A Reader Response
In the January issue, "Ask the Labs" responded to a question about "water white" polycarbonate. I'd like to clarify several points that are occasionally oversimplified:

Waterwhite polycarbonate has been available for more than one year. Our company has used only waterwhite since our inception three years ago. Mr. Bruneni noted polycarbonate can be identified by a "rainbow" effect from scratch protection coatings on polycarbonate. This effect is simply the residual color that results when there is a difference in refractive index between coating and substrate (a so-called "oil slick" effect). Our company uses scratch-protection coating that has a superior index match between coating and substrate, avoiding this residual color.

The best way to tell new "waterwhite" from earlier polycarbonate is to place the lenses on a white piece of paper. Some lenses will look "dingy" rather than blue. They were made from older resin versions that came yellow from the resin manufacturing process. These pellets were mixed with blue-colored or magenta-colored pellets to offset the natural coloration, resulting in this dinginess. These points about the resin and scratch coatings may be minor, but they clarify matters that may have been oversimplified in the past.--Robert R. Zeidler, VP/COO, Oracle Lens Manufacturing Corporation

Q. It's Hip to Be a Small PAL
Many of our progressive patients want to choose the hip new small frames. We always try to steer them into a larger frame because we know most manufacturers recommend a minimum fitting height of 22-24mm. Is any lens manufacturer planning a PAL design for fashionable, small-sized frames?

A.
Here's one case where we can give you some good news. One company, American Optical, has just introduced a lens designed specifically for small frame geometry. Called AO Compact�, the lens has a recommended fitting height of 17mm. International standards define corridor length as the vertical drop from the fitting cross to that point of the corridor where the nominal add power, less one-eighth diopter, is attained. The manufacturer claims a shorter International Standard Length corridor than other PAL designs.

Q. A Mathematical Paradox: How 1.66 + 0 = 1.67
For my high minus patients, I usually recommend high index lenses made from a 1.66 refractive index material. Recently, I have noticed lenses advertising a refractive index of 1.67. We want to give our patients the best lens for their correction. How much thinning of minus lens edges do we get by switching to this higher index?

A.
Seiko advertises an index of refraction of 1.67, while Optima lenses (among others) advertise a 1.66 index. These are the same products, both being made with a base resin called MR7. The difference in advertised index is due to the wavelength at which the index is measured.

European and Asian markets measure refractive index using a wavelength of 546.07 nanometers, which is the Fraunhofer mercury e-line. The U.S. market uses 587.56 nanometers, the Fraunhofer helium d-line for measuring refractive index. Since refractive index varies with wavelength, these two values are always different.

Every lens material currently used has a higher index of refraction when measured using the mercury e-line than when measured using the helium d-line. Therefore, when considering refractive index, you should note whether the manufacturer is using the mercury e-line, designated as *e or the helium d-line, designated as *d. Lenses advertised in this country should follow the official U.S. measurement. Based on refractive index alone, the Seiko product, *e = 1.67 would give exactly the same thickness as the Optima product *d = 1.66. However, different center thickness, different base curve or different surface figuration (i.e. spherical or aspheric front surface) would produce different edge or center thickness.

Your laboratory can provide the correct information on the thickness of different products.--Dan Torgersen, OLA Technical Director

Q. UV Protection and High Index
Which high index lens materials include UV protection?

A.
The newly published book Spectral Transmittance by Dan Torgersen reveals, among other things, the UV transmittance of the most-used lens materials. Those charts indicate the following for high index plastics:

Spectralite�: UV A = 1.3 %, UV B = 0.0

Spectralite Transitions III� faded: UV A & B = 0.0

Spectralite Transitions III darkened: UV A & B = 0.0

1.55 Plastic, Seiko Changers� faded: UV A = 0.1, UV B = 0.0

1.55 Plastic, Seiko Changers� darkened: UV A = 0.1, UV B = 0.0

RLX� 1.56: UV A & B = 0.0

MR6� (1.60 index) : UV A & B = 0.0

Polycarbonate, of course, also blocks 100% of UV A & B. EB

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