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Thread: CR-39 vs 1.6 weight in very low powers?

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    CR-39 vs 1.6 weight in very low powers?

    In general 1.6 is about 20% lighter than CR-39. Does the same apply to plano lenses or very low powers (0.25-0.75) or is there barely be any difference? In a lightweight titanium frame 20% lighter lenses would amount to about 10% less weight in total.

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    Most high index lenses have a higher specific gravity (heavier) than cr39. 1.60 has about the same as cr. Unless your 1.60 lenses are substantially thinner (less material), than cr, they will weigh about the same.

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    I've never really compared thickness in low power lenses between 1.5 and 1.6. According to lens thickness calculator even plano lenses are 20% thinner in 1.6 material. Is that really the case though?

    I'd prefer trivex for lighter weight but we don't offer that unfortunately. I'll have to ask around if we could make an exception.

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    What's up? drk's Avatar
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    This ^^^ is confusing, but I know it's true.

    I'm pretty sure if you had a neat 1 cm x 1 cm x 1 cm dice-sized-ice cube of each material, it would be fun to see how much they weighed. That would represent specific gravity.

    But to see how it would perform in a lens, you'd have to cut some lenses at the same power for each material and see how much they weighed. That would represent weight. Some would be thinner and lighter. Some woud be thiner and heavier! Some would be thicker and heavier. Some would be thicker and lighter!

    But my question to the board: See if it's right.

    If you compared say a 1.67 which is thinner but heavier to Trivex which is thicker but lighter
    @-3.00
    @-9.00
    @-15.00

    I belive that the winner at -3.00 would be the winner at -15.00, as the PERCENTAGE difference in weight would be constant. 15% lighter is 15% lighter at all powers. Does this sound right?

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    One eye sees, the other feels. OptiBoard Gold Supporter
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    Quote Originally Posted by Airegin View Post
    I've never really compared thickness in low power lenses between 1.5 and 1.6. According to lens thickness calculator even plano lenses are 20% thinner in 1.6 material. Is that really the case though?
    No. In the USA, it's lighter because it can be surfaced thinner and still be as impact resistant as CR39.

    In general, as the density decreases, the index of refraction decreases. Conversely, the density usually increases as the index increases. It all tends to be a wash, where 1.74 weighs about the same as 1.60 because although thinner, its density is .14 g/cm3 greater.

    When I discuss with my clients the use of lower index lenses (excluding cr39/crown glass) to minimize chromatic aberration, I emphasize that although the lens will be somewhat thicker, it won't be heavier!

    Best regards,

    Robert Martellaro
    Last edited by Robert Martellaro; 06-16-2020 at 09:24 AM.
    Roberts Optical Ltd.
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    Quote Originally Posted by Robert Martellaro View Post
    No. In the USA, it's lighter because it can be surfaced thinner and still be as impact resistant as CR39.

    In general, as the density decreases, the index of refraction decreases. Conversely, the density usually increases as the index increases. It all tends to be a wash, where 1.74 weighs about the same as 1.60 because although thinner, its density is .14 g/cm3 greater.

    When I discuss with my clients the use of lower index lenses to minimize chromatic aberration, I emphasize that although the lens will be somewhat thicker, it won't be heavier!

    Best regards,

    Robert Martellaro
    I was only talking about 1.5 vs 1.6 which both have about the same density (1.6 being slightly less dense). In that specific case thinner definitely means lighter!! But yes, 1.67 and 1.74 will end up weighing about the same as 1.6.

    I was just wondering if 1.6 plano lenses are in fact 20-25% thinner than 1.5? Or does that only apply to higher powers?

    Then there's the difference in Abbe value between 1.5 and 1.6. Does it really make any difference in clarity at all for very low powers?
    Last edited by Airegin; 06-15-2020 at 02:45 PM.

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    What's up? drk's Avatar
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    In my thinking, plano lenses are as thick or thin as you want them to be.

    And light dispersion (chromatic aberration) is a factor of how long the light stays in the material. But it has to be non-parallel surfaces (prismatic or + or - meniscus) in order to disperse laterally.

