Q. A FLAW IN THE EYE OF THE BEHOLDER?
I have dispensed a lot of polarized sunglasses, but never wore them. I just got my first pair of polarized lenses and I love them. I do notice one unusual effect. The glasses are especially effective for driving, but I see peculiar checkerboard patterns in the windows of automobiles. Is this a flaw in the lenses?

A.
Not at all. Here's what you are seeing: You may find your lab still has an instrument dating back to the 1950s called a "polariscope." They were originally made by American Optical and consisted of two polarized lenses separated by about four inches with an incandescent light bulb behind the rear lens. The instrument dates back to an era when many frames were metal and all lenses were glass. When slightly oversize glass lenses are mounted in a metal frame, they often later crack or chip from internal stress created by frame pressure. Labs found they could view finished glasses between polarized lenses and visibly see internal stress in the lenses.

What you see in automobile windows viewed through polarized sunglasses are stress patterns. You see them because auto glass is tempered for resistance to breakage. You'll notice patterns more in side windows than in windshields. What you see is perfectly normal and proves your sunglasses really are polarized.

Here's a postscript that may interest you. Metal frames are once again popular and oversize lenses are still common. Unfortunately, this creates internal stress in the lens. Since plastic lenses don't chip or break under pressure, the problem is mostly ignored today. This is unfortunate because stress in plastic lenses creates warpage, which can create visual problems for the wearer. A few skilled lab technicians still check all lenses in metal frames in a polariscope to make sure there is no stress in the lenses.

Q. SKULLDUGGERY IN THE MARKETPLACE
We edge lenses in our office. Some of the 1.60 high index stock lenses we are getting recently don't seem to be 1.60 index. The lenses are imported and are not a major brand. The envelope claims they meet FDA requirements. How can we be sure we are getting what we request?

A.
You raise two important points: misrepresenting the index and compliance with FDA drop ball requirements. Of the two issues, the second could have the most serious consequences for you.

You can be sure no recognized lens producers will jeopardize their reputation by misrepresenting the refractive index of a stock lens. If you know the company whose name is on the envelope, you may be willing to accept their labeling. The FDA disclaimer on the envelope is also important because it tells you that the manufacturer has regularly batch-tested every production run to make sure the lenses meet FDA drop ball requirements. The wording on the package is a legal notice that the lenses can be edged, mounted, and dispensed without requiring individual drop ball testing.

However--and this is a big however--if sample lenses from each day's production were not individually tested by the manufacturer (with records to prove it), you are required to individually drop ball test every one of those lenses after edging. This becomes a critical issue if litigation ever results from a broken lens. Currently, there is a flood of plastic stock lenses coming in from third world countries that are re-packaged in the United States by importers. There are stories circulating that some imported stock lenses are mislabeled and do not pass drop ball testing. U.S. Customs does not verify FDA compliance or the refractive index since most lenses are repackaged after going through customs.

For your own protection, you need to be sure the index is not misrepresented and that all plastic lenses you dispense meet the FDA standards.

Q. Mirrors: No Flash Mirrors: in the Pan
We had a patient come in asking for a mirror coating that is clear. Does such a coating exist, and what would be the purpose of dispensing it? I think it's called a flash coating.

A.
Yes, there is such a thing as a "flash" mirror coating. It is produced by applying a lighter mirror coating to clear or lightly tinted lenses. Traditional full mirror coatings (the "highway patrolman" look) are applied over dark sun lenses, enhancing the mirror look.

Flash mirror coatings on clear lenses transmit 80 percent of light through the lens. In appearance, the lens would have a barely visible surreal or mother-of-pearl look. Dispensers can produce attractive cosmetic lenses by having flash mirror coatings applied to clear lenses. They can then tint (solid or gradient) the lenses and produce some stunning effects.

Unusual "Venice Beach" effects can also be created by using a full mirror and, before tinting, block off portions of the back surface with tape in star or other shapes. After tinting, wherever the tape was on the back surface, the mirror effect will be fainter. A flash mirror gives these dramatic lenses a softer, more subtle look.

As you can see, mirrors can be a useful dispensing tool.

--Edwin Ellefsen, Opticote, Chicago EB

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