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Thread: Ground In Prism

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    Ground In Prism

    I am trying to visualize this better and could use some help please. Let's say you have 2 identical spherical lens blanks. In one, you grind the back to produce a conventional plus lens. On the other, you grind the back at an angle to produce a plus lens with prism. On the first lens, the optical center is at the thickest portion of the lens. On the second lens, however, the thickest part is skewed toward the edge and no longer coincides with the geometric center. So what I am not getting is what defines the optical center of the second lens with ground in prism? Is there still some point which can be identified where light will pass straight through? I just cannot visualize where this would be. Thanks.

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    OptiBoard Professional Kujiradesu's Avatar
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    If you are grinding prism into a lens, you need not worry about the optical center only the prism refernce point.

    Prism reference point: the point on a lens where prism is as prescribed.
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    What's up? drk's Avatar
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    It will be nowhere if there's just prism.

    It may be somewhere if the lens has power, because power's a variable prism in and of itself. More on that in a minute.

    First of all, you don't care where the optical center is, with prescribed prism. You only care what prism the patient gets at their "MRP", or, split p.d. Let's be practical.

    But if you want to "do optics", consider a high plus lens with ground BI prism. If you look off to the edge of the lens, it will have TWO prism components...the first will be the BI prism that was ground. Say that's 2^ for our example.

    But you'll also get prism from the power portion. Let's say you look temporally, and you will get BI prism from that. So looking temporally you get double BI!

    But if you look nasally, you would get BO prism from the power component. Maybe just enough to cancel the BI ground prism. And there would be an "optical center" where a ray would pass undeviated.

    Fun, huh?

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    I'm digesting what you guys are telling me okay. Let's say you don't know the desired PD or desired prism, and you check an unknown pair of glasses on the lensometer - won't you always get "some" reading for PD and prism regardless?

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    OptiBoard Professional Kujiradesu's Avatar
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    Quote Originally Posted by go_hercules View Post
    I'm digesting what you guys are telling me okay. Let's say you don't know the desired PD or desired prism, and you check an unknown pair of glasses on the lensometer - won't you always get "some" reading for PD and prism regardless?
    How I was taught to neutral was: PD is what it is, if there is prism horizontally you include that in the PD (i.e. OD 30/ OS 37, theres some amount of horizonal prism, but the way we reproduce it is by using this PD); vertically, you put all prism in the eye with a weaker power at 90. ( i.e. OD +1.00 Sph; OS +2.00 Sph dot the OS on center, then move over to the OD after centering horizontally the target lines are 0.5D BU then the RX for the OD will include 0.5D BU)
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    That makes sense, I understand what you're saying. What still gets me though is how a lensometer can deduce prism, since prism varies at different PD's (or DBC's to be correct).

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    Quote Originally Posted by go_hercules View Post
    I am trying to visualize this better and could use some help please. Let's say you have 2 identical spherical lens blanks. In one, you grind the back to produce a conventional plus lens. On the other, you grind the back at an angle to produce a plus lens with prism. On the first lens, the optical center is at the thickest portion of the lens. On the second lens, however, the thickest part is skewed toward the edge and no longer coincides with the geometric center. So what I am not getting is what defines the optical center of the second lens with ground in prism? Is there still some point which can be identified where light will pass straight through? I just cannot visualize where this would be. Thanks.
    Move the lens in the lensometer until the mires are in the center of the reticule. That's the OC for both lenses.

    Of course with prescribed prism we want the light rays to be deviated, so we can ignore the OC and concentrate on the Prism Reference Point.

    Start by measuring the monocular PD. Mark the lens with a large plus sign using a lens centration chart, and then place the lens on the spectacle table of the lensometer so that the plus sign is aligned the vertical and horizontal lines around the lens stop. You can now measure the value for prescribed or unwanted prism.

    Hope this helps,

    Robert Martellaro
    Roberts Optical Ltd.
    Wauwatosa Wi.
    www.roberts-optical.com
    ~~~~~~~~~~~~~~~~~~
    An expert is a man who has made all the mistakes which can be made, in a narrow field. -Niels Bohr

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    Quote Originally Posted by go_hercules View Post
    I am trying to visualize this better and could use some help please. Let's say you have 2 identical spherical lens blanks. In one, you grind the back to produce a conventional plus lens. On the other, you grind the back at an angle to produce a plus lens with prism. On the first lens, the optical center is at the thickest portion of the lens. On the second lens, however, the thickest part is skewed toward the edge and no longer coincides with the geometric center. So what I am not getting is what defines the optical center of the second lens with ground in prism? Is there still some point which can be identified where light will pass straight through? I just cannot visualize where this would be. Thanks.
    The OC is still at the thickest point in the prism lens, it's just that you no longer want the patient looking through the OC. The fact that the OC is no longer in front of the normal gaze is the reason there is a prismatic effect. In a spherical lens, whether you thicken one edge by grinding or by decentering makes no difference (there may be a practical difference in getting the blank to cut out). In an aspheric lens, decentering for prism doesn't work. The geometric center will now not coincide with either the OC or the position of the patient's pupil.

