ASK THE LABS
By Joseph L Bruneni

AR & Photochromics

Q Do you know whether AR coating affects how well plastic photochromic lenses darken?

�Rosalie Lee, O.D., Anderson, S.C.

A Yes, AR will always have some effect on photochromic lenses. There are two issues to consider. First, an AR coating can reduce some transmission of UV light; and UV, of course, is what activates most photochromics. Transitions originally advised against applying AR on their lenses. Later, they developed a list of some 50 coaters who had altered their AR formula to avoid blocking UV. Lenses that use the �in-mass� photochromic technology claim no problem with AR. The other issue to keep in mind is that AR will increase light transmission through the lens by some eight percent, so photochromic lenses with an AR coating don�t get quite as dark. But, they�ll be eight percent lighter indoors.

Darker Polarized Lenses

Q A patient wants dark polarized lenses. A lab tells us polarized lenses can only be tinted green or gray, and variable degrees of tint darkness are not available in polarized lenses.

A The lab may assume that hot dye will damage the film within the polarized lens. There is no problem dying polarized lenses as long as the temperature of the dye is kept to 200� or less. Polarized lenses come in at least two shades of gray and two shades in brown (green is not much used). The lighter �A� shades in gray and brown are produced specifically to be dyed darker or for tinting to other colors. A final choice for the patient could be mirror coatings, which are often used for mountain climbers and/or skiers to provide dark lenses for high altitudes.

Thinner Safety Glasses

Q I heard that the 3.0 mm center thickness for industrial safety might be reduced to 2.0mm or less. What�s the latest information?

A We have been advised that industrial safety prescription lenses in poly have been approved for 2mm thickness and the new standard is expected to be published shortly. We�ve also been told that the producers of Z87 safety frames have all submitted their frames for testing with 2mm poly lenses and have been approved. It is wise, however, to wait until you see notice the new standard is in effect before dispensing prescription poly lenses at 2mm.

�Joe Marcella, director, industrial; Safety Sales, Essilor Laboratories

Editor�s Note: Late word informs us that the ANSI (Z87.1) does not identify polycarbonate by name but instead specifies a testing protocol that only poly currently can pass. PPG, producer of the new Trivex lens material, states that Trivex lenses will also meet the new 2mm industrial testing requirements.

Big Frames, Big Pals

Q Some of the new frame styles seem to be getting larger. Will fitting progressive lenses in larger lens sizes be good or bad for wearers?

A In terms of near vision, larger lens sizes will provide PAL wearers with a larger reading area. Short corridor PALs developed for small frame sizes have a harder design, of necessity. Larger lens sizes will give fitters more options in choosing PAL designs and that is an advantage. The only downside to larger lenses is that patients could notice a slight increase in peripheral distortion just above the 180 line. The likelihood of that happening is slim to none. In general, larger sizes will benefit PAL wearers and make the fitter�s job a little easier.