There are a variety of available lens style and you must specify the type of lens as well as the material you require. The most common lens types are:
Single vision lenses have only one focal length they are corrected to. This is traditionally for distance, but it can be set for near, or intermediate. The optical center of a single vision lens is the thinnest part of a minus lens and the thickest part of a plus lens.
Bifocals have two separate focal distances they are corrected to. This is traditionally distance and near, but can be set for distance and intermediate, or intermediate and near. The distance optical center of a standard bifocal, or trifocal lens is the thinnest part of a minus lens and the thickest part of a plus lens. The optical center of the segment is a preset depth into the segment itself. Segment centers cannot be spotted on a lensometer for bifocals, or trifocals unless the distance power of the lens is Plano, or 0.00. Straight Top (ST), Flat Top (FT) and D Bifocals are all different manufacturer names for the same thing. The reading portion of a multifocal is called the “Add”, or the segment. The number designation is the width of the segment measured at its widest part. The most common bifocals used today are (in order): ST 28, ST 35, Round 25, or round 28 and Executive.
Trifocals have three focal different distances they are corrected to. The intermediate is traditionally set at 50% of the total add power, but can be adjusted in some styles to accommodate different working distances. Straight Top Trifocal (STT), Flat Top Trifocal (FTT) and D Trifocals are all different manufacturers names for the same thing. The first number in a trifocal gives the height measured from the top of the bifocal part of the segment to the top of the trifocal portion. The second number is the width of the segment measured at its widest part. The most common trifocals used today are (in order): STT 7x28, STT 8x35, STT 10x35 and the Executive Trifocal. The lens identified as a Trifield is more commonly called an Executive Trifocal.
Progressive Power Lenses
Progressives provide the three focal different distances that you get from trifocals, but do it in a continuous, gradually increasing manner, rather than jumping from focal area to focal area like a bifocal, or trifocal. Today’s new computer corrected designs offer excellent vision and much wider optical zones than in the past. In addition there are some specialty progressives designed for small frames such as the Varilux® Panamic, Solamax, AO Compact and Outlook. Most progressives (not all) come with scratch coating. Check with your lab to see if this requires an additional charge. The most accurate method for telling what the add power of a progressive power lens is to locate the temporal side layout marking. Just under this mark, the manufacturer will have engraved the add power.
Occupational lenses serve special purposes, having an additional segment at the top of the lens allowing the patient to see at near above as well as below and are often called “double segs’. If the bottom portion is a trifocal the lens is called a quadrafocal. The standard separation between the top and bottom segments is 13-14 mm and the upper seg can be bifocal, or trifocal strength.
Better known as cataract lenses, aphakics are very high power lenses that were once the only way to restore sight when the patient’s own crystalline lens clouded (a cataract), and was surgically removed from the eye. Modern techniques now allow the surgeon to implant into the eye a substitute lens when surgery is performed, and as a result aphakic lenses are declining in usage. Aphakics however are still available in a variety of styles. Aspheric styles (many curves on the same surface) are preferred for clearer sight in both single vision and multifocal. The style shown is an aspheric lenticular with a round bifocal and looks like a large button on a flat lens.
Most of these lenses have distance and others do not. The main purpose is to provide intermediate (arms length) in the upper portion of the lens and standard reading power in the bottom. Do not let their designation as Computer Lenses confine you to computer use only. Anyone whose work/hobby visual requirements are higher than normal will benefit from these designs, generally as a second pair. Examples: mechanics, cooking, crafts, music, shooting pool etc..
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 need to know the different materials, their designs and be able to explain their features and benefits to the patient.
Plastic Hard Resin - CR-39
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.
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.
Hi Index Glass
The word Index refers to the Index of Refraction of a given material. Generally, the higher the index of refraction, the thinner the lens. 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. Nothing is “scratch proof”. Reduce thickness as index increases. AR is more scratch resistant on all types of Glass because of the harder surface.
Weight increases as index increases. While 1.60 can be hardened and will pass the “drop ball” test, 1.70, 1.80, and the new 1.90 indexes may not meet ANSI standards for impact resistance and will probably require a waiver from the patient and doctor. Increased chromatic aberration basically demands an AR coating. No aspherics available.
Hi Index Plastic
The term Index refers to the Index of Refraction of a given material. The higher the index of refraction, the thinner the lens. 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 - Hi Index 1.60 - Hyper Index 1.66 and the new Hyper Index 1.74. 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.
This mid index lens is a brand new monomer that is set to challenge polycarbonate as “the lens for safety purposes. With its higher abbe, better scratch resistance, ease of tinting and aspheric design, we finally have an alternative to polycarbonate for safety and children’s eyewear.
Mid Index 1.54 to about 1.58
This mid index range makes a noticeable improvement over standard plastic, but is most effective in certain products that include aspheric design at no additional charge. Check with your lab on their preference here. The abbe value on this range is in the “comfort zone” that does not require an AR coating, although AR is still and always recommended for best vision.
Hi Index 1.60
This is the index most commonly associated with Hi Index, but like the mid-indexes, is most effective in certain products that include aspheric design at no additional charge. The abbe value of most 1.60 would suggest AR to help with chromatic aberration. Some 1.60 products include aspheric design at no additional charge. Check with your lab on their preference here.
Hyper Index 1.66
There are several manufacturers of the 1.66, 1.67 index lenses. All of the lenses in this index are aspheric, with some of them also having an “atoric” design.
Features and Benefits:
All Hi Index lenses offer built-in UV and scratch resistance. Aspheric is built-in in some lenses and must be specified in others. Hi Index lenses provide thinner, lighter lenses than the older regular plastic lenses.
