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Thread: Prism and index of refraction

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    Prism and index of refraction

    Hello All

    Can anyone give me an explanation of how index of refraction affects prism in a lens? I have set up software and procedures for prism expressed in the tooling index and in one where everything was ground with the prism in its own index. A situation has come up where a more thorough understanding would be helpful, so anyu education would be appreciated

    Thanks
    Mike

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    Exactly the same as it effects power.
    Try to remember that a lens is nothing but two prisms placed base to base or apex to apex.

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    One more question

    So if I apply prentices rule to get the prism for Decentration it is in the index of the lens?

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    What's up? drk's Avatar
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    In Prentice's rule ("Prism diopters is equal to dioptric power of the lens multiplied by distance from the optical center in centimeters"), index is not a factor.

    The greater refracting ability of higher index materials (better light benders) is already accounted for in the power factor.

    So, it's the same for all indices.

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    What if the lens is a plano?;)

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    Same as if it were a flat sided prism. A prism is a flat sided triangular lens. A lens is just a couple of bent prisms.

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    Sorry Chip, my question was rhetorical. :D

    The index of refraction does effect prism, a plano cr-39 with 10Diopters of prism requires 10.7 Diopters ground into the lens, wheras a 1.8 index lens with the exact same specs requires 6.6 Diopters ground into the lens.

    Prism acts the exact same way as a lens power when it comes to refraction, the higher the index the less angle required to achieve the required bend in light.

    DRK's reply was 2 dimensional, I was trying to get him to use the dark side and think like a lab rat, no offense to the lab rats on board .:D

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    I thought that was what I said.

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    Only the 1st line was for you chip, sorry :)

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    braheem:
    I will now never know if I would have figured out the plano prism question :(

    My initial 2-D take on the question was re: decentration, a linear "surface" measure, but not thickness of the lens, which is essentially what you are asking, I believe, in so many words.

    I'm thinking even if I substituted some equations for terms in Prentice's rule, we'd never get lens thickness to pop out.

    If we were to calculate thickness of a prism, then index and apical angle would be germane.

    Thus, I hope I'm right on analyzing Prentice's rule, and I hope that your question of the special case of F = 0 nullifys it's use, and an entirely different equation must be called in to play.

    I believe that in the lab prism rings are used to surface prism, and that the rings are of a certain thickness calculated to give a prism-dioptric power at an assumed index of refraction.

    True or false assumption: a simple ratio of indices would suffice as a multiplier?
    E.g.: CR39 1-pd ring used for polycarb would be:

    1.50/1.59 x 1 p.d. = 0.94 p.d. in poly.

    So, at the basic theoretical level, we're dealing with thickness, not prism diopters, I think, right?

    Thanks for a good brain-teaser....
    Last edited by drk; 08-02-2007 at 01:12 PM.

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    Prism is afocal so in a sense should have nothing to do with power, drk you could get thickness out of prentices formula, but you would have to be hard pressed and it would still do nothing for you. Check out the paper in the downloads section for more on prisms and low powers and you could get all the answers you need.

    drk, yesy prism rings were the back in the day way of surfaceing prism, but nowadays the generator does it for you. True story I worked at an old store that got rid of an old coburn generator and we got more money for the rings thatn the generator.
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    Quote Originally Posted by HarryChiling View Post
    True story I worked at an old store that got rid of an old coburn generator and we got more money for the rings thatn the generator.


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    Quote Originally Posted by drk View Post
    braheem:
    I will now never know if I would have figured out the plano prism question :(

    My initial 2-D take on the question was re: decentration, a linear "surface" measure, but not thickness of the lens, which is essentially what you are asking, I believe, in so many words.

    I'm thinking even if I substituted some equations for terms in Prentice's rule, we'd never get lens thickness to pop out.

    If we were to calculate thickness of a prism, then index and apical angle would be germane.

    Thus, I hope I'm right on analyzing Prentice's rule, and I hope that your question of the special case of F = 0 nullifys it's use, and an entirely different equation must be called in to play.

    I believe that in the lab prism rings are used to surface prism, and that the rings are of a certain thickness calculated to give a prism-dioptric power at an assumed index of refraction.

