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Thread: Lens meter questions!

  1. #1
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    Confused Lens meter questions!

    Hi, I've been trying this question and just want to check if I'm doing anything wrong (or right) as the case may be!

    1. A lens meter (lens clock), calibrated for a refractive index of 1.523, is used to
      measure the front and back surface powers of a toric spectacle lens. Whenapplied to the front surface of the lens the lens meter gives values of +5.00Dalong 90 degrees and +7.50D along 180 degrees. When applied to thespherical back surface the lens measure gives a value of -3.00D. The centrethickness of the lens is 1cm and the refractive index is 1.700. What is theback vertex power of the spectacle lens? Express your answer in negativesph-cyl form with the powers correct to two decimal places. (11 marks)




    I get +6.20/-3.41x90?

  2. #2
    Master OptiBoarder MakeOptics's Avatar
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    Quote Originally Posted by Hannah Doidge View Post
    Hi, I've been trying this question and just want to check if I'm doing anything wrong (or right) as the case may be!

    1. A lens meter (lens clock), calibrated for a refractive index of 1.523, is used to
      measure the front and back surface powers of a toric spectacle lens. Whenapplied to the front surface of the lens the lens meter gives values of +5.00Dalong 90 degrees and +7.50D along 180 degrees. When applied to thespherical back surface the lens measure gives a value of -3.00D. The centrethickness of the lens is 1cm and the refractive index is 1.700. What is theback vertex power of the spectacle lens? Express your answer in negativesph-cyl form with the powers correct to two decimal places. (11 marks)




    I get +6.20/-3.41x90?
    Covert the curves from your lens clock's index to the materials index

    Front@090 = +5.00 * (1.70 - 1)/(1.53 - 1) = +6.60D
    Front@180 = +7.50 * (1.70 - 1)/(1.53 - 1) = +9.91D
    Back = -3.00 * (1.70 - 1)/(1.53 - 1) = -3.96D

    Now lets solve with thickness
    Last edited by MakeOptics; 03-22-2016 at 01:08 PM.
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    I see! Thank you so much, I'm thinking this forum is much more worthy of my £9,000!

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    Master OptiBoarder MakeOptics's Avatar
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    Quote Originally Posted by Hannah Doidge View Post
    I see! Thank you so much, I'm thinking this forum is much more worthy of my £9,000!
    I think an education through the UK in optics is worth the pittance. I didn't really understand optics until reading works coming out of the UK, most of the US based stuff outside of Optometry is rote memorization of formula's it's left to the student in some cases to understand the underlying theory and root of the equations. In the UK I have noticed that most of the work builds upon theory and proofs in the US equations are just given without their domains or ranges. A common case is many of the US based formula's pertain to Gaussian optics, with third order and higher being treated as exotic with very little literature.
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    Quote Originally Posted by MakeOptics View Post
    In the UK I have noticed that most of the work builds upon theory and proofs in the US equations are just given without their domains or ranges. A common case is many of the US based formula's pertain to Gaussian optics, with third order and higher being treated as exotic with very little literature.
    There has been a decent amount of R&D and publications about higher order aberrations and their coorelation to aspheric and free form optics over the past 10-15 years.

    You won't find them in ophthalmic optics though. Subscribe/join OSA/SPIE.

    All the popular optical design software(s) available are slowly implementing ray tracing support, tolerancing, and design optimization into their packages.

    For now, the main problem isn't so much the design or understanding, but the metrology.

    You can somewhat easily design a lens that's better than something else with Zernike or some other higher order polynomial. The problem is confirming you made what you designed.

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    Master OptiBoarder MakeOptics's Avatar
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    Quote Originally Posted by ml43 View Post
    There has been a decent amount of R&D and publications about higher order aberrations and their coorelation to aspheric and free form optics over the past 10-15 years.

    You won't find them in ophthalmic optics though. Subscribe/join OSA/SPIE.

    All the popular optical design software(s) available are slowly implementing ray tracing support, tolerancing, and design optimization into their packages.

    For now, the main problem isn't so much the design or understanding, but the metrology.

    You can somewhat easily design a lens that's better than something else with Zernike or some other higher order polynomial. The problem is confirming you made what you designed.
    Well put. I am a member of the OSA the value is there with I think 20 article downloads a year that's a value of roughly $600, I was spoiled for years living near the National Library of Medicine so could pull unlimited amounts of research as time permitted. Then I moved near Johns Hopkins, so I paid a student every year for his username password for the schools VPN which again allowed me to pull unlimited amounts of research. Now the best thing I have is OSA for the 20 articles, I pull them sparingly but still feel anemic with only roughly 2 research articles per month, and even then sometimes the summary is a bit misleading and I pull a dud, or the research lead nowhere tangible.
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    Quote Originally Posted by MakeOptics View Post
    Well put. I am a member of the OSA the value is there with I think 20 article downloads a year that's a value of roughly $600, I was spoiled for years living near the National Library of Medicine so could pull unlimited amounts of research as time permitted. Then I moved near Johns Hopkins, so I paid a student every year for his username password for the schools VPN which again allowed me to pull unlimited amounts of research. Now the best thing I have is OSA for the 20 articles, I pull them sparingly but still feel anemic with only roughly 2 research articles per month, and even then sometimes the summary is a bit misleading and I pull a dud, or the research lead nowhere tangible.
    What types of articles/optics are you interested in?

    If you only want to learn more about geometric optics of aspheric/free form single lens systems,

    Clifford Algebra is a nice place to start, along with some type of modeling/computing software such as MatLab.

    Message me if you want more in-depth info/leads.

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