Q. What's Up With Glass?
Even though I know glass has superior optics over most other materials, I had never thought about using glass for most of my patients due to the weight restrictions and the legal ramifications. My lab is now pushing some new glass lens products and I wanted to know your opinion of the benefits of glass lenses. What is my legal responsibility to the patient? Are these "new" glass lenses safer than the ones I dispensed 10 years ago?

A.
Several events have conspired to once again give glass lenses a place in the sun. The first was the development of new glass materials that can be surfaced thinner than the usual 2.0mm. The first was Thin & Dark�, a fast acting photochromic glass. The most recent was a clear glass called Clears 1.6(tm). A new series of aspheric lenses made from the 1.6 index Clears material has just been released. Lenses make of either of these materials can be processed to 1.5mm centers. A second influencing event has been the fashion trend towards smaller frames. As a result of these two events, acceptably lightweight spectacles can be made with glass lenses.

Keep in mind that only in the United States has glass lenses shrunk to single digit percentages. Glass is the predominant lens material in the rest of the world.

There are no longer minimum thickness requirements for dress lenses. If a lens passes the drop ball test, it is a safe and legal. Glass producers have processed glass lenses as thin as 1.0mm that, following chemtempering, readily pass standard drop ball tests. Even though the tested lens would be legal, you probably won't find a lab that will produce lenses that thin. A good percentage of the 1.0 glass lenses they tested would pass the drop ball test, but their refusal is prompted by the fact that that lab pays for the ones tat don't pass drop ball testing.

Q. Bending Light
When should prism thinning be used for progressive lenses? Won't prism thinning create more problems with the newest "as worn" progressive designs than it offers cosmetically?

[Editor's note: The curvature of the front surface of a progressive increases toward the bottom of the lens, producing the desired add power. Producing a progressive lens that has sufficient thickness at the bottom often leaves the top edge unattractively thick. Prism thinning refers to the lab process of grinding down base prism in both lenses to balance the thickness between the top and bottom edges.]

A.
The effects of prism thinning on lenses designed for "position of wear" will likely depend upon how the lenses are manufactured. Custom atoric lenses, for instance, can simply have their optimal atoric (rear) surface computed by simply treating the prism thinning as prescribed prism. In these cases, the optics of the lenses will be modified according to adjust for the effects of prism thinning.

Other "position of wear" lenses, which are not as customized and use simple toric (rear) surfaces, may not compensate for the optical effects of prism thinning quite as accurately, but the effects are usually negligible, so it probably won't be an issue.

You already know that the patient acceptance of prism thinning is good (for prism powers commonly used for prism thinning, anyway). Other effects would include any optical aberrations produced by prism and reflections for low-and minus-powered lenses. The thinking on prism thinning has changed, generally as computer software had become more adept at handling lens thickness (particularly for progressives) in general.--Darryl Meister, ABOM, SOLA Optical Technical Marketing

[Editor's Note: A SOLA Technical Marketing paper titled "Understanding Prism Thinning" is available by calling SOLA at 800-333-7652.] EB

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