Ask The Labs
By Karlen McLean, ABOC, NCLC, and Martine Bruneni

BIREFRINGENCE MADE EASY
Q What does birefringence mean in terms that I could explain to my patients?

--Tom Hicks, owner/optician, Oxford Opticians, Oxford, Ohio

A Birefringence is an anisotropism of the index of refraction. It is most commonly a stress in a lens cause by the extreme pressure of injection molding that is seen as distortion. Wherever your patient sees color in the lens is where stress has compromised the index of refraction.

The best way to demonstrate this pressure-stress distortion is to use a polarascope or colmascope, or by using two sheets of polarized film. You can prove this distortion is from stress by pressing on the lens. Extra stress on the lens causes birefringence.

You can explain birefringence to your patient by using this analogy: Consider making a layered cake, where all the layers are not fully baked. Thus, the cake will vary in consistency and taste.

--Nick Niejelow, president and CEO, Optima, Stratford, Conn.

REFLECTIONS ON GLARE
Q Is there a real difference between glare and reflection?

--Sandi Shannon,owner/optician, Classic Eyes Optical, Ocala, Fla.

A Yes, there is a difference. The answer lies in cause and effect--reflection is the cause and glare is the effect. Reflections come in a variety of forms regarding eyewear. They can be as simple as the reflections from the surface or internally in lenses, or can be blinding, as in sunlight reflected off wet pavement. In each case, these reflections create glare.

It is important to understand the types of glare, their effects and causes, and ways to fix them. Distracting glare occurs when light reflects from the lens surface or from within the lens, from sources such as street lights or car headlights at night, which result in squinting and fatigue. Distracting glare can be removed by using quality AR coatings.

Discomforting glare occurs when the eye is unable to adapt naturally (e.g., moving from the shade into sunlight). Reduce or eliminate discomforting glare with light tints, such as grey 1 polarized or photochromic lenses.

Disabling glare occurs when light intensity becomes extreme (above 10,000 lumens) and blocks vision. Reduce or eliminate disabling glare by using dark tints, such as photochromic or grey or brown 3 polarized lenses.

Blinding glare is caused by light reflecting off smooth, shiny surfaces such as water, sand or wet pavement. This type of glare is blinding and blocks vision. The only lenses that eliminate blinding glare are polarized lenses.

-- David Rips, president and CEO, Younger Optics, Torrance, Calif.

FADE Factor
Q I've heard that tint samples kept on display in typical dispensary lighting can fade over time. What is the average life span of tint samples? Do I need to replace them, or can they be re-tinted?

-- Cindy Elkin, optician/owner, Point of View, Falls Church, Va.

A We recommend that tint samples be changed every year to 18 months. During that time, fading will be minimal and will not affect sales in any way. At that time, you should tint or order new samples. You could re-tint existing samples, but they've likely been handled by hundreds of people and may have scratches that degrade the quality of the tint and the appearance of the lens. Also, re-tinting is not worth the hassle of color correction, so it's easier to start with new lenses.

Changing tint samples every year to 18 months gives you the opportunity to re-evaluate the ranges of tints that you show. New colors come into fashion all the time, and there is a growing market for computer and sporting tints. So, slow-moving colors could be dropped from the sample range and new tints introduced to liven up your selection.

--Tim Rivett, manager, Brain Power Incorporated (BPI), Miami

If you have any questions you'd like to have answered in Ask the Labs, send them to Karlen Cole. Fax: 215-643-1705. E-mail: colek@boucher1.com. An archive of past Ask the Labs columns can be found on the Eyecare Business Website at www.eyecarebiz.com.