    Yes, there's probably minimal transverse (?) chromatic aberration, too, but I'm not sure about that, and it doesn't matter.

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    https://www.laramyk.com/resources/ed...ens-materials/
    Just fun info at the bottom chart, pretty well-known.

    Darryl's expert take on lens form and weight is interesting, about halfway down:

    http://64.50.176.246/cecourse.php?url=high_powered/

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    Quote Originally Posted by drk View Post
    https://www.laramyk.com/resources/ed...ens-materials/
    Just fun info at the bottom chart, pretty well-known.

    Darryl's expert take on lens form and weight is interesting, about halfway down:

    http://64.50.176.246/cecourse.php?url=high_powered/
    Thanks, some good information there. Which makes me now wonder about light transmission. Reflectance rates of different materials are compared with no AR but how do they all compare with AR? CR-39 with AR has a light transmission rate of 99.2%. Is it lower for 1.6 with AR?

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    What's up? drk's Avatar
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    Good question.

    Can AR work equally efficiently regardless of the task it faces?

    I'm going to guess that, yes, they can vary the thickness or quality of the coating for a certain substrate index to make it close to equally efficient. But that's a guess.

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    Quote Originally Posted by drk View Post
    And light dispersion (chromatic aberration) is a factor of how long the light stays in the material.
    For ophthalmic lenses and visible light, there is linear relationship between the speed of light through a material and the refractive index, but not with the material's level of dispersion, although most but not all high index lenses are more dispersive than lower index materials.

    Best regards,

    Robert Martellaro
    Roberts Optical Ltd.
    Wauwatosa Wi.
    www.roberts-optical.com
    ~~~~~~~~~~~~~~~~~~
    Science is a way of trying not to fool yourself. - Richard P. Feynman

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    Quote Originally Posted by Airegin View Post
    I was just wondering if 1.6 plano lenses are in fact 20-25% thinner than 1.5?
    Comparing refractive index only, (1.50 - 1.00) / (1.60 - 1.00) = 0.83 or 17% thinner. But the surfacing center or edge thickness inputed will be the deciding factor, not the refractive index. One might use 1.60 for a very low power because it's more impact resistant, but not because it's thinner or lighter. If I wanted the lightest weight and safest lens in a low power I'd choose Trivex, if available.

    Or does that only apply to higher powers?
    For simplicities sake, above ±1.00 D.

    Then there's the difference in Abbe value between 1.5 and 1.6. Does it really make any difference in clarity at all for very low powers?
    No, except for hypersensitive types, or very high prescribed prism values.

    Hope this helps,

    Robert Martellaro
    Last edited by Robert Martellaro; 06-16-2020 at 10:10 AM.
    Roberts Optical Ltd.
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    www.roberts-optical.com
    ~~~~~~~~~~~~~~~~~~
    Science is a way of trying not to fool yourself. - Richard P. Feynman

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    Quote Originally Posted by Robert Martellaro View Post
    Comparing refractive index only, (1.50 - 1.00) / (1.60 - 1.00) = 0.83 or 17% thinner. But the surfacing center or edge thickness inputed will be the deciding factor, not the refractive index. One might use 1.60 for a very low power because it's more impact resistant, but not because it's thinner or lighter. If I wanted the lightest weight and safest lens in a low power I'd choose Trivex, if available.

    For simplicities sake, above ±1.00 D.

    No, except for hypersensitive types, or very high prescribed prism values.

    Hope this helps,

    Robert Martellaro
    Thanks, that's very helpful. So if I get it right even plano lenses will be lighter in 1.6 vs 1.5 even if it's only 1 gram per lens?

    I can't order Trivex where I work unfortunately.

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    Quote Originally Posted by Robert Martellaro View Post
    For ophthalmic lenses and visible light, there is linear relationship between the speed of light through a material and the refractive index, but not with the material's level of dispersion, although most but not all high index lenses are more dispersive than lower index materials.

    Best regards,

    Robert Martellaro
    That does raise an interesting question, what does dictate dispersion? I have to say, despite my education/training, I don't think I've ever heard a good technical explanation for that.

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    One eye sees, the other feels. OptiBoard Gold Supporter
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    Probably from different additives mixed with the monomer. One additive might make it tint better, another to increase heat resistance, etc. A witches brew so to speak.

