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Thread: Progressive Lens Terminology

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    Progressive Lens Terminology

    Maybe it's an old school/new school thing, but I've been hearing the term seg height applied in a few different ways. When it comes to progressive lenses, is seg height defined and most commonly used as the distance between the center of the pupil and the bottom of the lens? Or is seg height the beginning of the progressive addition on a progressive lens? What is the most commonly used term for the distance between the pupil and bottom of the lens in a progressive lens?

    Thanks!

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    Honestly, I think there's room for debate here.

    Classically, segment height was used (before the advent of PALs) as a measurement of the height of the reading segment in a bifocal/trifocal (duh...).

    If we interpret the classic definition of segment height, then technically, it ought to mean the measurement from the bottom of the lens box to the beginning of power change in a PAL. But I believe most people use segment height as a measurement from the bottom of the lens box to the pupil center. I suppose either is correct, but I think more people will say pupil center.

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    OptiBoard Professional Kujiradesu's Avatar
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    Progressives have no traditional segment. Ergo no segment (seg) height. Progressives have a fitting height. The fitting height is often measured from the pupil center to the lower eyewire. The fitting height is used to place the fitting cross, or major reference point of the progressive lens. Depending on the progressive design the power progression starts at the fitting cross or a few mm below it.
    Optical Cross: n. crucifixion apparatus used by the New Jersey State Board.

    "It is not knowing, but the love of learning, that characterizes the scientific [person]." -Charles Sanders Peirce

    "A concept is a brick. It can be used to build a courthouse of reason. Or it can be thrown through the window. -Gilles Deleuze

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    So it sounds like a borrowed term (borrowed incorrectly), but everyone uses it as a synonym for the fitting height. What does ANSI, or ABO list as the proper term for the distance between the pupil and bottom of the lens?

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    OptiBoard Professional Kujiradesu's Avatar
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    ANSI: lists various reference points that are of importance to a progressive lens. Distance Reference Point, Fitting Cross/Fitting Point, Near Reference Point, and Prism Reference Point. These reference points are not what you are looking for.

    The reference you are asking for is Pupil Height. A term popularized by Barry Santini, and which may or may not coincide with the Fitting Point.
    Optical Cross: n. crucifixion apparatus used by the New Jersey State Board.

    "It is not knowing, but the love of learning, that characterizes the scientific [person]." -Charles Sanders Peirce

    "A concept is a brick. It can be used to build a courthouse of reason. Or it can be thrown through the window. -Gilles Deleuze

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    Master OptiBoarder OptiBoard Silver Supporter Barry Santini's Avatar
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    I did not popularize it. I used it to defend against the great unwashed’s incorrect use of seg height

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    OptiBoard Professional Kujiradesu's Avatar
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    Optical Cross: n. crucifixion apparatus used by the New Jersey State Board.

    "It is not knowing, but the love of learning, that characterizes the scientific [person]." -Charles Sanders Peirce

    "A concept is a brick. It can be used to build a courthouse of reason. Or it can be thrown through the window. -Gilles Deleuze

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    OptiBoard Professional Kujiradesu's Avatar
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    Quote Originally Posted by Barry Santini View Post
    I did not popularize it. I used it to defend against the great unwashed’s incorrect use of seg height
    And lo the Lord said let thy Pupil Height be thy sword.
    Optical Cross: n. crucifixion apparatus used by the New Jersey State Board.

    "It is not knowing, but the love of learning, that characterizes the scientific [person]." -Charles Sanders Peirce

    "A concept is a brick. It can be used to build a courthouse of reason. Or it can be thrown through the window. -Gilles Deleuze

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    Master OptiBoarder OptiBoard Gold Supporter
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    Quote Originally Posted by Kujiradesu View Post
    And lo the Lord said let thy Pupil Height be thy sword.
    Excalibur!

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    OptiBoard Apprentice JGor's Avatar
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    Yeah. Progressive lens terminology is just that. Terminology for Progressive lenses. Why is there a need to try and translate bifocal terms to a PAL? Therein lie the seeds of madness.

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    Master OptiBoarder DanLiv's Avatar
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    Quote Originally Posted by Kujiradesu View Post
    The reference you are asking for is Pupil Height. A term popularized by Barry Santini, and which may or may not coincide with the Fitting Point.
    Yes, pupil height is what we measure, fitting height is what we fit. Most of the time the two are the same, but from time to time we all alter fitting heights from measured pupil heights.

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