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Thread: the speed of light was fun lets try another..

  1. #1
    Master OptiBoarder Jeff Trail's Avatar
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    Since you guys like to go back and forth on and throw optical theories about then I thought I would let you in on a raging debate me a couple of OD's an MD and Retinal Specialist have been bashing about for the last few years..
    It's prism :-) It all started over Fresnel press on prism and has grown from there..
    namely the idea that an elderly guy was prescribed 30^ base out (yikes) well here is my point of view.. the corneal apex forms the anterior pole. opposite is the posterior pole on the temperal postion to the optic nerve..now the optic axis joins the 2 poles. (so far easy right?) ..NOW the fovea centralis set temporally and below the posterior pole..so the visual AXIS and the optic AXIS are NOT coincident!! ..OK so far, right? ..think of a "X" ..stretched, the visual axis at one point, the optic axis at the other side then they cross at the "equator" (well close to it) then spread out again at the fovea centralis..now here is where we get into the "crux" of the hmm "debate" .. My point is that you have to break down the fovea centralis (rod & cones) ..the RODS are very sensitive to light, they are responsible for low light conditions (night vision etc.) the rods do NOT "see" the color spectrum per se so they produce "colorless vision" ..now CONES do break it down (blue, green, red) ..though low light source is not enough to get you to "see" color in low light conditions with the cones... NOW to get to the "meat" of it.. Cones & rods both connect with the bipolar cells. The cone fiber is dominate while the rods fibers are devided...
    Now we breakdown the fovea.. it's about 16 to 1 (rods to cones) I'm going from memory but it's like 110 million to 6 million (something real close to that.. but the crux is where they are located and the concentration..the rods are located centrally and drop off rather quickly, there are NO rods at a small area in the central fovea zone.. while the rods are at maximum around 5 mm away from the fovea..
    SO .. I think you can hit it with prism (the rods) more so the the cones..BUT there is a limit to the amount of diviation of "light" and still get a actual image that can be of any use.. The way I see it you can "bend" more through prism but you would be flashing across the rods and cones and only the perception of "brighter" light would be acheived but NOT any usuable images.. while less prism would give you not as great "acuity" but give you an actual image that you can "see" .. amybe "blurry" but it sure beats just "brighter light" in my opinion...
    The second part of my theory is that the time difference of reproduction of the rods and cones.. rods can do it far qiucker then cones.. so I've been batting around the idea that if you actually stimulate the rods and cones with prism (even in the case of MD if diagnossed soon enough) then you can give it a slight "push" and stimulate regrowth and "trick" the eye to thinking (I know it has no brain but give me some slack) that even though centrally you have loss you bend light across the fovea at an angle to hit the rods and cones...and then gradually cut back the amount of prism to get images over a time period..
    well anyone want to add to the debate? .. any idea's? ... am I off my nut? (optical nut :rolleyes :) Well you guys seem to have opinions.. how about it..

    Jeff "just, still, a lab rat.. but a curous one" Trail

  2. #2
    Master OptiBoarder Shwing's Avatar
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    But Jeff, if one were to solve an optical issue such as this, then what fun would there be in optics??

    As I posted to Pete a while back, if surgery were the answer, then we would be out of business, and the world would be a better place.

    If I had been born 500 years ago, I wouldn't have made it past age 15, due to my ammetropia. (think rock thrown at you, but you don't know where from...)

    As I comment to people: "if everyone were perfect, I'd be out of a job." This applies not only to refractive errors, but also (and thus a continuance of)spectacle adjustment.

    If you could buy it off the shelf like butter (or margarine, or oleo) then great, it's for all... (it ain't butter, it's...)

    Huh?

    Shwing;-]

  3. #3
    Master OptiBoarder Darryl Meister's Avatar
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    Hi Jeff,

    I don't know that I followed your logic entirely, but it seems to me that you are proposing the use of prism to move the image across various regions of the retina to accomplish certain visual effects.

