now that so many of us live long enough to experience it. Through history much of the detailed artwork was probably done by artists with extreme myopia.
Even today a lot of detailed rug weaving in the third world is done by children who can see the threads without spectacles. With several Diopters of accommodation the actual specs on the eye focal length and size were always pretty loose.
Did you choose optimum focus for distant vision or for reading? They asked my wife for her preference and she asked me for advice. I said "neither; you want hyperfocal correction, so go for 1 meter." (Prescription -1 Diopter.) Her doctor was surprised, but want along with it. They hit it for one eye and only missed by 1/4 Diopter for the other. The result has been excellent! Most of the time she does fine without glasses, but has progressives to perk up her vision at the near and far limits. She doesn't need prescription sunglasses and will be all set if she ever goes diving with a mask.
My own cataracts are changing so slowly I may never need to have them swapped out, but I'll select hyperfocal if I ever do.