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Thread: question for darryl

  1. #1
    opti-tipster harry a saake's Avatar
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    Darryl, as you know there has been some discussion on this board as to prism diopters vs degrees, which i have to admit is my favorite term. However assuming the aboslute truth is prism diopters and that one equals apx 1.75 in power, then the question seems to me, is in a practical sense, how would you measure one with what instrument, where it would show you 1.75 on some sort of scale. Every vertometer i have ever seen has been marked 1,2,3 etc. progressively on circular rings. Now if that stands for 1.75, fine and dandy, but what good does it do you from a practical standpoint?

  2. #2
    Master OptiBoarder Darryl Meister's Avatar
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    Hi Harry,

    It's probably best to define a couple of terms in my own language, first. "Prism" simply refers to the deviation of light through a refracting element (such as a lens). "Prism power" simply quantifies this deviation. This is an angular deviation, meaning that the deviated light makes a certain angle with its original path.

    Since we are dealing with an angular deviation, prism power is probably most easily described using degrees (the most common unit of angular measurement). However, because of various reasons, including the way doctors refract patients using a flat surface or wall, it is generally more convenient to measure the "tangent displacement" of a prism -- not its angular deviation. (Although the two are proportional to each other for small deviations.) This is a measure of how far the prism moves an image across a flat plane. This is also why Prentice's formula is so simple when expressed in prism diopters.

    The "unit" of tangent displacement used for prisms is referred to as a "prism diopter." This represents a displacement of 1 unit over a distance of 100 units (e.g., 1 centimeter over 1 meter). As I pointed out in an earlier posting, however, the prism diopter isn't a true unit of measure since its effective value changes as the deviation increases. Meaning that 10 prism diopters is not equal to 5 times 2 prism diopters. However, for the small deviations used with spectacle lenses, the differences are negligible.

    In any event, you can convert back and forth between the two with this formula:

    Prism Diopter = 100 * tan Degree

    1 prism diopter is roughly equal to 0.57 degrees. Most ophthalmic instruments (including lensometers and vertometers), surfacing programs, and prism rings are calibrated in prism diopters, not degrees. The term "degree," when used now in conjunction with prism, is generally a misnomer.

    As a side note, the apical angle of a prism IS measured in degrees. The deviation produced by a prism is roughly proportional to the apical angle (for small deviations), so the apical angle can also be used to describe prism power.

    Anyway, I hope this helps.

    Best regards,
    Darryl

  3. #3
    Bad address email on file Darris Chambless's Avatar
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    Redhot Jumper

    Okay Darryl,

    After reading your posting (and taking a hand full of Tylenol afterward for the brain cramp :-) I have to ask this question. Wouldn't it make more sense to measure prism in degrees?

    Here is what I mean; since (as you implied) every other power measurement is approximately, close to, in the neighborhood of, or roughly an amount. Why not measure in degrees since this is as close to an exact measurement of deviation that you can get? In otherwords why say diopters = X with some variables, when all you are after is the degree of deviation you want for an image.

    I think I'm with Harry on this one. It would seem to me that using degrees would be much more acurate and with fewer variables.

    Just my thoughts,

    Darris C.

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    opti-tipster harry a saake's Avatar
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    Dang Darryl, hell of an answer, but i,m with darris, plus i had to be on percodan all day..harry

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    Master OptiBoarder Darryl Meister's Avatar
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    Well, I can't say that I've ever questioned the usefulness of the prism diopter.

    Keep in mind a couple of points I made earlier, though...

    1) For the small prisms we generally work with in ophthalmics, the two units are proportional to each other in any case. So it really doesn't serve much of a benefit to use degrees over prism diopters (or vice versa).

    2) A degree represents quite a bit of prism (almost 2 prism diopters worth). We deal with smaller prisms in ophthalmics more often than we do larger ones. To put that amount into perspective, the ANSI Standard for vertical prism would read 0.19 degrees, instead of 1/3 prism diopters.

    3) Prentice's rule, which is also based upon centimeters of displacement over meters of distance, lends itself readily to the prism diopter unit. It would be a bit more unwieldly in degrees.

    However, I really can't say that I have any terribly compelling reasons why one should use prism diopters -- other than the fact that it is now recognized as the internationally accepted unit of prismatic deviation.

    Best regards,
    Darryl

    [This message has been edited by Darryl Meister (edited 06-10-2000).]

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    sub specie aeternitatis Pete Hanlin's Avatar
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    Considering the fact that I'm going to have to speak about prisms for a two hour CEC, I'm enjoying any discussion of prism...

    I think the diopter is a useful measure of prism because it is based on base ten formulations (like the metric system that we use for convergence formulations). If any argument could be made for a change, I think it would be for switching from diopters to centrads (which maintain accuracy when dealing with higher amounts of prism).

    If I'm not mistaken, using degrees to describe a prism is not unlike using vergence to describe focal power.

    Pete

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    opti-tipster harry a saake's Avatar
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    Well pete, as i,ve stated before and since i am not aware of any instrument that measures prism diopters and comes up with a figure saying so, it seems rather redundant. When you apply a lens to your focimeter to measure prism imbalance, do you not use the circles? It seems to me when you do, that its merely an irevelant assumption that it is 1.75 prism diopters down, up or whatever the case may be. Also when you send back a reject that lets say has an imbalance of less then 1, but more then .75 are you going to sit there and calculate out how many prism diopters its off and send it back to the lab with a note, i need you to move this oc 1.5432 prism diopters up. I think you would get a lot of strange looks. Degrees is the only way to go from a practical standpoint...harry

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    Objection! OptiBoard Gold Supporter shanbaum's Avatar
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    Originally posted by harry a saake:
    Well pete, as i,ve stated before and since i am not aware of any instrument that measures prism diopters and comes up with a figure saying so, it seems rather redundant. When you apply a lens to your focimeter to measure prism imbalance, do you not use the circles? It seems to me when you do, that its merely an irevelant assumption that it is 1.75 prism diopters down, up or whatever the case may be. Also when you send back a reject that lets say has an imbalance of less then 1, but more then .75 are you going to sit there and calculate out how many prism diopters its off and send it back to the lab with a note, i need you to move this oc 1.5432 prism diopters up. I think you would get a lot of strange looks. Degrees is the only way to go from a practical standpoint...harry
    I'm lost... those circles in your lensmeter are diopters, not degrees. And regarding degrees being "the only way to go", au contraire - the entire industry uses prism diopters to describe and measure prescribed prism, in an unusual display of consistency.

