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Thread: Digital Fresnel prizm

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    Digital Fresnel prizm

    Hello All,

    Could high prizm 20 PO be digitally surfaced into a lens? I mean instead of applying Fresnel patch? Any company doing that?

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    Master OptiBoarder optical24/7's Avatar
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    Chadwick Optical can do some amazing work, But TMK they don't make a Fresnel surface on the entire lens. They do make a Peli style Fresnel "strip or ribbon" if you will out of CR39 that are 8x22 mm's in size and are cemented into a patients Rx lens at the upper and/or lower lid margins. It's designed pretty much solely for visual field expansion for Hemianopic patients.

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    Thanks for info. This is general curiosity question, but connected with real life problem for young women, she has -3.5OD and 20 PO. I don't understand why it could be a problem for modern digital surfacing: back side, may be hard coat will degrade it a bit, but still better than 3M patch. May be calculations for surface is a problem?

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    Master OptiBoarder optical24/7's Avatar
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    A couple of factors to keep in mind;

    Digital generators cut lenses like it's making a phonographic record: it spins the lens and uses a stylist to cut. To make vertical (or horizontal) cuts like Fresnel types, the generator would have to keep the lens stationary with the cutter moving to cut straight lines. (they currently don't do that.)

    If someone was going to, the most practical way to create a Fresnel design built on the lens surface would be to cast the design on the convex, and surface the power on the concave. There's simply is not enough need out there to to justify the cost in designing the equipment and building up an inventory of semi-finished blanks in the various prism amounts, base curves and materials needed.

    Fresnel surfaces are extremely hard to clean. They look pretty dog gone dirty after 6 months or so. A disposable replacement is pretty practical.

    The biggest help you can provide is a high abbe value lens like Trivex or CR to reduce the LCA's (Lateral Chromatic Aberrations) created by the prism.

    PS; Is the patient 20D out OU or total? If total, splitting the prism will help with cosmetics and is doable on conventional generators.

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