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9 Prism diopters Base Out per eye?

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    #16
    Originally posted by HarryChiling View Post
    If you look at a prism as

    1Dprism = 1cm/1m = 1cm/100cm

    so for 9 diopters of prism

    9Dprism = 9cm / 100cm

    If you were to look at the abgle that the light deviates

    tan(dev) = 9cm / 100cm
    tan(dev) = 0.09
    dev = 5.14o

    now the vertex distance of a pair of glasses averages about 13mm and the globe diameter averages about 27mm so you have 40mm from the back of the lens to the retina

    tan(5.14) = PDcomp / 40mm
    PDcomp = 40mm * tan(5.14)
    PDcomp = 3.6

    If you were to further divide that by the 9D you would get

    3.6 / 9 = 0.4

    There is your answer my friend.

    Thanks Harry. I just found it in the second edition of System for Ophthalmic Dispensing. You are right, but they assumed 30mm from the back surface of the lenses to the center of rotation of the eyes. They came up with .29 or .30.

    Jeff

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      #17
      Yeah if you were to change from the corrected 26.5mm to 30mm

      tan(5.14) = PDcomp / 26.5mm
      PDcomp = 26.5mm * tan(5.14)
      PDcomp = 2.38

      If you were to further divide that by the 9D you would get

      2.38 / 9 = 0.26

      tan(5.14) = PDcomp / 30mm
      PDcomp = 30mm * tan(5.14)
      PDcomp = 2.7
      If you were to further divide that by the 9D you would get

      2.7 / 9 = 0.30

      So you really can't go too wrong either way. It's actually pretty simple geometry and worth doing whenever you have high amounts of prism.
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        #18
        It appears that that is the one new thing I learned today. In all my years, I have never compensated a Pd on any prism job.

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          #19
          Originally posted by obxeyeguy View Post
          It appears that that is the one new thing I learned today. In all my years, I have never compensated a Pd on any prism job.
          Same. Thanks for the education, all.
          Andrew

          "One must remember that at the end of the road, there is a path" --- Fortune Cookie

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            #20
            So help me totally get this. The new lay-out PD would be 25/23 or 25.5/23.5 depending on 2.38 interpretation??

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              #21
              Originally posted by gemstone View Post
              Are you saying you need to ajust the PD .4 mm for 9 diopters of prism?

              No you would multiply .3 x 9=2.7mm moved in for each eye.

              Original pd 27.5/25.5

              New pd 25/23

              Jeff

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                #22
                Originally posted by obxeyeguy View Post
                It appears that that is the one new thing I learned today. In all my years, I have never compensated a Pd on any prism job.
                A new one on me too. Wonder if they know about that at Durham Tech yet.

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                  #23
                  Originally posted by gemstone View Post
                  A new one on me too. Wonder if they know about that at Durham Tech yet.
                  They should, it's apparently in their book.
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                    #24
                    This all sounds great. It really does. But, what were the old PDs and how will moving or changing them affect adaption?

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                      #25
                      Originally posted by gemstone View Post
                      A new one on me too. Wonder if they know about that at Durham Tech yet.
                      We just figured it out yesterday.;)

                      Originally posted by HarryChiling View Post
                      They should, it's apparently in their book.
                      Well it is not our book, it is Dr. Brooks' book.;)


                      Do not forget to reduce the prism amout by:

                      0.25 X -1.96 = 0.49^ BI OD
                      0.25 X -1.86 = 0.47^ BI OS

                      So reduce the prism by 0.5^ OU for a total of 8.5^BO each eye.

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                        #26
                        I can follow the math, I just need to understand the principle.

                        Follow each step and tell me where I go astray:
                        1. A patient's optical axis will go through the optical center of a lens, regardless of where it is, unless they want to see double.

                        2. If the optical centers are in front of a "rest position" for the eye muscles, then no abnormal muscle contraction is exerted.

                        3. If a person is eso, like this example, their eyes naturally want to cross. Moving the O.C.s to the "rest position" (per the prescribed prism) allows comfortable vision.