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    I think I am getting what you are saying. Sounds like the exact same set of glasses could be considered to have prism or no prism strictly depending on the reference point assumed. Am I finally thinking this right?

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    Quote Originally Posted by Robert Martellaro View Post
    Move the lens in the lensometer until the mires are in the center of the reticule. That's the OC for both lenses.

    Of course with prescribed prism we want the light rays to be deviated, so we can ignore the OC and concentrate on the Prism Reference Point.

    Start by measuring the monocular PD. Mark the lens with a large plus sign using a lens centration chart, and then place the lens on the spectacle table of the lensometer so that the plus sign is aligned the vertical and horizontal lines around the lens stop. You can now measure the value for prescribed or unwanted prism.

    Hope this helps,

    Robert Martellaro
    ^^^This

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    Quote Originally Posted by go_hercules View Post
    I think I am getting what you are saying. Sounds like the exact same set of glasses could be considered to have prism or no prism strictly depending on the reference point assumed. Am I finally thinking this right?
    If it's a refracting lens, yes.

    If it's a plano prism, no.

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    Thank you.

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    Quote Originally Posted by go_hercules View Post
    I think I am getting what you are saying. Sounds like the exact same set of glasses could be considered to have prism or no prism strictly depending on the reference point assumed. Am I finally thinking this right?
    All lenses that aren't plano are a set of prisms in concentric form. Usually you take a PD and decenter to put the OC in front of the eye so that there is no prism effect, but as soon as the patient looks away from the OC there is prism. With luck, both eyes have a roughly similar Rx so that the prism that both eyes experience is yoked; base in OD, base out OS for example. If the patient is antimetropic, it's harder for them to look away from center. Take that lens that you mentioned in your original post, move it in any direction, and you have just created prism. If a doctor prescribes prism, it means prism above and beyond what is already inherent in Rx lenses.

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    Quote Originally Posted by Robert Martellaro View Post
    Move the lens in the lensometer until the mires are in the center of the reticule. That's the OC for both lenses.

    Of course with prescribed prism we want the light rays to be deviated, so we can ignore the OC and concentrate on the Prism Reference Point.

    Start by measuring the monocular PD. Mark the lens with a large plus sign using a lens centration chart, and then place the lens on the spectacle table of the lensometer so that the plus sign is aligned the vertical and horizontal lines around the lens stop. You can now measure the value for prescribed or unwanted prism.

    Hope this helps,

    Robert Martellaro
    Robert,
    This is an interesting method that Ive never thought of; however, is this intended as a method of neutralization or just a way to say give an OD a good prism starting point? I just worry that by using this method the PD couldve been measured more narrow or wide when manufactured and by extention the prism will be off as well.
    Last edited by Kujiradesu; 04-23-2017 at 08:34 AM.
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    Quote Originally Posted by Kujiradesu View Post
    Robert,
    This is an interesting method that Ive never thought of; however, is this intended as a method of neutralization or just a way to say give an OD a good prism starting point?
    Kujiradesu,

    Both of the above, along with verification. It's the only way I know of to determine the value for prism in an existing pair of eyeglasses, especially when there's enough refractive/dioptric power to influence, sometimes significantly, the value for prism, prescribed or not.

    Hint: When marking the lens, use the corneal reflex, not the pupil center.

    http://www.opt.indiana.edu/v665/CD/C...on/CH4/CH4.HTM

    I just worry that by using this method the PD couldve been measured more narrow or wide when manufactured and by extention the prism will be off as well.
    Sorry, I didn't make it clear that when we mark the eye's pupillary axis in the primary gaze, it must be done in situ, that is, with a preadjusted frame. This is how we discover the degree, if any, of unwanted or prescribed prism in an existing pair of eyeglasses.

    Best regards,

    Robert Martellaro
    Last edited by Robert Martellaro; 04-24-2017 at 11:20 AM.
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    Robert, original poster back here - so just to put it to rest in my mind, I think I understand that if someone handed you a pair of specs and asked you to determine the amount of prism, it would be impossible without knowing the intended PD.

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    To make it simple, yes. That shows that you understand the concept. Without a reference point (the lens MRP from the patient), how do you know?

