Originally Posted by
Darryl Meister
Prism in a progressive lens should be verified at the prism reference point, not at the fitting cross. This is also the location at which prism is toleranced in optical standards. There are no limits on prism-thinning provided in optical standards, although popular lab management software will generally allow the lab to set reasonable limits based upon clinical judgment.
For a high plus progressive lens with a high add power like this, up to 3.0 prism diopters of prism-thinning at the prism reference point is not uncommon, particularly with higher fitting heights. Given that the PRP of Varilux lenses is 4 mm below the fitting cross, there would be an additional 0.4 cm * +3.00 D = 1.2 diopters of prism due to the location of the fitting cross.
If you are uncomfortable with the amount of prism-thinning involved, or the wearer has presented with symptoms of discomfort while wearing the lenses, you can always discuss the use of less prism-thinning with your laboratory. Of course, this will most likely result in a significant increase in lens thickness and weight with this particular job.
Also keep in mind that, had this been a single-vision job, the optical center would have typically been left along the datum (180) line of the lens. Since the pupil center is 6 mm above the datum with this job, the wearer would have experienced a significant amount (0.6 cm * 3.00 D = 1.8 prism diopters) of vertical yoked prism, either way.
At least one study has shown that excessive vertical yoked prism may result in initial wearer discomfort and excess postural adjustments (see, for instance, Sheedy and Parsons (1987) "Vertical yoked prism - Patient acceptance and postural adjustment" in Ophthal. Physiol. Opt.). However, over time, many spectacle wearers exhibit significant prism adaptation.
Best regards,
Darryl
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