    True or false assumption: a simple ratio of indices would suffice as a multiplier?
    E.g.: CR39 1-pd ring used for polycarb would be:

    1.50/1.59 x 1 p.d. = 0.94 p.d. in poly.

    So, at the basic theoretical level, we're dealing with thickness, not prism diopters, I think, right?

    Thanks for a good brain-teaser....

    1.0 prism diopter CR ring in poly would be 587/498 = 1.18

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    What's up? drk's Avatar
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    Whoops. Thanks for inverting me.

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    Welcome to the dark side

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    prism problem

    Quote Originally Posted by mtarleton View Post
    Hello All

    Can anyone give me an explanation of how index of refraction affects prism in a lens? I have set up software and procedures for prism expressed in the tooling index and in one where everything was ground with the prism in its own index. A situation has come up where a more thorough understanding would be helpful, so anyu education would be appreciated

    Thanks
    Mike
    Mike - this depends upon how you are dealing with the prism. Are you using rings ( as in the original Coburn phenolic system) or are you blocking it using alloy, as in a prism blocker?

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    Richard Smith

    Mike, this is my first submission to this site, so forgive me if I am clumsy! Are you using rings, or a prism blocker? If you are using phenolic rings ( as per the original Coburn system) or a prism blocker, the advice is different. Do you know by chance Joe Tusinski, who lives in Muskogee/Tulsa?

    Kind Regards,

    Richard

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    The prism that is called for the grinding aspect will differ with different indexes, as will the thicknesses - the prism being read will not change. As the index gets higher the less prism that your lab ticket will call for but will still give the correct prism in your final product. Hope this helps. Gary

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    ATO Member HarryChiling's Avatar
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    Gary, the reference index for your grinding and tooling is often times 1.53. o just as you would convert over to the true power with:

    Dtool=[(1.53-1)/(n-1)]*Dmaterial

    The same goes for the Prism:

    Ptool=[(1.53-1)/(n-1)]*Pmaterial
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    Quote Originally Posted by richard smith View Post
    Mike - this depends upon how you are dealing with the prism. Are you using rings ( as in the original Coburn phenolic system) or are you blocking it using alloy, as in a prism blocker?
    Interesting thread, albeit an old thread. After several long conversations with my lab rat friend I started been studying the surfacing side of production. Specifically the concepts and formulas for custom lens production. I was reading about prism and prism rings and could not find a formula to determine the actual prism ring design as it pertains to index and blocker diameter. Does anyone know this formula and/or have a link to read more about prism rings?

    Thanks,
    Doc

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    Doc, you need to hire a good hard core lab rat. :D

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    I believe I figured it out, 4 cups of coffee later and the following formulas:

    P = 100 tan(d)

    where "d is the angle of deviation

    d= a(n-1)

    where "a" is the true angle for a given index

    The plug in

    Tan "a" = O (prism base)/outer ring diameter

    If the prism ring apex thickness (X mm) then the true base prism ring thickness would be X mm + O (prism base).


    Please correct me if this is wrong!


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    deleted
    Last edited by DocInChina; 06-09-2008 at 01:12 AM.

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    Doc

    Generally a prism ring is 50mm in diameter and if you look at a side profile you will see it is like a wedge. The thickness difference between each side produces the prism needed.

    t is the thickness difference in mm
    d is the diameter in mm
    p is the prism power

    So if you have a 2^ prism ring with a diameter of 50mm what is the difference in thickness. The index used is 1.53, standard tooling index.


    If the thinner side measures 2mm then the thicker side will measure 3.9mm

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    Thanks, LG

    Much easier formula than the way I did it.

    Doc

    Quote Originally Posted by lensgrinder View Post
    Doc

    Generally a prism ring is 50mm in diameter and if you look at a side profile you will see it is like a wedge. The thickness difference between each side produces the prism needed.

    t is the thickness difference in mm
    d is the diameter in mm
    p is the prism power

    So if you have a 2^ prism ring with a diameter of 50mm what is the difference in thickness. The index used is 1.53, standard tooling index.


    If the thinner side measures 2mm then the thicker side will measure 3.9mm

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