ASK THE LABS
By Joseph L Bruneni

A SUGGESTION FOR PRISM SEGS

Q I have been making eyewear for almost 25 years and early on had a request for prism at near only in a bifocal. I came up with an easier approach, though it too requires skill. Ben Franklin had the right idea, and following it, I had a lab grind pairs of top and bottom corrections using matching base curves. I created top and bottom patterns, ground off the edges to match (getting them flat was the tricky part), and glued them together. The net effect was beautiful because there was no ledge on the front since the front surfaces had identical curves. I edged them to fit the frame after gluing them, making that part of the work easy.

This customer had demanded executive style bifocals but I had another idea that is easier, if not as aesthetically pleasing. We now order FT-45 bifocals and if the add power is high enough, decenter the segments using Prentice�s Rule to create prism. The seg is wide enough to do this without sacrificing usable reading area. For example, a prescribed +2.00 add power can yield two prism diopters, base in, if ordered with 10mm added to the seg inset. Example: order 2mm decentration for distance and 12mm for near. The distance O.C. will be on the visual axis, but the near is shifted 10mm x 2D = 2 prism diopters each eye. The 45mm seg still leaves an ample 22mm usable reading area. While not for all cases, when it falls in range it is an elegant solution to the problem.

�J. Scott Nagel, FNAO/ABO, Park West Vision, Brooklyn, N.Y.

Editor�s note: Greg Stephens from the University of Houston College of Optometry also suggested this sensible approach for creating prism segs.

LARGE FRAMES

Q There are very limited selections for the customer requiring a large frame, especially one made of plastic. I have a customer in need of a plastic frame with a 16mm keyhole bridge, 150mm temple length and 54-56 eye size. Out of the hundreds of frames that I have searched, I have yet to find a frame that meets all of these requirements.

�Harry A. Brown, product manager, BASCO

A ArtCraft has a frame called Passport 15 that comes in a 55-17-150 size (a single millimeter off the bridge size he indicated). Would this be close enough? Oh, and it is a keyhole bridge!

�Mike Franz, Art-Craft Optical, Inc., Rochester, N.Y.

Q Our lab recently purchased a new photometer. After calibrating the unit, we noticed that most polycarbonate lenses registered an 8 percent or above (UV transmission). The manufacturer of the photometer said the lenses were manufactured with insufficient UV-absorbing additive. The lens manufacturer�s reply was that the photometer was not accurate enough. Why the disparity?

�Ken Sambuchi, lab technician, Pearle Vision, St. Louis

A Ask the Labs receives more questions on this subject than any other. In a paper written by Corning scientist H.L. Hoover, it was pointed out that the only universally recognized instrument for measuring UV is the spectrophotometer, and no retail office or laboratory could justify the cost of such an instrument. The meters you describe never provide valid comparisons of different products or give any UVB information or true UVA absorption data.  In the OLA�s book, �Spectral Transmittance of Lens Materials,� data for polycarbonate shows transmittance is zero for both UVA and UVB. The OLA�s technical director reminds us that the material tested must be a true plano lens or data will be skewed. One valid way to calibrate any type of UV meter is to check a plano poly lens which should read zero UV transmission.

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