    I would say that this is generally not practical. Although prism deviates the image across the retina, the eye makes a compensatory movement to re-align and place the image of interest back on the fovea centralis for critical vision. (Or at least within a small region surrounding it called Panum's fusional area.) Visual acuity drops off rapidly away from the fovea, so you wouldn't use this region for critical vision. Incidentally, I believe that this is related to those bipolar cells you mentioned -- and the number of photoreceptors linked together within a given region of the retina, which creates an area called the 'receptive field.' Receptive fields become larger away from the retina.

    Spectacle wearers literally have to be trained to use regions of their retina away from the fovea for critical vision. For instance, low vision patients with macular loss can practice "eccentric fixation" to use regions of the retina slightly away from the fovea for reading and critical vision. This takes a great of effort at first. You can get an idea as to how this works by fixating on an object while trying to read some print to either side of it (without looking away from the object).

    The refractionist who prescribed that much yoked prism might have prescribed it for eccentric fixation, or perhaps to compensate for some sort of musculoskeletal disorder. Yoked prism is also used periodically for certain binocular vision disorders.

    Best regards,
    Darryl

  4. #4
    Master OptiBoarder Jeff Trail's Avatar
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    Originally posted by Darryl Meister:
    Hi Jeff,

    I don't know that I followed your logic entirely, but it seems to me that you are proposing the use of prism to move the image across various regions of the retina to accomplish certain visual effects.
    Especially in impairment cases (MD, ocularhistoplasmosis, retinal Pigmentosis, etc.)

    I would say that this is generally not practical. Although prism deviates the image across the retina, the eye makes a compensatory movement to realign and place the image of interest back on the fovea centralis for critical vision. [/I]
    Ah but in cases where central vision is impaired you do not get it, to try fusion, especially if it is OU ... we have been tinkering with this for a few years now and have gotten some surprising results, and EVEN the MD agree'd so (be it very grudgingly :))


    (Or at least within a small region surrounding it called Panum's fusional area.) Visual acuity drops off rapidly away from the fovea, so you wouldn't use this region for critical vision.[/I]
    Right, but I was posting some of the pathology to show where the rods and cones are at and the area they covered and what they did, and you can strike the fringe of the cones and since a macula problem is hardly ever aspherical (more irregularly shaped) you can find the farthest point in and direct your prism there..very time consuming but with a trial frame I can work with some cases and get some remarkable results ..



    Spectacle wearers literally have to be trained to use regions of their retina away from the fovea for critical vision. For instance, low vision patients with macular loss can practice "eccentric fixation" to use regions of the retina slightly away from the fovea for reading and critical vision. This takes a great of effort at first. You can get an idea as to how this works by fixating on an object while trying to read some print to either side of it (without looking away from the object).[/I]
    These are the guys we have been trying to tinkering with most often.. Compared to the alternative (holding it close and trying to fixate at an angle and have very limited field, then I think we have had some pretty good results..doesn't hurt to try. I know the MD said I was wasting my time does not say that as often now when he chaired them before and AFTER I tinkered, it may not be practical and I do it more or less for free and on my own time, but worth it if I can do more then just "over plusing" the heck out of someone or handing them a magnifier and telling them "sorry nothing we can do"

    The refractionist who prescribed that much yoked prism might have prescribed it for eccentric fixation, or perhaps to compensate for some sort of musculoskeletal disorder. Yoked prism is also used periodically for certain binocular vision disorders.[/I]
    I just mentioned it because that was what started all this debate we have been having for the last few years I STILL contend that ground prism is going to give you better optics any day of the week then press on... sorry I'm just stubborn on this one :) Also I still think you can accomplish more, at times with LESS prism then more, I'm just going on what I think, but from my tinkering if you go to far in prism you always seem to get the same answers.."oh it is brighter" but I am going for images not "brightness", so bending it so far where it cross the receptors and all you get is "dark and light" is not my goal.. Maybe I'm off my nut on this one, but I have a good reason to do it , my Dad has ocularhistoplasmosis and that's the MAIN reason I dug into all this in the last few years, selfish? probably... but I'm going to keep trying all the tricks I know..
    Thanx Darryl.. I had hoped more people would have jumped into it but I think anatomy and pathology is not the strong points on this site :)

