LENSES

LENS MATERIALS

CR39

Cr39, also referred to as Standard Plastic, is the least expensive and most often used material in eyewear.  Cr39 offers an abby value of 58, which is quite high.  This means that cr39 lenses offer exceptional clarity.  Cr39 is the most affordable material, making it a great choice for patients with budget constraints.  Cr39 accepts tints and anti-reflective coatings well.  Cr39 has a refractive index of 1.50, the lowest of the conventional lens materials.  Because of this, cr39 tends to be the thickest lens material, and is often avoided when dealing with higher power prescriptions.  Cr39 has a low tensile strength, so it is a poor choice for grooved or drilled frames.  Cr39 is available in more designs than any other material, so if you are having trouble fitting a patient with a particular progressive design, cr39 is often a good fallback choice if the desired material is unavailable.  Cr39 is available with photochromatic properties, as well as in polarized designs along with the standard clean lens.  Clear cr39 is the only commonly used material that does not include full UV blocking other than crown glass.

Trivex

Trivex is a unique material, and is relatively new to the optical world.  Trivex has a abby value of 44, thus providing great optical clarity.  It has a refractive index of 1.53, which places it in the mid-index range, but Trivex really is in its own class.  It shares many of the properties with polycarbonate, but is not chemically reactive.  It's higher index makes it a thinner choice than cr39.  Trivex has exceptional tensile strength, and is an excellent choice for both grooved and drilled mountings.  Trivex is limited in design availability, but is available in the standard designs for single vision, bifocal, and progressives.  Trivex is slightly more expensive, typically, than either cr39 or poly, but less expensive than high index lenses. Trivex accepts tints very well.  Because of its 1.53 index, it is thinner than plastic, but often many of the benefits of trivex are diminished by higher scripts and added thickness.  Still, for many prescriptions, Trivex is an excellent choice, combining affordability, optical clarity, and durability.

Mid Index

Mid index lenses, encompassing everything with a refractive index from 1.51 through 1.59, compromise a relatively small amount of the optical lens market.  They offer affordability, but suffer from the same shortcomings as cr39.  They are poor choices for drilled and grooved mountings.  They offer an abby value of around 37, so the optical clarity is somewhat diminished compared to cr39 or trivex.  Mid index lenses are not readily available in all common lens designs, but they are available in the standard single vision, bifocal, and progressive designs.  With an affordability close to that of trivex, mid index lenses are often overlooked in place of trivex.

Polycarbonate

Polycarbonate, or "Poly", is the second most common material in optical lens production today.  Poly is typically only slightly more expensive than cr39, but includes built in UV protection that cr39 does not.  Poly has exceedingly high tensile strength, making it an optimal choice for industrial or safety lenses.  It also holds up very well in grooved mountings, but can be suspect in drill mounts as the drill holes leave the poly exposed and it may chemically react with a number of common chemicals and split/crack/"spider web".  Poly is readily available in most lens designs, and can easily be found in both photochromatic and polarized lenses.  Uncoated poly will not accept any tint, so a factory scratch coat is required to tint poly, and often it tints poorly or cannot be tinted as darkly as other materials. Poly has an abby value of 30, making it the least clear material of the common materials listed here.  However, despite its poor optical clarity, its low price point, accessibility, and safety make it a great choice for many patients.  Poly also boasts an refractive index of 1.59, making it a good choice for patients with a higher prescription as it will be thinner than cr39, trivex, or mid-index lenses.

High Index

High index lenses, made up of lenses with a refractive index of 1.60 through 1.74, are the thinnest and most expensive lenses of the common materials listed here.  High index lenses have abby values ranging from 32 to 41, giving them average to slightly below average clarity.  The main draw of high index lenses is their thinness with high prescriptions.  The higher the index, the more difficult it becomes to tint correctly.  High index lenses are available in most of the common designs, and have limited availability in photochromatic and polarized designs.  High index lenses with an index of 1.60 or 1.67 are acceptable for grooved and drilled mountings.  The 1.70 and 1.74 are not recommended for those mounting styles.  The 1.70 and 1.74 index lenses typically require an AR coating from the manufacturer, and are therefore not able to be tinted.  As a result of the expense, high index lenses are typically relegated to use by patients with fairly high prescriptions.

Glass

Glass, available in a number of designs, is the oldest lens material available.  It offers exceptional clarity with an abby value of 59 for crown glass, as well as unparalleled scratch protection.  However, glass is very hard to produce, and is very expensive.  Current aftermarket hard coats can offer scratch resistance close to that of glass on all the other materials.  Glass typically cannot be grooved or drilled safely, so its usefulness is very limited.  As a result of these limitations, glass is normally only recommended for use with patients who already have glass lenses and refuse to change, or when a lens design is not available in any other material.

Drivewear

Produced by Younger Optics, Drivewear is a unique lens type.  Made of cr39 or poly, it is a polarized transition lens available in single vision, straight top 28, and Image progressive designs.  It transitions though both UV and visible light, making it a good choice for driving use.  Drivewear can delaminate based on the placement of the polarization wafer, so it's use in grooved and drilled mountings is not recommended.  With the introduction of the poly material, this may be subject to change.  In any case, Drivewear is a unique lens design that makes an excellent choice for patients looking for a second pair of prescription eyewear for use outdoors.

LENS DESIGNS

Single Vision

Single vision lenses are designed with just one focal length (typically distance or near/reading) prescription.  Usually this is the first type of lenses a patient will need.  The patient's prescription is available equally across the lens.

Bifocal

Bifocal lenses are basically single vision lenses with an area of increased magnification for reading use.  The most common type is the straight top, or d-seg, and the most common bifocal width is 28mm.  However, bifocals are available in designs from the standard straight top to curve tops, round tops, and even blended designs, all ranging in width from 22 to 45.  Also available are "executive" bifocals, where the seg line runs the entire width of the lens.  Add powers range from .75 diopters up to 8 diopters and beyond.

Trifocal

Trifocal lenses are similar to bifocals, but feature two segmented areas instead of one, creating both a reading area along with an "intermediate" area for working on things at arm's length.  Trifocals range in width from 25 to 35mm, and vary in height between seg lines from 7 to 14mm. The higher the height between segs, the larger the intermediate area.  The standard trifocal has a height of 7mm, and a width of 28mm, making it a 7x28 trifocal.

Progressives

Progressive lenses are similar to a trifocal without visible seg lines.  They feature an intermediate "corridor" or "channel" that leads to the reading area of the lens.  Due to placement of the corridor, correct PD measurements are exceedingly critical.  There is no standard progressive design, and each lens manufacturer has their own designs.  For help in selecting to proper progressive design for your patient, please contact the lab.  Progressive lenses are available in all materials, and are slowly replacing trifocals and bifocals in the marketplace.  Progressives do come in three basic design categories. First is the standard length corridor progressive, and this include progressives with a minimum fitting height of 18 or more.  Second are short corridor progressives, and these have a minimum fitting height ranging from 13 through 17.  Third are "occupational " lenses, and are designed without much distance, but an abundance of intermediate for computer work along with standard near for up-close reading.  Fitting heights vary by manufacturer for occupational lenses, but most require a minimum b measurement of 30 or more.

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