Ask the Labs
? & A
By Joseph L. Bruneni

CONFUSED ABOUT IMPACT TESTING
Q I see references to impact testing for lenses that refer to a "high mass, low velocity" test and other references to "low mass, high velocity" testing. Which is the better test to use for ophthalmic lenses and why?

A Both tests are important because lens materials and frame retention systems may react differently to each test. The "high velocity, low mass" tests reflect impacts with flying particles, such as a rebounding nail or fragment thrown by a grinder. Total energy usually isn't great, but the lens must be able to withstand a large force concentrated in a small area. While this is a severe test for the lens, it is not a severe test for the frame or the retention system.

The "low velocity, high mass test" tests the lens, frame, and lens retention system. This test reflects impacts such as a hammer rebound or walking into the end of a small diameter pipe. The most severe tests are in ASTM standards for sports, which reflect energy typically seen in various sports and combine relatively high velocity with moderately high mass. The majority of industrial safety spectacles that pass both the high velocity and high mass tests of ANSI Z87 cannot withstand any of the impact tests of ASTM F803 (squash ball, racquetball, and tennis ball at 90 mph, lacrosse ball at 45 mph, or baseball at 55 mph [for youth baseball]). That is why sports eyewear must conform to ASTM F803 requirements rather than ANSI Z87.

Paul Vinger, MD, Tufts University

Editor's Note: Younger Optics has movie files on its Website showing high mass, low velocity tests. The files are found at:
www.youngeroptics.com/products/trilogy/ StressMovie.html

HARD VS. SOFT
Q Evaluating progressive lenses, how can we tell whether a PAL has a "hard" design or a "soft" design? Which is better?

A Conventional progressives have progressive curves on the front side of the lens, so if we can visualize that surface, we can determine a hard or soft lens design. The best way to do this is with a contour map. Lots of lines on the map close together indicate a steep mountain. Progressive plot lines close together indicate that the power changes rapidly. When lines are further apart and fewer in number, they indicate a softer design. Hard designs offer larger areas of unchanged power for distance and near. Soft designs offer easier adaptation with less "swim." While some manufacturers publish contour maps or plots for their progressives, most do not. This leaves ECPs to evaluate progressives by trial and error on patients.

When to use Trivex
Q Our lab tells us about lenses made of the new Trivex material. We would like to know when it makes sense to use this material instead of any others. Is there a rule of thumb to use in deciding when to use lenses made of this material?

A Trivex lenses are produced by the manufacturers as all-purpose lenses, but this super-tough new material offers special advantages in certain situations. For instance, the material has a relatively high Abbe value and is appropriate to use when impact protection similar to polycarbonate is important, but the patient has a strong correction where Abbe problems could arise (7.00D or higher). It has the lowest specific gravity, which can produce lighter eyewear. One area where it shines is for rimless mountings. Characteristics of the material eliminate stress points around screw holes, providing greater stability and better performance for rimless. They also claim superior optics because of the material's high Abbe value. The index is relatively low (1.43), but it can be safely surfaced to 1.0mm centers which helps thin the edges.

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.