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Thread: Prism Base Direction

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    Prism Base Direction

    Hi all! I'm getting prism mixed up in my head. I know the basics - light is displaced toward the base of the prism and the eye always turns in the direction of the apex. Where I'm having trouble is in the practical application. For instance, a textbook I'm reading states that if one eye turns upward, a prism may be placed with its base down before that eye. It would allow the person to see objects easier but it wouldn't "force" the eye to a normal position. Wouldn't you want to put the prism base up to get the eye to not turn upward?

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    Quote Originally Posted by Trina View Post
    Wouldn't you want to put the prism base up to get the eye to not turn upward?
    The eye turns upward because of a muscle imbalance or deficiency. The goal of the prism would be to direct the light passing through the lens to where the eye is looking, as it lacks ability to turn down to a normal posture.

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    I have also cut lenses with prism that displace light for people who have field of vision loss. Usually was yoked prism to project the image into the part of the retina that was not damaged. Was life changing for a few folks.

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    Ghost in the OptiMachine OptiBoard Silver Supporter Quince's Avatar
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    Quote Originally Posted by Tallboy View Post
    I have also cut lenses with prism that displace light for people who have field of vision loss. Usually was yoked prism to project the image into the part of the retina that was not damaged. Was life changing for a few folks.

    I never even thought of this as an option! I don't work for an OD so I'm a little out of the medical loop, but I will remember this for future application.
    Have I told you today how much I hate poly?

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    Basic rule with prism: Light bends towards the base...object move towards apex. So in your example, the eye position or gaze has moved upward requiring the image to be displaced in a manner in which the object appears in the same position for both eyes. Most opticians begin to get confused when considering + and - powers. Base direction will determine displacement based on the following:

    Click image for larger version. 

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    Thanks everyone! I guess my confusion was in application - I figured that the prism was being used to "force" the eye into a more natural position instead of allowing the eye to stay at whatever position it naturally falls into and just moving the image there.

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    Quote Originally Posted by Trina View Post
    Thanks everyone! I guess my confusion was in application - I figured that the prism was being used to "force" the eye into a more natural position instead of allowing the eye to stay at whatever position it naturally falls into and just moving the image there.
    Right!!
    You could place base down on the 'high' gaze eye only, or base up on the 'good' fellow eye only, or some BD and some base up on each eye to balance the vertical prismatic imbalance found in this situation.

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    Quote Originally Posted by Trina View Post
    Thanks everyone! I guess my confusion was in application - I figured that the prism was being used to "force" the eye into a more natural position instead of allowing the eye to stay at whatever position it naturally falls into and just moving the image there.
    Think bed specs...



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