HANDS ON
ASK THE LABS
by Karlen McLean, ABOC, NCLC

IDENTITY CRISIS

Q: I have a new patient who got her last eyewear elsewhere. I can't get the place to verify what lens material she's wearing. How do I tell the difference between polycarbonate, Trivex, and high-index lenses?

A: You can tell the difference between poly and high-index by dropping the lens a few inches onto a table and listening to the sound. Poly has a sharper and more hollow sound than high-index. High-index has a flatter sound.

It's difficult to tell if a lens is poly or Trivex. If the patient is getting a smaller frame, you can rub a little acetone onto the edge of the lens. Acetone will damage poly but will not affect Trivex.

If you have a roughing wheel or hand-stone, you can shave a little off the lens edge. Poly will gum up where Trivex will shave off. We don't recommend either of these methods because they will alter the original lens.

If you aren't careful, these methods can make the lens unusable. It's always best to find out the original order and see what was originally dispensed.

— Glenn Hollingsworth, general manager, Robertson Optical Laboratories, Inc., Atlanta

PREMIUM CHECK

Q: How does one determine if Trivex lenses are installed in a frame versus polycarbonate lenses?

A: I recently returned Trivex lenses for a remake of the left lens. After the remake, I weighed the lenses and found that the new lenses weighed 15 percent less. Without the lenses in hand, it's almost impossible to say. If I had the lenses, I'd look for evidence of birefringence. "Fringes" may be found on poly lenses and are seen as a purplish haze on the lens surface, especially under fluorescent light.

Also, poly lenses tend to have a tinny sound when you tap them on a metal object like a coin or ring.

These are easy, low-tech ways of checking between poly and another plastic lens material (including 1.5 and Trivex).

But, the only way to truly identify the lens material is by checking the index of refraction with a spectrophotometer.

— Clay Musslewhite, director, product marketing, HOYA Vision Care, North America