Q. What's the Truth?
We see advertising for photochromic glass lenses that claim 93 to 98 percent UV absorption rate. When we check the lenses in our UV meter, we see a much higher transmission of UV light. Who should we believe?

A.
Commercial UV meters are particularly ill-suited to measuring photochromic lenses. The problem is, readings are usually taken when the photochromic lens is in its clear state. Instead, UV transmission should be measured in the darkened state, as the lenses are when in the presence of UV light.

Darkening the lens outdoors and then attempting to quickly transfer it to the UV meter doesn't work. Photochromic lenses clear too fast.

Only a laboratory spectrophotometer can accurately measure UV absorption of ophthalmic lenses. A 1988 article by H.L. Hoover on UV meters and a 1991 research paper on transmittance of photochromic lens materials is available from Corning by calling 1-800-821-2020. Request OPO-430.--John Van Zanten, Corning, Inc.

Q. Kids' Lens Thickness
Legally, what is the minimum edge thickness on plus lenses for children? If there are no guidelines, what do you recommend?

A.
There are no official requirements for center or edge thickness values. One of the most critical limitations of thickness is its effect on impact resistance, a requirement in the United States. This isn't an issue for plus lenses, since their center thickness values are generally robust enough to provide acceptable impact resistance in plastic materials -- especially polycarbonate.

The ideal edge thickness of a plus lens is affected by factors such as weight/cosmetics, impact resistance, retention (in the frame), flaking, and mechanical stability. These factors are important for both adult and children's frames.

With finished (stock) lenses, edge thickness is governed by initial diameter and edge thickness of the blank. The edge thickness of finished plus lenses varies from 1 to 2mm. When using finished lenses for small frames, it is cosmetically beneficial to choose the smallest blank size available. For a small frame, edge thickness of an edged lens will be thicker than the initial edge of the lens blank -- and usually substantial enough to ensure good impact resistance, retention, and mechanical stability.

For semi-finished lenses, the manufacturer's processing recommendations for thickness should serve as the minimum thickness value. When in doubt, it is best to be too thick as opposed to too thin -- especially for a child.--Darryl Meister, ABOM, SOLA Optical USA

Q. How Do You Prove What You Can't See?
We are sometimes asked by a patient how to determine if their sunglasses or street glasses block ultraviolet rays. We tell them that all our sunglasses block 100 percent of UV and feature first-quality lenses. Is there any reasonable way we can prove this statement to our patients?

A.
A very useful tool recently became available that provides dramatic proof of UV protection in spectacle lenses, either sun lenses or conventional lenses. It consists of a plastic card with a treated white strip that darkens from lavender to purple when exposed to UV light. The darkening depends on the amount of UV exposure. Placing a pair of glasses over the strip and exposing them to direct sunlight will quickly determine if the lens transmits any UV.

Regarding demonstrating quality lenses, there is a quick but brutal way to demonstrate the optics of a plano sunglass. Hold the sunglass to be tested in front of a chart projector. If the lenses are quality ground and polished lenses, the letters on the chart will be dimmer but sharp and clearly defined. If the lens quality is anything less, the letters on the eye chart will be blurry and indistinct. You might keep a cheap pair of sunglasses in the office to demonstrate the difference between the quality sunwear you carry and the lesser quality found on drugstore racks.--Duane Ferguson, President, Polycarbonate Lens Council

* The UV Sunsensor card is available through laboratories or can be ordered from the Polycarbonate Council at 800-944-6206.

Q. What's the Point?
We're having an argument in our office about the logic of ordering scratch coating on a lens if it's going to be covered with an anti-reflection coating. Is there any reason to order scratch coating for a lens that is to be AR coated?

A.
As illogical as it may seem, there are very sound reasons for applying scratch coating on a lens to be AR coated. In fact, most coating labs prefer to apply AR coatings only on scratch-coated lenses. A well-applied scratch coating provides a firm, smoother base for the AR and enhances the effectiveness and longevity of the AR coating.

An AR coating is a very thin, relatively fragile coating. Visualize the AR coating like a tissue. Imagine placing the tissue on a soft pillow. If you attempt to push on the tissue with your finger, your finger would tear through the tissue. Conversely, imagine placing a piece of cardboard between the tissue and the pillow. Now press on the tissue with your finger. The tissue holds firm and resists tearing. In general, any AR coating you order should always be applied over scratch-coated lenses. EB

Got a question you'd like answered in "Ask the Labs?" Send it now!