Anybody have idea how to sag a varifocal We sag at the very top of the blank. Anything easier??
Anybody have idea how to sag a varifocal We sag at the very top of the blank. Anything easier??
If it's a cast molded PAL, the front curvatures change below the 180 or PRP line. So above will yield a consistent BC.
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Top portions of many current lenses are aspheric so any sagging will be fruitless unless you are using it for thickness calculations, even then the results will be accurate as long as the shape of the final lens is similar to the distance between the outside posts on your sag gauge the more the difference varies the higher the margin of error. Unfortunately the best results come from using the supplied values from the manufacturer, either on the box, in the LMS, or provided through a data sheet.
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If your trying to sag for power you could clock the back of usually a spherically molded, caliper the thickness and read the back vertex power then calculate the front appropriately, with that known you could the sag the front and use the difference to figure the asphericity. That could give you a better set of numbers for front curve thickness and power calcs. The advantage for having the two front curve values is being able to compute a rotational asphericity which would allow you to apply that value to other finished lens sizes other than your sag gauge diameter making for a more accurate thickness calculation. The assumption that the lens is rotationally symmetrical is still going to produce some error, but it should be negligible.
I would only recommend doing this for lenses that are molded that you use regularly since it would be tedious to perform this action on every lens. I would also recommend documenting the values for each base and add then potentially putting together your own data sheet or loading into your LMS for future work. You could potentially share the data with the manufacturer if they're not to proud, it could save the next guy from having to go through a tedious process.
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Creator of the industries 1st HTML5 Browser based tracer software.
Creator of the industries 1st Mac tracer software.
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