|Hoya’s Super Hi Vision Ex3 is scratch-resistant and durable.|
Which anti-reflective (AR) treatment will you recommend to your patients today? An AR treatment can enhance the performance of a lens dramatically. Some AR lens suppliers and coaters usually carry out testing to determine the quality of their products. Here’s a look at some of the tests.
AR CRAZE/HEAL TEST
The AR Craze/Heal Test is designed to determine the extent to which an AR treatment will heal following an increase in temperature in a non-humid, dry heat environment. Lenses that exhibit defects are given a classification based on the percentage of their surface that exhibits visible crazing, and photographs are taken to document the results.
At one independent testing laboratory, this test is performed using four test lenses with varying powers or base curves. The lenses are placed in a 110ºF convection oven for 15 minutes and immediately inspected using a high intensity inspection lamp. If crazing is evident, the effects are photographed and the lens is re-inspected one hour later. The lens is again re-inspected after 24 hours and any effects recorded.
For lenses that do not show crazing at 15 minutes, they are placed to the side. After a 24-hour assessment, and if crazing is not evident, the lenses are returned to the oven for additional rounds of testing. The temperature is increased by 10ºF for each additional round until the maximum temperature reaches 170ºF. Once crazing is observed for a lens, no further heating or assessment is performed.
|Kodak CleAR and Clean ‘N’ CleAR treatments from Signet Armorlite are available on all Kodak lenses.|
Many of today’s premium AR treatments have an anti-static property. This procedure measures how well the lens surface resists the buildup of a static charge. A handheld electrostatic field meter is held perpendicular about 3cm above the center of a cleaned and inspected convex lens surface. After 30 seconds, its electrostatic field is measured and recorded.
The lens is then mechanically rubbed using a cotton cloth for 20 cycles (30 cycles per minute) using a 5-lb. weight. Thirty seconds after the rubbing, the lens’ electrostatic field is again measured and recorded, and an electrostatic field difference is calculated for each lens. These values are determined for both the control lens (an untreated, CR-39® plano lens) and the test lens.
The relative anti-static property is then determined. These ratings help determine which AR treatments exhibit the best anti-static properties.
One of several tests used for determining scratch resistance, the Bayer Test evaluates how effectively an AR treatment (or other coating, e.g., scratch resistance) resists abrasion. It does this by measuring the difference in haze (light scattering) that the lens has before and after being subjected to abrasion from sand. The higher the Bayer Ratio, the better the abrasion resistance of the AR lens. A Bayer Ratio of 5 means that the AR lens had five times the resistance to haze gain than an untreated lens. A Bayer Ratio of 4 or greater is considered to be a premium coating.
CROSSHATCH ADHESION TEST
|tzAvoRite, produced by Opticote, uses a hardcoating technique, an anti-reflective stack, and a topcoat.|
The Crosshatch Adhesion Test has become fairly standard for AR lens testing. This test deter mines the adhesion qualities of the AR coating layers to the lens. Cleaned and inspected lenses are placed concave side down and a crosshatch grid pattern is cut through the lens’ coating. Compressed air removes dust or particles and the grid is checked to ensure the incisions are even and void of chips. A 3-in. piece of tape of a specified grade is applied over the crosshatched pattern so one end extends past the edge by at least a half inch and air bubbles are removed from under the tape.
After 1½ minutes, the lens is held firmly in one hand while the other grasps the extended end of the tape and rapidly pulls towards the opposite side as close to a 180º angle as possible. The process is repeated two more times.
Test lenses are examined and rated for the amount of lens surface that has been exposed. Three classifications define: 1. crazing, 2. delamination of an inter-layer (partial removal of the coating), and 3. delamination, (when the complete coating detaches for the lens substrate). Ratings are given within each category, ranging from no visible crazing or delamination to severe (fern-like crazing) over any part of the lens surface, or complete delamination of individual layers over the entire lens surface.
This test can be used on a clear lens, in which case, it is called an initial adhesion test. If it is performed on a tinted lens, it is a post-tint adhesion test. If performed after boiling in salt water, it is known as the boiling water adhesion test. No matter when the test is performed, the assessment criteria are the same.
The Tumble Test is an abrasion assessment that attempts to determine the effects of normal wear and tear a patient will give in a year’s time. The test consists of an untreated, CR-39 plano lens (which is used as the control lens) and three AR lenses. Different media (sand, wheat bran, foam, 20/40 grit, 40/80 grit, emery, and Scotch-Brite®) are introduced into a barrel and mixed with a natural sawdust-type material. The lenses are added to the barrel and tumbled for 20 minutes. As they fall repeatedly in the barrel, lens surface abrasion occurs.
A BYK Gardner Haze-Gard is used to measure light dispersion through the lenses before the test and again after the test so a comparison can be made. Because of the abrasion they have acquired during the test, dispersion will be higher due to increased haze. The difference between the haze measurements before and after tumbling is then calculated. These figures are used to establish a ratio of how many times more abrasion resistant the control lens is to the test lens.
When considering a new AR treatment, make sure you ask for the testing data. Once you’ve decided, do a wearer trial with about 10 patients since they are the ones you’re trying to help.
Randall L. Smith is the Opticianry Program Director at Baker College in Jackson, MI.
WHERE TO FIND IT
Carl Zeiss Vision Inc.
800-358-8258 • zeiss.com/lenses
Essilor of America, Inc.
800-542-5668 • essilorusa.com
HOYA VISION CARE
877-528-1939 • hoyavision.com
866-492-6499 • nexusvisiongroup.com
Pech Optical Corp.
800-831-2352 • pechoptical.com
800-248-6784 • opticote.com
Seiko Optical Products of America
800-235-5367 • seikoeyewear.com
Signet Armorlite, Inc.
800-950-5367 • signetarmorlite.com