    (In reality, you can often tell within reason, because the prescribed prism usually outweighs the power-component's prism induced from not knowing where to put the lens on the lens table. That is, no matter where you move the lens, the mires may always deflect to some general location, and that would closely approximate the prescribed prism amount. But don't get off-topic.)

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    Quote Originally Posted by drk View Post
    To make it simple, yes. That shows that you understand the concept. Without a reference point (the lens MRP from the patient), how do you know?
    Sorry to nitpick, but, more correctly, if we align the Prism Reference Point (Major Reference Point is an archaic term), with the wearer's distance PD, and do so within the tolerances defined by Ansi's Z80.1-2010 standards, we're pretty much good to go.

    In reality, you can often tell within reason, because the prescribed prism usually outweighs the power-component's prism induced from not knowing where to put the lens on the lens table.
    Not with the RXs I get!

    Best regards,

    Robert Martellaro
    Roberts Optical Ltd.
    Wauwatosa Wi.
    www.roberts-optical.com
    ~~~~~~~~~~~~~~~~~~
    An expert is a man who has made all the mistakes which can be made, in a narrow field. -Niels Bohr

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    I took it to mean that he was asking if one can precisely determine prism amount in a power lens without knowing the reference point (of any name).

    As to point #2, if it's a low power lens (say +1.00) with a decent amount of prism (say 2^) I think you're going to see very little deviation of the mires when you wiggle the lens around on the lensometer. It's a "near plano power prism", of sorts...

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    Quote Originally Posted by drk View Post
    I took it to mean that he was asking if one can precisely determine prism amount in a power lens without knowing the reference point (of any name).
    We would need the distance PD, and the pupil heights, both transferred to the eyeglass lenses.

    FWIW, Prism Reference Point has been the accepted term by all of the relevant agencies for at least ten years. Major Reference Point was dropped because it was sometimes used incorrectly and was lacking in specificity. The German word is Prismenbezugspunkt.

    As to point #2, if it's a low power lens (say +1.00) with a decent amount of prism (say 2^) I think you're going to see very little deviation of the mires when you wiggle the lens around on the lensometer. It's a "near plano power prism", of sorts...
    True. However, if the lenses were decentered 5mm per, and the lenses (SV) were reversed (how many times have we seen that happen!), we would have insufficient data to come to a meaning conclusion.

    go_hercules,

    Did we answer to your questions? I could post some fun examples if you wish. Let me know.

    Best regards,

    Robert Martellaro
    Roberts Optical Ltd.
    Wauwatosa Wi.
    www.roberts-optical.com
    ~~~~~~~~~~~~~~~~~~
    An expert is a man who has made all the mistakes which can be made, in a narrow field. -Niels Bohr

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    What's up? drk's Avatar
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    What? My 1987 optics course is obsolete?

    What about "level MRP"? Obsolete, too?

    What about my "Members Only" jacket?

    My Renault Encore Alliance?

    Next you're going to tell me Kevin Bacon and Patrick Swayze aren't cool anymore...
    Last edited by drk; 04-24-2017 at 11:42 AM.

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    Quote Originally Posted by drk View Post
    What? My 1987 optics course is obsolete?
    Just some of the terminology.

    What about "level MRP"? Obsolete, too?
    There you go.

    What about my "Members Only" jacket?
    Oh yeah.

    My Renault Encore Alliance?
    One of my optician friends drove an AMC Pacer.

    http://sfcitizen.com/blog/wp-content...80451-copy.jpg

    Imagine an Iranian Jew and a mostly Sicilian Catholic (recovering) driving around in a Pacer, playing pool mostly at Marquette U's taverns.

    BTW, the Renault Encore and Pacer were both made in Kenosha Wi. about fifty miles south of here.

    Next you're going to tell me Kevin Bacon and Patrick Swayze aren't cool anymore...Yes!
    Keven Bacon was never cool!

    Cheers,

    Robert
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    www.roberts-optical.com
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    OptiBoard Professional Kujiradesu's Avatar
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    Quote Originally Posted by Robert Martellaro View Post
    Keven Bacon was never cool!
    Totally off topic. Many years ago, I was in the lobby of The Angelica Film Center in NYC. I had bought my ticket and was heading for my theater when coming out of the same theater is Kevin Bacon with a ballcap pulled down over his eyes. I see him and we lock eyes, I recognize him, and Im not an ******* so I didnt stop or make a big deal I just nodded and continued on to my movie. The look that crossed his face in that instant, I swear he was dissappointed that I didnt make a fuss and ask for his autograph.
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    OP here. Yes, I think my question was answered, thank you everyone for the help.

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