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Thread: Help with prisms!

  1. #1
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    Help with prisms!

    Hi guys,

    I'm going through a few questions in my book. For the following, I don't understand differential prisms or really anything in these questions. It doesn't seem to be anywhere in my books! Can anyone do them, so I can see the working, or can someone point me in the direction of formulas etc pleeease?
    There are as follows:

    5) A prescription reads:

    RE 3.5 base up, 1.5 base out
    LE 6.5 base down, 7.5 base out

    I need to equally divide the prism between the 2 eyes.

    6) Another prescription reads:

    RE 5 base up
    LE 9 base out

    I need to equally divide the prism between the 2 eyes.

    7) Calculate the total prism in each eye, and the differential prism, when the wearer of the following prescription looks at an object through points 12mm to the left (from the wearer's perspective) and 8mm up from the distance optical centres.

    RE +6.75D
    LE +1.50D

    8) Calculate the total prism in each eye, and the differential prism, when the wearer of the following prescription looks at an object through points 10mm to the right (from the wearer's perspective) and 8mm up from the distance optical centres.

    RE -2.25D
    LE -5.50D

    9) Calculate the total prism in each eye, and the differential prism, when the wearer of the following prescription looks at an object through points 6mm to the right (from the wearer's perspective) and 10mm up from the distance optical centres.

    RE -3.50D
    LE +2.50D

    10) A prescription reads RE +5.00D, LE+5.00D. If the wearer os this prescription looks through a point 8mm above the optical centres, will she or he experience any differential prism? If so, how much? If not, why not?

  2. #2
    ATO Member HarryChiling's Avatar
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    Questions 5 and 6 you can sum up the prism and then divide by 2

    5) A prescription reads:

    RE 3.5 base up, 1.5 base out
    LE 6.5 base down, 7.5 base out

    I need to equally divide the prism between the 2 eyes.
    3.5 up and 6.5 down = 10 total difference
    1.5 out and 7.5 out = 9 total difference

    So:

    RE 5.0 up 4.5 out
    LE 5.0 up 4.5 out

    7) Calculate the total prism in each eye, and the differential prism, when the wearer of the following prescription looks at an object through points 12mm to the left (from the wearer's perspective) and 8mm up from the distance optical centres.

    RE +6.75D
    LE +1.50D
    well it's 12 to the left and 8 up so:

    RE = +6.75 * 12 / 10 = 8.1 out
    RE = +6.75 * 8 / 10 = 5.4 down

    RE 8.1 out 5.4 down

    LE = +1.50 * 12 / 10 = 1.8 in
    LE = +1.50 * 8 / 10 = 1.2 down

    LE 1.8 in 1.2 down

    The trick here is that the prism in reference to the side of the eye is going to chage your bases orientation on the horzontal but on the vertical 90 is up and 270 is down no matter how you slice it.
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    Quote Originally Posted by Whitwoo View Post
    10) A prescription reads RE +5.00D, LE+5.00D. If the wearer os this prescription looks through a point 8mm above the optical centres, will she or he experience any differential prism? If so, how much? If not, why not?
    The wearer will not experience any differential prism as his/her eyes are in balance.

  4. #4
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    Quote Originally Posted by Whitwoo View Post
    9) Calculate the total prism in each eye, and the differential prism, when the wearer of the following prescription looks at an object through points 6mm to the right (from the wearer's perspective) and 10mm up from the distance optical centres.

    RE -3.50D
    LE +2.50D
    Begin by working out the vertical (v) and horizontal (h) components of prism in each eye, presenting them side by side helps (however the nature of this post i have to just do one eye after another. I've also attached a drawing from your parameters which aids in working out the base direction in your workings, have a look!

    The triangles I've drawn on, represents the prism of a plus and minus lens (R minus, L plus respectively) relative to the vertical and horizontal. The base is the bold lines. And thus you can work out its direction away from the oc from your point (in your case, 6mm to the right and 10mm up).

    To calculate prism, use Prentice Rule P = cF where, c = distance away from optical centre IN CENTIMETRES (ie, 10mm = 1cm), F is power of lens in Dioptres.

    RE
    Pv = cF
    = 1 x 3.5
    Pv = 3.5Δ Base UP

    Ph = cF
    = 0.6 x 3.5
    Ph = 2.1Δ Base OUT


    LE
    Pv = cF
    = 1 x 2.5
    Pv = 2.5Δ Base DOWN


    Ph = cF
    = 0.6 x 2.5
    Ph = 1.5Δ Base OUT


    Then calculate the differential prism effect, by compounding them together. You should have info in your texts for 'how to'.. but if you're stuck,
    up and down = add together (ie 1 up R 3 down L = 1+3 = 4)
    out and out = add together (ie 1 out R and 3 out L = 1+3 = 4)
    in and in = add together (ie 1 in R and 3 in L = 1+3 = 4)
    in and out = subtract (ie 1 in R and 3 out L = 3-1 = 2 Base Out L)

    In this question, you still have to associate the Base Up direction to an eye, it is safe to pick the eye with the most prism component when labelling the Vertical DPE (in this case, the R eye had 3.5, so more than L eye which had 2.5 prism dioptres).

    You do not need to specify an eye when giving Horizontal DPE.

    V DPE = RE Pv + LE Pv
    = 3.5 + 2.5
    V DPE = 6Δ Base UP RE

    H DPE = RE Ph + LE Ph
    = 2.1 + 1.5
    H DPE = 3.6Δ Base OUT

    The answers are in bold, however be sure to provide your workings with your assignments.
    Attached Thumbnails Attached Thumbnails prism question 9.bmp  
    Last edited by Memoir; 07-05-2008 at 12:19 AM. Reason: typo

  5. #5
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    Quote Originally Posted by HarryChiling View Post
    So:

    RE 5.0 up 4.5 out
    LE 5.0 up 4.5 out
    This is a once in a lifetime opportunity to correct Harry Chiling! I believe the left eye would be 5.0 down b/c up and down are the compounding directions for vert. prism.
    Whew, that was fun! ;)
    :cheers::cheers:

  6. #6
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    You right the left eye will be base down

  7. #7
    ATO Member HarryChiling's Avatar
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    Quote Originally Posted by EyeFitWell View Post
    This is a once in a lifetime opportunity to correct Harry Chiling! I believe the left eye would be 5.0 down b/c up and down are the compounding directions for vert. prism.
    Whew, that was fun! ;)
    :cheers::cheers:
    I'd give you a positive for that but it says I got to spread some around, that was a typo nice catch.
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  8. #8
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    I figured it was an oversight/typo rather than a lack of knowledge. :::bows to your optigenius:::

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