Ask the Labs
By Joseph L. Bruneni

MORE ON WHICH WAY IS UP

Q I would like a clarification of "Which way is up" in your February column. It may be semantics, but I would suggest that Jim Lemmons has the alignment issue 90 degrees off. The stria in a polarized lens typically are designed to form a vertical bar gradient to filter out the light being reflected off a horizontal surface. Light reflected off a horizontal surface has been polarized, i.e., aligned horizontal. The vertical bar gradient prevents this light from passing through the lens.

A The principle involved in polarization is simple. Explaining it is more involved. Jim Lemmons stated that the polarizing film in a polarized sun lens is placed "so that the stretch lines are horizontal in the lens when mounted in the frame." He now suggests visiting a University of Colorado Website that explains how a polarizing filter is made up of horizontal parallel strands of long molecules. The energy of horizontal light components (reflections from surfaces like water or pavement) are absorbed or blocked. The site is www.colorado.edu/physics/2000/polarization/polarizationII.html.

ANOTHER NEW ITEM

Q Some lab people were telling us recently about "point files." They said these were going to revolutionize ophthalmic lenses and they had something to do with computer software for labs. We'd like to know more.

A Point files are creating excitement behind the scenes and were the topic at a recent meeting of the California Optical Laborato-ries Association. A few labs now have a new type of generator that can cut almost any type of surface, from aspheric to atoric or the curve of a progressive lens. With the right type of polisher, labs can produce progressive lenses from slabs of plastic. The equipment requires precise instructions from a computer. A point file is a set of computer instructions telling the generator how to cut progressive curves, for example. Currently, the best labs fabricate lens surfaces accurate to one tenth of a diopter. But point file instructions produce surfaces accurate to one hundredth of a diopter. Further, they enable labs to produce customized brand name progressives. The progressive curves are on the back surface and the distance portion is totally atoric. Point files are purchased from the lens designer and can only be used once. One manufacturer is currently delivering point files and other manufacturers plan to offer them.

Progressive Fitting Heights

Q My question pertains to progressive heights and where to measure them. We were having a discussion on the sales floor as to whether a progressive should be measured to the bottom of the frame directly under the pupil or to the bottom of the lowest part of the frame regardless of where that is, as would be the case in an aviator frame. We had several arguments for both sides. Could you settle this for us?

--Catherine M. Warme, Pearle Vision Franchise, Waterville, Maine

A A good question that illustrates a problem some people may be overlooking. The answer is simple: The lab making the lenses will treat the fitting height for progressives the same as they do a bifocal height. They measure to the lowest part of the frame, whether it's directly below the pupil or not. Imagine a box drawn around the lens shape. The fitting cross height is calculated from the bottom edge of the box around the lens. A dispenser measuring from the pupil center down to the rim of the frame in an aviator frame can end up with progressive lenses positioned too low in the frame. Here's a tip: Don't forget to add for the beveled groove in a metal or zyl frame.

If you have a question you'd like to have answered in Ask the Labs, send it to Joseph L. Bruneni. Fax: 310-533-8165. Phone: 310-533-4975. E-mail: joe@bruneni.com. Or mail questions to: Vision Consultants, 2908 Oregon Court, #I-2, Torrance, CA 90503. An archive of past Ask the Labs columns can be found on the Eyecare Business Website at www.eyecarebiz.com.