    Ideally the additive won't negatively effect material performance in other areas, but I imagine that would be the biggest challenge, taking from Peter, and not paying Paul.

    Best regards,

    Robert Martellaro
    Roberts Optical Ltd.
    Wauwatosa Wi.
    www.roberts-optical.com
    ~~~~~~~~~~~~~~~~~~
    Science is a way of trying not to fool yourself. - Richard P. Feynman

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    Quote Originally Posted by Lelarep View Post
    That does raise an interesting question, what does dictate dispersion? I have to say, despite my education/training, I don't think I've ever heard a good technical explanation for that.
    the varying wavelengths go through the material at different speeds where the curve of the lens is more severe such as in a high minus lens toward the peripheral edge. The colors divide, and the patient sees them when their eye gaze focuses through that part of the lens. General rule; the higher the index the lower the ABBE value, the more dispersion.

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    Per Darryl's Spectacle Optics program, a 50mm diameter plano CR-39 weighs 5.3 grams, MR6 1.60 4.0 grams. However this assumes a standard center thickness of 2mm on CR-39 and 1.5 on 1.60. If one got the CR-39 down to 1.5 as well then for the same volume of material the lower density CR-39 would be lighter, ever so slightly.

    Quote Originally Posted by drk View Post
    If you compared say a 1.67 which is thinner but heavier to Trivex which is thicker but lighter
    @-3.00
    @-9.00
    @-15.00

    I believe that the winner at -3.00 would be the winner at -15.00, as the PERCENTAGE difference in weight would be constant. 15% lighter is 15% lighter at all powers. Does this sound right?
    Looks like not. This looks like exactly what high index are supposed to be for, that as power dramatically increases the gains in thinness overtake the losses in density. CR-39 is always the ultimate loser, but note at -12.00 (the program only goes up to 12 otherwise I would have given you your -19.00) the lightest of all, trivex, is not very light, just because a -12.00 trivex is super thick.

    And bonus, this surprised me, check out how poly is even at 1.5 CT. Grind that down to 1.0 and it could end up being the lightest even at -19.00!


    material -3.00 -9.00 -12.00
    CR-39 (CT 2.0) 7.9g
    14g 18.1g
    poly (CT 1.5) 5.6g 10g 12.6g
    trivex (CT 1.5) 5.4g 10.1g 13.1g
    MR6 1.60 (CT 1.5) 6.2g 11g 13.8g
    MR10 1.67 (CT 1.5) 6.1g 10.4g 12.8g
    1.74 (CT 1.5) 6.3g 10.3g 12.6g
    Last edited by DanLiv; 06-18-2020 at 01:12 PM. Reason: added g and CTs

  18. #18
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    Man, that's good stuff.

    The upshot, to me, is that it all seems to balance out, with weight, by and large, except for CR39.

    So, I'll try to remember:
    "Any high index material should reduce the weight compared to CR39, about the same, at any power."

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    Quote Originally Posted by DanLiv View Post
    Per Darryl's Spectacle Optics program, a 50mm diameter plano CR-39 weighs 5.3 grams, MR6 1.60 4.0 grams. However this assumes a standard center thickness of 2mm on CR-39 and 1.5 on 1.60. If one got the CR-39 down to 1.5 as well then for the same volume of material the lower density CR-39 would be lighter, ever so slightly.



    Looks like not. This looks like exactly what high index are supposed to be for, that as power dramatically increases the gains in thinness overtake the losses in density. CR-39 is always the ultimate loser, but note at -12.00 (the program only goes up to 12 otherwise I would have given you your -19.00) the lightest of all, trivex, is not very light, just because a -12.00 trivex is super thick.

    And bonus, this surprised me, check out how poly is even at 1.5 CT. Grind that down to 1.0 and it could end up being the lightest even at -19.00!


    material -3.00 -9.00 -12.00
    CR-39 7.9
    14 18.1
    poly 5.6 10 12.6
    trivex
    5.4 10.1 13.1
    MR6 1.60 6.2 11 13.8
    MR10 1.67 6.1 10.4 12.8
    1.74 6.3 10.3 12.6
    Just to be clear...for me. The chart shows materials at all the same CT?
    The numbers across are weight in grams?

