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Thread: oblique cross prism calculation

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    oblique cross prism calculation

    Hi, does anyone know of a formula to calculate oblique cross prisms as you you would calculate oblique cross cyls which normally occur with a over refraction?

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    ATO Member HarryChiling's Avatar
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    Split them into their horizontal and vertical components and they are additive.

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    If I understand the question, and Harry's answer, you would consider each of two prisms as separate, resolve each into horizontal and vertical components, then add the horizontal to horizontal, and vertical to vertical. You could then compound the resulting prisms into a single base orientation.

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    ATO Member HarryChiling's Avatar
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    Quote Originally Posted by Dave Nelson View Post
    If I understand the question, and Harry's answer, you would consider each of two prisms as separate, resolve each into horizontal and vertical components, then add the horizontal to horizontal, and vertical to vertical. You could then compound the resulting prisms into a single base orientation.
    Bingo, if you want you could take all the steps and combine them into one ultra complicated lookign formula, but then you loose the meaning behind it.

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    Quote Originally Posted by HarryChiling View Post
    Split them into their horizontal and vertical components and they are additive.
    Thanks for the reply Harry sorry I didn't reply before but I was sailing the high seas!
    I understand the reply like Dave suggested but if one prism was base in the other base out would one counteract the other?

    Rob

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    Quote Originally Posted by rob in uk View Post
    Thanks for the reply Harry sorry I didn't reply before but I was sailing the high seas!
    I understand the reply like Dave suggested but if one prism was base in the other base out would one counteract the other?

    Rob
    That is correct, if one eye has BASE IN and the other eye BASE OUT that would cancel each other out if same power. If different power then you subtract each other.

    In terms of Oblique Prism, that would be measuring Horizontal Prism and Vertical Prism.

    In phoropters we are able to measure horizontal and vertical prism separately.

    When I prescribe prism, I neutralize the vertical prism first, using the lowest as possible prism first, and then I try to find the lowest amount horizontal prism.

    Then I split the prism in both eyes.

    For example, during the trial frame testing I find the patient needs 4 UP OS and 6 BO OS.

    I would split this way

    OD: 3 BO and 2 DOWN
    OS: 3 BO and 2 UP

    Of course if you find vertical prism split 2 UP OD and 2 UP OS, that would yield relative no prism.

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    Thanks that as I thought

    Rob

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