    Jeff"Pis$ed I had to miss the Vegas show" Trail


  5. #5
    Hi, I'm an optometrist with some experience in Low Vision.
    The use of prism to move an image is an old idea and is most often used to expand the peripheral field of vision with patients who have intact retina but cortical vision loss ie stroke victims. If they have lost the vision on the right side due to brain damage, then one tries to shift the images on the left retina (right side of world) unto the right retina (left side of world)with yoked prism. Since the brain is still working for the right retina, they see. This meets with middling success.

    Your idea of moving central fixation rarely works since the eye will move to place the image on centre. Eccentric fixation can be trained but it takes a very motivated patient, lots of practice and lots of time. Prism can be used in the training to give the idea, but ultimately, if the training works the prism isn't needed. And, since the visual acuity drops off sharply, the central vision has to be very bad (worse than 20/200) before the non diseased eccentric view is better than the diseased central view.

    Macular degeneration that has progressed to that level usually also has a large diameter central loss and scarring so the eccentric point is then placed a longer way out where the acuity is even worse.

    For these severe cases, referral to a Low Vision Clinic that can provide closed circuit TV viewers or spectacle mounted telescopes is in order.

    Nice to see you were thinking about the problem from the anatomical angle.

    ------------------
    Judy B

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    Master OptiBoarder Darryl Meister's Avatar
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    Originally posted by judyb:
    The use of prism to move an image is an old idea and is most often used to expand the peripheral field of vision with patients who have intact retina but cortical vision loss ie stroke victims. If they have lost the vision on the right side due to brain damage, then one tries to shift the images on the left retina (right side of world) unto the right retina (left side of world)with yoked prism. Since the brain is still working for the right retina, they see. This meets with middling success.
    Those almost sound like hemianopia spectacles???

    Best regards,
    Darryl

  7. #7
    sub specie aeternitatis Pete Hanlin's Avatar
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    Hi everybody (as Dr. Nick on the Simpsons would say)!

    Just to interject something that's rather simplistic into this, well, interesting thread... You do realize that the relative visual acuity through 30 diopters of prism would be < 20/40 (assuming the individual could actually see 20/20 if no prism were present) due to the effects of chromatic aberration? This is assuming you use crown glass or some other material with an Abbe around 60- a high index lens of any kind would make the lens about unusable (I would presume).

    I've read about specialized prismatic lenses that induce the effects drunkeness has on vision (they use them with teenagers in driving school to point out the effects of D.U.I.). Apparently the lenses displace light to other areas of the retina in such a manner as to require extra effort to maintain proper visual comprehension.

    Most of the prism we deal with in low vision applications concerns bifocals with prism added to the segments (to facilitate convergence with high power adds).

    Regardless of index, it would be interesting to see if a manufacturer could come up with a material with an ultra high Abbe value. Any thoughts on whether a material of say >70 Abbe might be possible, Darryl?

    Pete

  8. #8
    Master OptiBoarder Darryl Meister's Avatar
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    Originally posted by Pete Hanlin:
    Regardless of index, it would be interesting to see if a manufacturer could come up with a material with an ultra high Abbe value. Any thoughts on whether a material of say >70 Abbe might be possible, Darryl?

    Pete
    Hi Pete,

    Although I don't know what the feasibility of creating a super high Abbe material would be without consulting one of our chemists, I can say that it would generally be unnecessary. Studies have shown that a CR-39 lens (with an Abbe of 58) would not appreciably affect visual acuity until the power was well above +/-6.00 D. Increasing the Abbe would likely require a lower refractive index, which would be an undesirable compromise in most instances.

    Best regards,
    Darryl

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