Two-mirror device for strabismus correction

Discussion in 'Optometry Archives' started by David Paterson, Apr 1, 2005.

  1. Has anyone heard of read of anything similar to this? Any comments?

    My friend with strabismus is a former chief engineer, and I have a PhD
    in engineering, which I hope will lend credence to what I'm about to

    Strabismus is always associated with three or fewer rotations -
    inward/outward turning, up/down turning, and torsion of the eye. Every
    mathematician knows that the result of two rotations applied
    successively is a single rotation along an inclined axis. So the same
    is true of three.

    Also, two reflections always equals one rotation. The angle between
    the mirrors is half the rotation angle. So the rotation of light rays
    associated with strabismus can be corrected either by a prism or by a
    pair of plane mirrors.

    The correction of strabismus using a prism is well known; correction
    using a pair of plane mirrors is less well known.

    My friend has a 35 degree in-turning and 35 degree torsion. Through
    simple vector calculus (dot and cross products) I showed that this
    equals a single 52 degree rotation with axis (0.69, 0.69, -0.2) where
    x is horizontal, y is vertical and z away from the eye.

    I've built him a two mirror hand held correction device (images
    attached are from video). The angle between the mirrors is 52/2 = 26
    degrees. By rotating the two dowels the mounting can be used for
    in-turning and out-turning eyes with a small amount of up/down
    turning, and for any degree of rotation from -75 to +75 degrees. The
    mounting, field of view and mirror shapes could all be improved. In
    theory, the field of view can be expanded to be as good as that of
    normal spectacles.

    It works; and doesn't cost much.

    If I may be so bold, I suggest that two-mirror devices are a viable
    alternative to prisms for other sufferers when the resultant rotation
    from strabismus is large.

    Looking forward, I see versions of this that are not hand-held, and
    versions that simultaneously correct for long/short sight by using one
    plane and one curved mirror. Making a curved mirror is not formidable,
    it's just a lens with a metal coating on one side.
    David Paterson, Apr 1, 2005
  2. A two mirror system is like a periscope and is getting inpractical for
    everyday use if prisms solve that particular problem.

    I have played around with this kind of system. A none strabismic
    person can see normally with their eyes in a variety of different
    positions with no obvious binocular problems. There is a name for
    this phenonema which escapes me.

    Similarly you can place identical photos on the floor and move them
    around quite a bit with your feet and yet still get binocular vision
    providing you dont rotate them.

    where your system would be interesting is if you could vary the system
    so that around a small degree of variation the system continually
    created movement of the strabismic eye and therefore encouraged moves
    towards normal use of the eyes. So that eventually the eye was able to
    see normally without the mirrors.

    I have attempted to point out to experts here that eye movements are
    made via large movements which are essentially matched by each eye and
    also smaller movements which are only created via feedback from each
    eye and its required relative position needed to produce accurate
    binocularity. Most here seem to think this commonsense observation is
    impossible because they believe that eye movements are yolked together.
    Therefore via that reasoning strabismus must necessarily be a major
    neurological fault. However strabismus can be seen to vary with
    stress etc etc etc.

    Sounds interesting!



    Where such a device defi
    andrewedwardjudd, Apr 1, 2005
  3. oh by the way. You might want to look at the drawings for a
    'myopter' which allows none strabismic eyes pointing directly ahead to
    be able to read books etc. Its a head mounted thing and looks like a
    small welding eye guard:) It uses a prism and a two way mirror to
    split the incoming images and then enable recombination at a different
    angle. I am not an optical expert but if you use mirrors you may get
    strange effects because the strabismic eye is not in the same
    plane/axis/what not as the target? I found that felt very peculiar
    myself for bigger angles. The myopter solves that problem.

    This thing is made by a guy who advertises on the web.

    andrewedwardjudd, Apr 1, 2005
  4. Mike Tyner wrote
    to do to
    break the suppression and make the wearer actually _use_ the corrected

    I have wondered if a way to solve this problem is to have some kind of
    device similar to one of those virtual reality game helmets. Most of
    the technology is already available off the shelf.

    The suppressing eye could get opportunities to see when the seeing eye
    sees only a blank image.

    Since kids like computer games and it seems best to catch this in
    childhood it seems like an idea worth pursuing.

    andrewedwardjudd, Apr 1, 2005
  5. Thanks mike and andrew, great feedback.

    I forgot to give you the hyperlink to images. It's and 02
    and 03. This is just a first version, remember. I have hopes of
    increasing the field of view in version 2. I've also bought a Dick
    Smith magnifier that looks remarkably like the Myopter, in the hope
    that I can attach mirrors to it.

    The Myopter is a great looking system, with beamsplitter and 3
    mirrors. Two of them could be realigned on skew axes for it to work
    with strabismus with Z-torsion. That's not impossible.
    everyday use if prisms solve that particular problem.

    Yes, and no. I've figured out that a prism can be used to solve my
    problem, but it would have to be aligned at a very peculiar angle
    because of the eye torsion, not something easy to do with normal
    spectacles. Further, the prism would have to have a diopter of 125 to
    130, which is well beyond the range normally prescribed.
    so that around a small degree of variation the system continually
    created movement of the strabismic eye and therefore encouraged moves
    towards normal use of the eyes. So that eventually the eye was able to
    see normally without the mirrors.

    I hadn't though of that, for the reason that my friend with strabismus
    got it at age 60, and the affected eye is totally fixed in place,
    can't move at all. He's now 80 and would like to avoid surgery.

    Yes, the variation could easily be done, by rotating the two dowels
    holding the mirrors, continuously from 75 or so degrees of divergence
    all the way down to zero divergence. The only drawback of that is that
    the mirrors would have to be more nearly circular and this would
    reduce (perhaps halve) the total field of view.
    break the suppression and make the wearer actually _use_ the corrected

    In this case the eye with strabismus has better vision than the other
    (around the z axis) your market will be small but very, very
    Most of those problems are acquired, so they already have healthy

    The torsional correction around the z axis is exactly what I set out
    to fix. For my friend I'm correcting his 35 degree torsion. I can't
    yet see any limit on how big a torsion can be corrected, 90 degree
    looks possible.
    David Paterson, Apr 1, 2005
  6. looking at picture two i thought you might be a Kiwi. Thats got to be
    rimu! ???
    andrewedwardjudd, Apr 1, 2005
  7. David Paterson

    Rich Guest

    Hi Andrew,
    In regard to the the use of a dual mirror device and the question of
    binocularity, I built a similar device (although for a different
    purpose), for which I had to take into account a little-known
    phenomenon called Panum's fusional area. To quote from my paper:
    "...Panum's fusional area, which in stereopsis allows the image to be
    pulled apart by some 2 degrees before being broken up into two separate
    images. The images are actually pulled apart on the retina, but a
    supra-retinal function maintains perception of a single image".
    If your are interested, you can read it on my Web site at

    Rich, Apr 3, 2005
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