Few professionals doubt the benefits of anti-reflective coatings on clear, un-tinted lenses. Add color, a mirror coating, or polarization to the lenses, however, and the conversation shifts to a debate over whether or not to coat tinted lenses. Supporters are divided: some believe that coating the backside of the lens is sufficient, while others say it is important to coat both sides.

We went to the experts at several labs and coating facilities for answers to your questions about the whats, hows, and whens of adding AR coating to the backside of tinted sunwear lenses.

POLY FOR HUNTING

Q Hunting and shooting for sport is a big activity in my area, and I routinely fabricate tinted polycarbonate lenses for my customers who are involved in these sports. Some customers complain of glare and reflections while shooting, and I am wondering how much an anti-reflective coating on the backside of the lenses would help?

A In shooting, accuracy is the name of the game; and as with all lenses, adding an AR coating to the backside will help to greatly sharpen the clarity of images.

Since the material is polycarbonate, make sure that a good hard coat is applied prior to AR coating. For an ideal hard coat under-layer, choose a scratch resistant coating that is thermally cured rather than UV cured. Thermally cured coatings are harder, and an AR coating will adhere better to them.

The impact resistance of polycarbonate, the proper tint for the situation (from vermilion to yellow kalicrome), and the virtual elimination of reflections as a result of AR coating the backside of the lenses will produce jobs that are right on the mark.

--Michael G. Smith, lab manager,
Schmidt Optical Laboratories

LIGHT TRANSMISSION

Q I want to provide the benefit of the reduction in reflection that an AR coating provides on sunglass lenses, but I am concerned about the increase in light transmission the coating allows. I have heard that the amount of light transmitted will depend on the density of the tint in the lens, so I tint the lenses dark and find they come back faded. Can you explain why this is happening?

A When sending a tinted lens for AR coating, expect a 5 to 10 percent reduction in the color density. The amount the tint will actually fade will vary slightly depending on the coating system used. This fading is a result of the evaporation of heat during the degassing stage of the process. To maintain the desired density (and transmission overall), "over-tint" lenses by approximately 10 percent before sending them to the lab. For example, if the desired density is 70 percent or greater--that is, providing light transmission of 30 percent or less--tint the lens at 80 percent absorption.

--Jack Jue, president, I-Coat Company

THE POLARIZED PROCESS

Q Can an AR coating be applied to the backside of both laminated and encapsulated types of polarized lens, and is the process the same?

A Yes, AR coating can be applied to both types of polarized lenses. The primary concern in coating lenses treated with the laminated process is that the AR coating process will cause the laminated polarized wafer to separate from the lens.

To prevent the wafer from separating from the lens, a low temperature deposition process is used. Polarized lenses are coated at approximately 40 to 50 degrees Celsius, whereas the average temperature for AR coating CR 39 material is 250 degrees. Additionally, polarized lenses are inherently hydrophilic--and since an AR coating has a hydrophobic property, applying AR to any polarized lens will be helpful to wearers participating in water sports.

--Richard Eckstien, CEO, Vision Coat0

Coat Both Sides?

Q When coating tinted lenses for use as sunglasses, is it important to coat both the front and backside of the lenses? If so, why?

A If your goal is maximum effectiveness, then yes, applying AR coating to both the front and back sides of tinted lenses is a must. While backside AR coating reduces the mirror effect of the lens from light entering the rear (such as a sun shining in your car's rear window), light is also reflected at a lower intensity at the back side of the front surface of the lens, once again disturbing the wearer. Although the intensity of these reflections is lower, it is a distinct advantage for your patient to virtually eliminate these reflections by getting an AR coating the front surface as well.

--Grady Culbreth, president,
AR Council of America EB