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Thread: Dispensing Aspheric lenses

  1. #1
    Bad address email on file QDO1's Avatar
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    Dispensing Aspheric lenses

    When fitting an aspheric it is useful to realise that in fact it is like fitting two things at once - the physical or geometric centre of the aspheric portion of the lens, and the optical centre of the lens. Usually these are coincident with each other



    To understand this further, a description of an aspheric lens is useful. Aspheric means "NOT spherical". This means that the shape of the lens surface is not homogeneous or even, but varies across the lens surface. Put mathematically the surface does not have a formula:



    Radius = X



    X being the same radius all over the surface



    Generally speaking the aspheric surface is found on the front of the lens. when checking the lens position the lens on the focimeter so that you are aligned through the geometric centre or middle of the aspheric side of the lens. in this area of the lens, the power ought to measure exactly what the prescription was prescribed as. As we move away from the geometric centre of the aspheric surface, the prescription alters - inline with the way the lens surface changes power. The aspheric lens designer specifies a portion, which will be the central region of the aspheric side of the lens to be guaranteed 100% full prescription; the rest of the lens will have a different prescription. The size of this portion depends on what formula the designer used to formulate the shape of the curves



    This sounds terrible you say, well actually, the lens designer can correct some of the unwanted aberrations that a regular spherical lens exhibits at the edges. That’s right - we don’t just dispense aspherical lens designs because they are thinner, but because they improve the quality of vision. The fact they are often thinner is a nice side effect of the process of eliminating some of the peripheral aberrations that spherical lenses can exhibit



    There are many types of aspherical surface, and it is wise to think that "Aspherical" describes "ANY" lens that is NOT spherical. So some of the low base aspherical lenses might exhibit quite poor peripheral optics as a result of only being designed for thinness



    I have mentioned the peripheral portions and central portions of an aspheric lens as being different from each other; this is the reason why it is imperative that the centre of the aspheric side of the lens is accurately centred monocularlary in terms of height and OC. To that end one ought to compensate the height to allow for the frontal angle of the frame. What we are trying to achieve is the alignment of the optical axis of the eye, with that of the geometric centre of the aspheric side of the lens



    This accurate centring of the aspheric side of the lens, throws up an issue with prescribed prism. Some dispensers like to achieve prismatic effect by decentring the lens. Clearly this will move the optimum centre of the lens away from the optical axis of the eye, which causes unwanted optical problems for the patient. To maintain the alignment of the aspheric geometric centre with the pupil and have prism in the lens, we need to order the lens with surfaced prism. Typically the lens will have a aspheric front side, and rear surface with the prism and cylinder surfaced in it

    Patients who have migrated from a regular spherical lens, or another aspherical lens may find the periphery of the lens feels "odd" for a while, after collecting thier new spectacles. This is normal, and is part of the normal adaption process. This ocours, because as the lens is different in its periphery, the periphary of the image it presents to the eye is different too




    In summary


    • The aspheric surface must be aligned with the optical axis of the eye. That means mono heights and OC’s must be specified
    • Prism must be ordered by surfacing, and NOT achieved with decentration
    • Aspheric means “Not-Spherical” which describes a multitude of non spherical surfaces
    • There are many different types of aspheric lens, some make the lens thinner, some improve optics, some both make a lens thinner and improve optics
    • When checking an aspheric lens, the power generally reduces towards the edges of the lens, so the power on the optical centre ought to be the one prescribed
    • There may be an adaption time for new spectacles
    Last edited by QDO1; 12-22-2005 at 06:12 AM.

  2. #2
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    I appreciate the elaboration on Aspheric lenses here, quite the in depth description.

    Slightly off topic - I have a pt. with rx as follow:

    -0.25 / -3.00 x 90
    +2.75 / -2.75 x 15

    Requires a single vision distance set of specs, I have been in the habit of taking heights with every dispense of -1.00 cyl and above, this pt. however has OCs of 30.5 / 34.5 and measured heights of 19.5 / 23.5 (above rim), now naturally I would keep the heights at exactly what they are measured at and not split (not sure why people get in the habit of splitting) and use an aspheric lens to balance out the lenses as best as possible - thickness and weight imbalance wise.

    But I just wanted to get some input as to what would be the most beneficial path for the pt. in these kinds of cases, vision wise? Would there be other methods of dispensing that would be of better aid to him? Different lenses or different techniques, as he has struggled - in the past, with head movements giving him instability.


    Regards,

    Sam

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