                        4. As a general rule, a monocular distance pupillary distance measurement is not related to any patient's resting eye position in the glasses when he or she is viewing binocularly. It's kind of an artificial reference point.

                        I.e., unless they're perfectly "orthophoric", if glasses are made to a monocular p.d., any small amount of exo or esodeviation present will necessitate the patient make a slight bit of compensatory muscle exertion. This is commonly ignored if the person's muscle deviation is minor. If it's ignored when the patient's muscle deviation is major, the patient will experience eyestrain from needing to move the eyes to the monocular p.d. measurement. Their choice is strain or double vision. This is axiomatic.

                        5. The whole point in prism is to move the image to where an eye is going to "like it". Specifying monocular p.d. in these cases is merely the reference point from which you measure the prescribed prism amount. The optical center is what the patient will be looking through (actually zero prism).

                        6. If you could accurately draw some spot on the lens where the eye's optic axis intersects the lens when the eyes are binocularly viewing through any amount of prism (which you can't easily do) it would NOT go through the monocular p.d. measurement.

                        In the example above, the monocular p.d. measurement would be temporal to where the eye's optic axis intersects the lens. The monocular p.d. measurement spot would, of course, simply be where the correct amount of prism would be measured. It's a major reference point.

                        7. So I don't understand the concept of having to compensate for "where the eyes are going". They're going only one place: where they have to go to maintain a single image. And they do it all by themselves. :)
                        Last edited by drk; 08-16-2007, 04:16 PM.

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                          #27
                          Originally posted by Fezz View Post
                          This all sounds great. It really does. But, what were the old PDs and how will moving or changing them affect adaption?
                          I'll try a crack at that. Judging by the Add being a +2.00 I would say her previous pair if purchased about a year or two before this purchase was made with the wrong PD, she might have still been fine as the corridor would have been wider and may have accomadated the lack of precision, but as the Add gets higher and higher if the PD is not compensated and the corridor gets so narrow that the patinet is essentially viewing through the blended areas of the lens, she will start to notice that the lens isn't clear anymore. Also the higher the Add power the harder most desings seem to get exagerating the situation even more. If you start with a good Rx and compensate you can't go wrong.
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                            #28
                            Harry, your equation is simply to calculate the X-axis linear displacement of the eye's optic axis at a given distance from the eye (vertex distance), given an angular measurement of rotation. I know you know that.

                            I'm saying your equation is in reality only giving us the apparent linear movement of the "eye" to an observer standing in front of the patient. This comes into play when prism is prescribed cosmetically, for example in a densely amblyopic or blind esotropic eye that looks unattractively turned in. If you prescribe enough prism in the opposite/counter-intuitive direction (BI for esodeviation, e.g.) it will make the eye look straight to everyone else. Note: if the eye were not visually impaired, the patient's double vision would be exacerbated.

                            Since this is rarely the case, we don't hear about this much. Maybe Chip is aware of this.

                            You are misapplying it, here, I'm afraid.
                            Last edited by drk; 08-16-2007, 04:49 PM.

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                              #29
                              DRK you want to make sure that the eye is traversing in the correct position in the progressive lens. In other lenses this does not matter because you do not have a corridor to stay in.
                              The eye deviates 0.3mm for every 1^ of prism so if we left the PD alone the eye would not be looking through the correct location in the progressive.

                              So in this case by putting 9^ of prism BO in front of the eye the eye will turn in 2.7mm so we need to move the PD in more to account for this deviation so the eye will look in the correct position of the progressive.

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                                #30
                                Ok, playing Satan's Advocate here:



                                So who checks this completed job in?

                                Do you dot the lens and check it?

                                How thick or fine is the tip on that marker?

                                How accurate is your lensometer....really?

                                As Lensgrinder suggests, you reduce the prism by .49 OD, .47 OS----Do you call the prescribing Doc and suggest that change? Have him or her re-write it?

                                I am sure we all use labs that are capable of this type of accuracy.....


                                Ok...fire away!!!!!!!


                                :D:cheers::D:cheers::D

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