Lens Material and Design
Lenses today are available in a wide variety of material and designs. It’s our job to match the prescription and the patient’s visual needs with the best lens choices. To do this we provide information about the different of each materials, their designs and explain their features and benefits.
1. Wavefront Design & Freeform Processing
Wavefront and free-form technologies are leading the way to create the potential for lenses that offer patients the sharpest, clearest vision they’ve ever had. Wearers experience overall sharpness of vision, improved contrast acuity, improved nighttime driving vision, and seeing colors with greater richness and intensity.
Wavefront technology is used to analyze the way light travels through the eye. A painless, invisible and completely safe laser light is beamed into the retina at the back of the eye and then reflected back out. Paths taken by light rays traveling through the eye provide a three-dimensional optical map of the eye. This wavefront mapping helps practitioners detect aberrations not seen in conventional vision tests, which impact the quality of a patient's vision. Many corrective lenses cause distortions of light as it passes through the lens, which diminishes visual acuity. Progressive Addition Lenses, in particular, due to the nature of their design, are noted to deform light waves. Recently, wavefront technology has been used in new lens designs and manufacturing processes to correct these lens aberrations and improve visual performance.
Another major lens innovation is free-form processing. Using free-form surfacing techniques, an optical laboratory can cut complex curves onto the back and/or front surfaces of a lens blank creating the optimal prescription lens. Free-form generators can generate curves down to 0.01D, and the lens comes out of the generator ready to polish. The basic free-form surfacing system consists of a CNC (computer numerically controlled) generator, a soft pad lens polisher, and “point” file software that runs the CNC generator. Free-form processing is a faster and more accurate method of surfacing lenses so it not only produces more precise optics, but it also reduces processing times and helps improve delivery.
For years glass was the number one choice for lens materials. With the most precise and constant optics, glass is considered the most natural and comfortable vision. Untreated glass is not resistant to breakage, therefore ANSI standards state that all glass ophthalmi lenses must be either heat or chemically hardened to improve impact resistance. Many of the older glass seg styles and tints are no longer available or are only available at a high cost. Some colors can be obtained by coating the lens, although the cosmetics involved are questionable. Glass lenses are spherical design only and considered to be old technology by most dispensers. Crown glass has an index of 1.523 and is standard for glass lenses. Hi Index Glass lenses come in 1.60, 1.70 and 1.80 and 1.90 indexes. Because of their higher specific gravity, Hi Index glass lenses actually get heavier as the index increases, however today’s smaller frames can sometimes make this weight negligible.
Features and Benefits:
Excellent optics and the best scratch resistance of all materials. Remember, nothing is "scratch proof". Fused segments meaning there is no lip on top of the segment like all plastic material have. Glass lenses take AR coating excellently.
Glass is the heaviest material available as well as the lens "most likely to break". Glass can also pit and scar easily under different work environments. Aspheric designs should be coming out in the near future. Tints in glass lenses are built into the material, this includes glass photochromics. This will cause minus lenses to be darker around the edge and plus lenses to be darker in the center. To avoid this use another lens material or coat the lens for uniform density. Flat top bifocals and trifocals in glass photochromics are a different material from the rest of the lens and are clear causing the bifocal portion to be lighter in color than the rest of the lens. There is no UV protection in clear glass lenses.
The number one lens choice in today’s optical market. Plastic’s excellent optics, comfortable abbe, lightweight, ease of tinting and durability make it the most common choice among dispensers. There are more lens styles and blank sizes available in plastic today than any other lens material. Improved scratch resistant coatings have helped sales and should always be recommended. Regular plastic lenses have an index of refraction of 1.498. Hi Index plastics range from an index of 1.54, up to the new 1.74. Unlike glass, that gets heavier as the index gets higher, plastic Hi Index lenses generally offer lighter weight than regular plastic lenses. TIP: We recommend AR coating on both glass and plastic Hi Index lenses as the higher indexes also mean lower abbe values. If you have a Hi Index non adapt, try coating the lenses, or moving them to a Mid Index. Look at Hi Index as 4 separate levels, Mid Index 1.54 to about 1.58 ( 25% thinner than standart lens ), Hi Index 1.60 ( 30% thinner than standart lens ), Hyper Index 1.67 ( 40% thinner than standart lens ), and the new Hyper Index 1.74 ( 50% thinner than standart len ). It’s best to specify the index and whether or not you want an aspheric design, unless you care to trust your lab with picking the “best choice” and then you need to let them know that’s the case. While polycarbonate is technically a Hi Index plastic material, poly is almost always treated separately from other lens types.
Features and Benefits:
Plastic is roughly one half the weight of a comparable glass lens. Plastic will accept tints easier than any other material, it can be treated for UV, scratch coatings are available, takes AR and almost any other lens treatment available.
While standard plastic is the most common lens sold today, it’s old technology. About 30% thicker than glass, plastic requires that scratch coatings and UV enhancements (at an additional cost), rather than being built into the material like the newer products. While available in aspheric design, the additional cost is the same as going to an aspheric designed Mid to Hi Index lens.
The most impact resistant of all lens materials used today and has been the best choice for safety eyewear, kid’s glasses and sports eyewear. Polycarbonate has the added advantage of being a natural UV filter, protecting to 380 nm UVA and all UVB. While it has one of the higher indexes, it has a low specific gravity making it lighter in weight than most other lenses.
The Optical Laboratories Association has a program called “Duty to Warn” available to help educate your patient on polycarbonate and their choices. Proper positioning (4-5mm below the center of pupil) on single vision, good frame selection (No more than 5mm of decentration per eye, preferably less) will help avoid problems. We recommend the addition of multi-layer AR coatings to help provide the best vision for the patient
Features and Benefits:
The safest, lightest and one of the thinner materials available. New super tough scratch coatings can make this the most scratch resistant material available for a small added charge. Now available in aspheric designs that widen the viewing area to about that of a Mid Index lens.
Does not tint as evenly, as dark, or as naturally as CR-39. Polycarbonate’s lower abbe number can result in more chromatic aberration than some other lenses. Proper fitting and aspheric design can correct this.