    Exactly WHAT is your problem with the prism diopter?


  9. #9
    sub specie aeternitatis Pete Hanlin's Avatar
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    when you send back a reject that lets say has an imbalance of less then 1, but more then .75 are you going to sit there and calculate out how many prism diopters its off and send it back to the lab with a note, i need you to move this oc 1.5432 prism diopters up
    Actually, I would send the job back with a note to move the OC up or down depending on the prismatic result desired. This is exactly where expressing prism in diopters is most helpful. I know if I order the OC of a -5.00 lens moved 2mm higher off the seg that the prismatic result at the seg will be an increase of 1^BD... A computation like that using degrees seems more difficult (to me, anyway :^).

    I have no doubt that the degree is a more precise measure of the effect of the lens on light- within the lens. However, the diopter seems to calculate the effect of the lens upon light over a specified distance with greater ease.
    Pete "IMHO" Hanlin

    [This message has been edited by Pete Hanlin (edited 06-12-2000).]

  10. #10
    opti-tipster harry a saake's Avatar
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    Robert, i have no argument with what the industry has adopted, but i am also sure if the industry said lets call it prism wavelengths, a lot of us would go along with that also. It kind of reminds me of a situation that happened to all of us radio operators a number of years ago when they changed megacycles to megahertz, for no other reason except to honor mr. hertz. Megacycles much better describes the case.. Pete, i have no argument about prism diopters describing the effect, but it seems kind of irrevelant. If your oc,s are 2mm off vertically, the only thing you want to do is to have the lab change it, in a manner they will understand in the simplest terms. I,ll challenge you pete to send a couple of jobs back to a lab and tell them its off so many prism diopters, and then tell them to send you back and answer about how many millimeters they are going to move it. Harry(the Nth degree Saake)

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    Objection! OptiBoard Gold Supporter shanbaum's Avatar
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    Originally posted by harry a saake:
    Robert, i have no argument with what the industry has adopted, but i am also sure if the industry said lets call it prism wavelengths, a lot of us would go along with that also. It kind of reminds me of a situation that happened to all of us radio operators a number of years ago when they changed megacycles to megahertz, for no other reason except to honor mr. hertz. Megacycles much better describes the case.. Pete, i have no argument about prism diopters describing the effect, but it seems kind of irrevelant. If your oc,s are 2mm off vertically, the only thing you want to do is to have the lab change it, in a manner they will understand in the simplest terms. I,ll challenge you pete to send a couple of jobs back to a lab and tell them its off so many prism diopters, and then tell them to send you back and answer about how many millimeters they are going to move it. Harry(the Nth degree Saake)

    As I recall, it was changed because the former term "megacycles" was, by itself, an inaccurate description, and "megacycles per second" was, well, long.

    I still don't get what you're looking for; the physiological impact about which we're concerned is image deviation, which is only indirectly related to decentration. That's why the standards set maxima for unwanted prism (however it's expressed), not unwanted decentration - except at the margin, where the concern is the ability to produce to the standard, as opposed to usability.

    The first thing I would expect a lab to do, upon receiving a pair of specs rejected due to inaccurate distance decentration, would be to compute the amount of prism induced thereby, unless it's obviously excessive. Labs do receive more than a few pair of rejects that in fact meet Z80 prism standards. That's why our software prints the allowable error in millimeters for the lateral and vertical meridia on the lab workticket.

    You can't possibly reject a pair of glasses for the OC's being off 2mm vertically unless you first determine how much prism is induced thereby.

  12. #12
    sub specie aeternitatis Pete Hanlin's Avatar
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    Pete, i have no argument about prism diopters describing the effect, but it seems kind of irrevelant. If your oc,s are 2mm off vertically, the only thing you want to do is to have the lab change it, in a manner they will understand in the simplest terms. I,ll challenge you pete to send a couple of jobs
    back to a lab and tell them its off so many prism diopters, and then tell them to send you back and answer about how many millimeters they are going to move it.
    You are correct, in answering your last question (about resolving prismatic imbalance), I used an example of how expressing prism in diopters is useful for determining the prismatic effect at the seg- bad example.

    So, back to your original question... you are positing that expressing the imbalance in degrees would be simpler for the lab to understand than an expression of diopters. Having managed labs for a few years, when I received a rejected job (not that that EVER happened ;)), I would first verify that there was imbalance in the lens. Naturally, this can be done by centering the stronger lens and then reading the imbalance in the weaker lens. In the case of a bifocal, where the o.c. above seg is specified, I can also just dot up the distance ocs and measure the mm above seg. Using Prentice's Rule, I could then verify the prism present at the specified oc hgt. Likewise, I can verify in/out prism in a DVO job by dotting the centers, comparing the measurement to the patient's pd, and then computing the prism using Prentice's Rule.

    When it comes down to it, Prentice's Rule can be worked in degrees... however, instead of multiplying the Rx diopters by the mm decentered and dividing by 10, you divide by 17.5 (which will work for small amounts of prism). That just seems more difficult to me.

    Pete

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