Polycarbonate’s impact resistance, low specific gravity and other attributes make this the go-to lens material for many ECPs.
|THE GO-TO LENS MATERIAL
Polycarbonate lenses remain a top material choice for ECPs. All lens styles are available in this material. In addition, polycarbonate fabricates well in all frame designs, including three-piece drill mounts and compression mount rimless frames, due to the material’s high tensile strength/modulus. Polycarbonate lenses offer the best of all worlds to patients as:
• a high-index lens creating a thinner lighter profile
• an excellent choice for three-piece drill mount or compression mount rimless, due to its high tensile strength/modulus
• superior impact protection for sports/outdoor eyewear
• a natural choice for children’s eyewear due to superior impact resistance
• a lightweight, comfortable material for dress eyewear due to its low specific gravity
• it is available in all popular lens styles and lens options
• thinness (index of refraction)
• Abbe value of 32
• durability (scratch resistance)
• availability (Rx range, lens design)
• ability to process the material
• tint and coat
• drill, notch and groove
Polycarbonate’s long technological history in the U.S. dates back as far as 1957 when it was used in the military for fighter jet cockpits, helmet visors and other applications where visual clarity and impact resistance were paramount.
Polycarbonate was introduced in 1978 to meet the need of impact resistance in industrial safety eyewear. With continued refinements in lens formulation and processing, polycarbonate lenses became a material choice for dress eyewear in the mid-1980s.
Many positive attributes contribute to polycarbonate’s popularity. A low specific gravity of 1.22g/cm2, in comparison to 1 for water, translates into lightweight comfort. Eye safety, always a concern for ECPs, results from its impact resistance. Polycarbonate lenses are also 66 times stronger than glass lens materials and 33 times stronger than resin lenses in a comparable Rx.
In addition, polycarbonate provides 100% UV absorption and has an Abbe value of 32. Optical benefits include an index of refraction of 1.59, providing a thinner lighter weight lens profile than a conventional resin lens choice.
Polycarbonate lenses should be fit similarly to other high-index lenses. Monocular PDs should be taken with a corneal reflection pupilometer or similar device. The frame should be pre-fit and adjusted as needed for patient comfort. Pantoscopic tilt should be between 8 and 10 with modest face form or wrap angle. In addition, vertex distance should be checked as needed with a distometer, and compensated Rx powers should be calculated if there is a significant difference in the refractive vertex with respect to the final fitting vertex.
Patient’s eyes should be centered within the selected eyewear and verified. As needed, a vertical optical center height should be taken when the patient’s pupil doesn’t correspond to the 180° datum line and specified on the lab order. These measurements help guard against the patient experiencing chromatic aberration which can occur with a high-index lens and low Abbe value.
Polycarbonate lenses should include an anti-reflective coating, which improves overall optics, cosmetics and brings the light transmission of the lens close to 100%. The coating of the polycarbonate lens can absorb tints up to and including sunglass shades. Polycarbonate lenses can also be ordered with an assortment of photochromic options. Improvements in polycarbonate coatings include a much harder scratch coating to help ensure that the lens lasts the life of the prescription.
Fabrication of polycarbonate lenses should be performed by the ECP according to the capabilities of the edger in the dispensary. If the operator is new to the fabrication of polycarbonate lenses, it is a must to thoroughly understand the machine using the owner’s manual, with supplemental questions to the manufacturer if necessary.
It is important to note the edger’s special processing modes for various index lenses, specific chuck pressures for edging high-index lenses, and block/pad considerations when edging polycarbonate lenses that have an anti-reflective coating. When disposing of polycarbonate edging waste (swarf), the ECP should check with the local municipality to ensure they are operating within local environmental laws and guidelines. (See “Easy on the Environment” OLP May 2017.) When in doubt refer to the specific lens manufacturer in regard to lens waste disposal.
DUTY TO WARN
Every prudent ECP should have a working knowledge of duty to warn. We live and work in a litigious society, and it is the responsibility of the ECP to have a discussion about the impact protection of lenses while highlighting the superior impact protection of polycarbonate lens material and then documenting that conversation. Of particular concern are children, individuals with functioning vision only in one eye and those with visual impairments or previous ocular injuries. All should be made aware of polycarbonate.
For children, many optical chains will, without exception, dispense only polycarbonate lenses to children. Some businesses classify a “child” as a youth 13 years of age or younger, while other companies raise that classification to 18 years or younger. To avoid potential litigation, every practice should have a duty to warn procedure in place.
Sports eyewear or outdoor eyewear that is worn to protect the eye against injury or trauma should naturally be equipped with this state-of-the-art impact material. In addition, the practitioner should make sure the protective device is the right size and fits properly for the activity at hand. Furthermore, the patient should be advised as to what the protective product is intended for in regards to sport, and again, this conversation should be documented accordingly because in a product liability suit an ECP could be held liable for lens material choice and product/fit.
Richard W. McCoy, LDO, ABOC, NCLEC, is program director, opticianry at Reynolds Community College in Richmond, VA.