By Judith Lee
Some things just go together, like soap and water, one hand washing the other - and lens cleaning and anti-reflective (AR) coating. In fact, those three combinations are very   similar. Water is not an effective disinfectant without soap, and soap won’t have the coverage or rinse off properly without water. One hand cannot wash itself effectively, and needs the other to complete the process.

“Because the AR coating is a process that deposits   at the molecular level and ‘grows’ the thin film coatings on the surface of the lenses, the preparation of the surface is paramount. This means clean and dry,”   noted Quantum president Norm Kester. “Any small, surface-borne contaminant can cause the adhesion of the coating to break down. Keep in mind that the thickness of the AR coating is 1/1000th the thickness   of a hair!”

One reason AR equipment makers are adamant about lens cleaning prior to AR processing is because AR is applied after the lens is hard-coated, which is a lacquer. Experts say the conditions required for AR to bond to a lacquer are different than the conditions needed for   a primer or lacquer to bond to a lens (as in hard coat).

Imagine a typical lens flowing through a surfacing lab, experiencing all kinds of contamination: finger prints, powders from slurry, ink marks, water stains, and even residual components of seemingly clean products like dishwashing detergent. All these contaminants will need to be removed in order to bond an AR layer “stack”   to the lacquer.  

“Cleaning a lens is a common notion to any ophthalmic lab technician. Let’s face it, cleaning lenses is an everyday standard practice in any surfacing lab and should by no means be a foreign concept to any qualified lab personnel. But what do we truly mean by ‘cleaning   a lens’?” asked Leybold Optics president George Kim, who then ticked off several possible definitions:   

     » rinsing a lens with water and a lint free wipe

     » spraying with an optical lens cleaner and a wipe with a high-quality optical cloth  

     » done by hand or by machine

“It is quite possible to remove many of these contaminants by ‘hand cleaning’. However, there are some points to consider with this type of cleaning when trying to successfully processing a lens through an AR coating lab.   Hand cleaning does not lend itself to reproducibility and is an acquired skill. Hand cleaning is slow. Most importantly, hand cleaning does not remove enough contaminants to ensure trouble-free and consistent adhesion of the AR treatment,” Kim said. 

He gets no argument from the AR experts at Satisloh.

“The lens goes through many steps in the lab and the front and back sides are exposed to polish, wax, alloy tape residue, finger prints and other surface contaminates. The success of an AR coating relies heavily on the cleanliness of the two surfaces, therefore one of the best methods is ultrasonic-assisted cleaning lines,” said Brian Peterson, Satisloh product manager of coating.

AR gurus at Zeiss say both hand cleaning and mechanical cleaning (not one or the other) are essential.

“Basic requirements for a mechanical wash line are ultrasonics and detergents, capability to heat the cleaning solutions, filtration, water quality and adequate rinsing, and drying capability. If the AR process isn’t ion-assisted, a chemical etch may be required in the wash process as well,” said Zeiss’s Robert Spirito. 

He noted that this is for washing for direct to AR. If there is dip coating before AR, the wash line requires multiple wash and etch capabilities.

Chemalux uses high pressure water to clean lenses. Owner Henry Zheng said this simplifies the AR process and makes the Chemalux AR process ideal for automated labs.

Charlie Seidel, director of Coburn’s Lab Works Group advised lab owners to consider what type of materials are left on the lens before cleaning.

“How was the lens blocked during the surfacing process? For example, if the lens was taped, then blocked, it will most likely have less residual residue than a lens which was blocked directly with wax or alloy. Lab owners should consider what residue is left on the lens before, and   even after, cleaning and adjust their AR process accordingly,” Seidel said.

The gold standard in lens cleaning includes high-powered cleaning with de-ionized water and a cleaning agent, and then ultrasonic cleaning to eliminate   all contaminants.

Kim, an engineer by training, said labs should not be tempted to skip or modify steps for economy or efficiency.

“What is de-ionized water and why use it? De-ionized water is just that. It is water that has gone through a physical process where its mineral ions have been removed. Since the most common impurities in tap water are ions such as calcium, chlorides, and sodium, we want to clean our lenses with water that has these impurities removed as all can leave residues that appear to the eye as water stains,” Kim explained.

To remove these contaminates, water is moved through specially prepared resin tanks that exchange positively and negatively charged ions for hydrogen and hydroxyl ions. The end result is pure water with very low electrical conductivity (or high resistance) and few dissolved solids. The use of clean, pure water will prevent a lens from becoming more contaminated than it already was from the surfacing and spin coating process.   Also, since de-ionized water is missing ions, it will seek out and absorb ions from   the lens surface. 

Kim noted that lab owners may wonder why a cleaning agent should be used after cleaning with pure,   de-ionized water.

“Pure water does not have the ability to reduce the surface tension of the surface of the lens or to remove organic   materials. The surface tension is what is helping to keep contaminants attached to the surface of the lens (like particles, finger prints, large water stains).   The use of basic pH to neutral pH cleaning solutions at an elevated temperature in combination with de-ionized water can help break the contaminant from the surface of the lens,” Kim said.

Ultrasonic cleaning actually is continuous, multiple, high-frequency bursts of energy. The high frequency is created by transducers bonded to the bottom and sometimes sides of a stainless steel tank. A transducer takes an electrical input and transforms it into a mechanical output via ceramic piezoelectric elements. These high frequency bursts of energy (when applied in a tank of water) creates waves of bubbles in alternating waves of positive and negative pressure. The alternating waves create a mechanical forces that clean the lens like when it is hand-cleaned, but the mechanical force of ultrasonics does not touch the lens.

There are both upfront and operating costs associated with having an automated ultrasonic cleaning machine in your lab. Prices for new equipment can range from $50,000 to more than $250,000 depending on the   capacity and options of the cleaning system. The cost to regenerate the de-ionizing tanks, replace filters, and   replenish cleaning chemicals can similarly range from   several hundred dollars to several thousand dollars per month. AR experts advise lab owners to take “the long view” on these costs.

“The amount you clean directly affects the amount of   lenses you will have to re-run. In most cases the cost   of buying proper cleaning equipment is much lower than the cost of re-running lenses that have been rejected   due to dirty coating issues,” said Coburn’s Seidel.   

Going forward, the industry experts are bullish on AR, which has shown reliable growth even through tough economic times. Advances in AR coatings and equipment have removed barriers of entrance to the AR   market, so that large national brands and private label brands are all being produced side-by-side in labs around the country.

Satisloh’s Peterson observed that labs can reap a number of benefits from offering in-house AR: “First, they control the job from start to finish and the service to the customer is in their hands. They now control the speed, cost and quality and are no longer reliant upon   a third party.”

He added that there is still a big potential for growth with smaller labs and small integrated retailers getting   into AR coating.

“If we look at the VCA statistics of the number of   Americans over the age of 18 with a prescription and   then realize that 30 percent   of them are being served   with an AR product that helps them see better, the   upside for AR is immense,” noted Kester.

He added that education to the consumer and ECPs are the keys to the growth of AR for the next decade: “There will be a point reached where the   consumer understanding of AR and demand for the product’s medical benefits will drive the growth. Once this happens, huge gains in market penetration will   be made.”


Labtalk June 2020