Q. The Thinnest Transitions
Am I correct in thinking that polycarbonate lenses are the thinnest lenses in Transitions, since polycarbonate lenses have a higher index than high-index plastic Transitions lenses (1.55)?

Also, how significant is the issue of polycarbonate lenses transmitting less light? I've been in practice for six years, and I've never had a patient who was unable to wear poly lenses because of reduced light.

A.
You're right. Polycarbonate is the highest index currently available in Transitions (1.59) so, yes, they will have the thinnest edges for minus. They offer another cosmetic advantage as well. Poly Transitions lenses can also be surfaced 0.5mm thinner than other materials and still pass the drop ball test.

Regarding light transmission, all higher index materials transmit less light than CR 39 or glass (generally, the higher the index, the less light transmission). When multi-layer AR coatings are applied, light transmission of all these materials increases to more than 99 percent. AR is always a valid recommendation for high index lenses. Light trans- mission loss with poly compared to conventional plastic is between one and two percent-so slight it is seldom ever noticed by the patient. Nevertheless, you will enhance their vision with AR, particularly at night. Originally, Transitions didn't recommend AR for their lenses but have since changed that policy, as AR coaters were able to alter their technology so coatings block less UV.

Q. Children's First Eye Exam
I noted in the Feb 2000 issue of Eyecare Business that you recommend that children have their first eye exam before they enter school. While this is technically correct, it gives the impression that the first eye exam should be at about the age of six years. This, unfortunately, would be too late to try to correct conditions like amblyopia.

More correctly, it is advised by the American Optometric Association and Canadian Association of Optometrists that children should have their first exam at least by the age of three and, if there is a family history of vision problems, at six months.

-Michael Nelson, O.D.
Waverley Eye Care Centre,
 Winnipeg, MB, Canada

Q. What About Blue and Red Light?
We all know that polycarbonate is effective in blocking UV at 380 nanometers, but at what light transmittance is polycarbonate effective at blocking the blue and red spectrums? Where on the light spectrum does blue and red light begin?

A.
The visible spectrum is a continuum of the colors that make up white light. Blue and violet are at the shorter end of the spectrum-in terms of wavelength-between 380 and 480 nanometers. Red is at the longer end of the spectrum, between 630 and 760nm. Ultraviolet radiation is immediately adjacent to the violet/blue end of the spectrum (380nm), and infrared radiation is immediately adjacent to the red end of the spectrum (760nm). Clear polycarbonate transmits all visible light more or less evenly and is not intended to block either blue light or red light, two of the three primary colors of the visible spectrum. A lens material blocking either color would no longer appear "clear" to the wearer. According to the OLA's Spectral Transmittance of Lens Materials guide, clear polycarbonate has an ultraviolet radiation (A and B) transmittance of 0 percent, and an infrared radiation transmittance of 85 percent. In addition, clear polycarbonate has a "blue light" transmittance, which is weighted by the "blue light hazard" function for certain environmental vision conditions of 88.2 percent.

-Darryl Meister, ABOM, SOLA Optical Technical Marketing EB

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