In the upper indexes a small number of patients may experience chromatic aberration and report seeing a “color fringe” in peripheral vision. This can usually be avoided with the choice of smaller frames, proper positioning and the addition of an AR coating.
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 technoloty by most dispensers.
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 matieral 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.
Anti-Reflective (AR) Coatings reduce reflections inside the lens, enhancing colors, clarity and light coming through to the eyes. Lenses without AR lose from 8% to 18% of
the light, detail and purity of colors because
of these reflections depending on the index
of refraction of the material. The higher the index, the more reflections! Ask for a demonstration of AR and non-AR side-by-side to "See" the difference!
Today’s AR is a vast improvement to the coatings we used even a few short years ago. Various layers of coatings, sometimes including the newer, tougher scratch coatings, primer coats and hydrophobic coatings like Sutherlin's Clarion XS have taken a good product and made it great. Plus, we’ve learned that we must have the correct combination of materials and coatings to work properly together. This enhancement must be demonstrated with good samples to really be able to show you the difference today's quality AR coatings like Clarion XS will make.
In the past, mirror coatings were most often seen in plano sunglasses. Solid mirror coatings make the front surface of lenses reflective, so that the eyes cannot be seen. While the flash mirrors used most often today are more “see through”, they tend to have more modern colors. Mirror coatings are available in both prescription and non-prescription lenses, and are popular with the young and old alike. These coatings work especially well with a back surface AR. Ask about Clarion XS Cool Blue, Clarion XS Cool Silver and Clarion XS Cool Gold coatings for the ultimate in sunwear vision, fashion and protection!
Scratch Resistant Coatings
Note the term “Resistant”, nothing is scratch proof. That said, scratch coatings are a definite plus, greatly enhancing the lenses ability to resist scratching. Other than stock lenses where the prescription is already in the lens, most scratch coatings are front surface only. Your lab can add either a standard, or in some cases a super tough back surface scratch coating at an additional charge.
Super Tough Scratch Coatings
The Super Tough Scratch Coatings are a fairly new enhancement and well worth the small extra cost they usually carry. This is especially helpful under an AR coating. Unlike XTC™, some coatings will not take a tint well, or at all. Check with your lab!
Glass lenses can be coated for color with a vacuum coating process similar to AR. If a patient’s old lenses are in perfect shape, they may be made into sunglasses using this process.
Note: AR coatings will cut about 5% off the absorption rate of all tints! Allow for this! Solids and gradients are used for two main purposes, cosmetics and to cut light. Light tints have been used for years to help with inside light while darker tints can serve as sun protection. Tints are rated from #1 - #3 with a #1 being a light tint (approx. 10% absorption rate), #2 a medium (approx. 40% absorption rate), and #3 (approx. 80% absorption rate), a sunglass shade and a #4 (approx. 90% absorption rate). You should order tints by transmission rates. Tints darker than a #3 can sometimes overly dilate the pupil and actually start to take away visual acuity from the patient, very much like the dilating drops the doctor uses on them for exam purposes.
Clear lenses are called “white” in all materials. Solid color tints have the same shade equally throughout the lens. Glass and some poly lenses have the tint built into the material and will appear darker where the lens is thicker and lighter where the lens is thinner. Plastic, Hi Index and most poly lenses are dyed and will have even color throughout the lens regardless of Rx.
Usually, the lower the index of the material, the better it will accept tinting. Poly, 1.60 and the 1.66 materials may not even get as dark as a #2. Gradient tints start darker at the top and lighten as the tint gets lower in the lens. A double gradient tint has a second tint starting darker at the bottom of the lens and lightening as it gets higher in the lens and eventually meeting the upper gradient tint. Your office may have several sample sets of suggested tints. Be sure you specify what is wanted, and if possible, send your sample to the lab so that an exact match may be made. Your sample sets should be updated periodically; tints do fade with time.
Photochromic Glass: Glass with photochromic properties darkens as illumination and UV increases. There are a variety of photochromic colors and intensities available. If a photochromic is to be ordered, please be sure to specify a color and type. (i.e Photogray Extra). On Minus lenses Photogray Thin and Dark will give a thinner, lighter lens that will be darker outside, lighter inside and not have any of the yellow cast most glass photochromics show. PHOTOGRAY ARE NOT AND SHOULD NOT BE SOLD AS A FULL SUNGLASS!!!!
In the car they will darken to about a #1.5 at most, outside in the shade to maybe a #2 and outside in the sun to about a full #3. If your patient is a sunglass wearer, tell them that Transitions will make an excellent addition to their sunglasses. If they don’t wear sunglasses these lenses will provide significant brightness control over white, or lightly tinted lenses.
Both glass and plastic are sensitive to temperature and will be darker in the cold than in warm temperatures. While it is true that sticking photochromics in the freezer will cause lenses to darken quicker than if you don’t, as soon as they warm up again they will revert to normal color changing. All this really gives you is a quick change when just out of the freezer and a cold face.
Ultra Violet Protection
Many authorities believe that Ultra Violet light is harmful to the eye. This low cost protection may be of great long-term benefit to the patient. Clear glass does not provide UV protection, a coating (very ugly) can be used. Plastic lenses do not naturally provide UV protection, but can be dyed with a special material to block UV rays. In plastic, this process does not discolor the lens. All Hi Index and Poly lenses block the required UV by nature of the material. No added treatments are required.
The laws of physics, the standards adhered to by the optical industry, and certain governmental rules and regulations, all conspire together to make some minus power lenses thicker at the edges than is considered attractive. The edges of these lenses can be:
Rolled and Polished
These treatments can make eyewear more cosmetically appealing. Have samples to show the different effects.