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    Note that the weight difference is not as big for plus lenses. For +2.00, 1.6 seems to be only about 8-10% lighter while for -2.00 it's closer to 20%.

    I'm surprised though that plano lenses and very low powers are also about 20% lighter. Something to keep in mind for customers with very low powers who want the lightest weight possible. It will be noticeable, even more so in lightweight titanium frames.

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    Quote Originally Posted by Don Gilman View Post
    Just to be clear...for me. The chart shows materials at all the same CT?
    The numbers across are weight in grams?
    Yes weight in grams, and CT is 1.5 except for CR-39 which is 2.0.

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    Quote Originally Posted by DanLiv View Post
    Per Darryl's Spectacle Optics program, a 50mm diameter plano CR-39 weighs 5.3 grams, MR6 1.60 4.0 grams. However this assumes a standard center thickness of 2mm on CR-39 and 1.5 on 1.60. If one got the CR-39 down to 1.5 as well then for the same volume of material the lower density CR-39 would be lighter, ever so slightly.



    Looks like not. This looks like exactly what high index are supposed to be for, that as power dramatically increases the gains in thinness overtake the losses in density. CR-39 is always the ultimate loser, but note at -12.00 (the program only goes up to 12 otherwise I would have given you your -19.00) the lightest of all, trivex, is not very light, just because a -12.00 trivex is super thick.

    And bonus, this surprised me, check out how poly is even at 1.5 CT. Grind that down to 1.0 and it could end up being the lightest even at -19.00!


    material -3.00 -9.00 -12.00
    CR-39 (CT 2.0) 7.9g
    14g 18.1g
    poly (CT 1.5) 5.6g 10g 12.6g
    trivex (CT 1.5) 5.4g 10.1g 13.1g
    MR6 1.60 (CT 1.5) 6.2g 11g 13.8g
    MR10 1.67 (CT 1.5) 6.1g 10.4g 12.8g
    1.74 (CT 1.5) 6.3g 10.3g 12.6g
    This 2008 paper by Ed De Genarro states that for spherical 50mm diameter lenses with the same center thickness,

    "Trilogy lenses are lighter than polycarbonate from +12.00D through-11.00D and equal in weight for -12.00DRxs.Comparedto1.67, Trilogy lenses are always lighter across the +12.00D and -12.00D Rx range"



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    What's up? drk's Avatar
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    Yeah, but how much are we talking?

    One gram = one paperclip. Balance a paperclip on the front of your glasses and tell me how your nose hurts.


    Dang it, I almost poked my eye out!

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    I have no idea how much. Just throwing some other info I found into the ring. Someone would have to contact Ed or younger to see how they came to their conclusions.

    I will say that I use a ton of trivex, probably in powers that most others would scoff at. Just anecdotally speaking, when I do order a job in high index, the uncut blanks feel like bricks comparatively. Obviously the frame is going to have a major effect on the end result. Who cares if the lens is ±1 gram if the frame weights 25grams? But, if the frame is only 3 grams, the lens weight will be more significant. There is a noticeable difference in wight in my hand glazing Lindberg frames with trivex vs high index, across a broad spectrum of powers.

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    Quote Originally Posted by drk View Post
    Yeah, but how much are we talking?

    One gram = one paperclip. Balance a paperclip on the front of your glasses and tell me how your nose hurts.


    Dang it, I almost poked my eye out!
    I have weighed a bunch of titanium frames since I'm looking to order a very lightweight pair for myself. Most weigh about 10 grams. I have very low powers and the CR-39 lenses from my current pair weigh about 6.5 grams each. So the total weight would be 23 grams with CR-39 and 20-20.5 grams with 1.6 lenses. That's at least 10% less. Not a ton but I bet it will be noticeable.

    I also recently ordered a new pair for my grandfather. His old pair was 29 grams, new one 26 grams (lighter frame). Only 3 grams difference but he says it feels quite a bit lighter.
    Last edited by Airegin; 06-18-2020 at 03:01 PM.

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