The Importance of Lens Cleaning After Spin Coating and Prior to AR

By George Kim
Cleaning a lens is a common notion to any ophthalmic lab technician. Let’s face it. Cleaning lenses is an every day and standard practice in any surfacing lab and should by no means be a foreign concept to any qualified lab personnel. But what is “cleaning a lens”?

Can it be classified by rinsing a lens with water and a lint free wipe? Can it be defined as spraying with an optical lens cleaner and wiping with a high-quality optical cloth? Is it something that is done by hand or by machine? Or, should it be given a more scientific definition and be quantified by measurable parameters? As an engineer by education and advocate of the benefits of AR lenses, this author would choose the latter proposed definition.

Cleaning a lens prior to hard coating is very important, most would say crucial, to the success of a good application of a hard coat. Depending on whether the lens will be processed via dip coating or by spin coating, the standard for a “clean” lens may differ. Nonetheless, a clean lens is important prior to application of any lacquer.

Cleaning a lens prior to AR (anti-reflective) processing is arguably more important than cleaning a lens prior to spin coating. The process of AR application to a lens is vastly different than the application of a primer or lacquer to a lens, so the requirements are significantly different. The conditions required for an AR to bond to a lacquer are different than the conditions needed for a primer or lacquer to bond to a lens. In this regard, a typical lens flowing through a surfacing lab will experience all kinds of contamination. Some examples are finger prints, powders from slurry, ink marks, water stains, and even residual components of seemingly good cleaning products like dish washing detergent. All these contaminants will need to be removed in order to bond an AR layer stack to the lacquer. 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 process 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. One also has to keep in mind why a lens gets a scratch. The most common reason is when an abrasive object comes in contact with it (touches it) and leaves a mark in the lacquer! To successfully process a lens through an AR process and retain high quality and high yield, a systematic approach must be used. The best way to achieve such a goal is to clean lenses with de-ionized water, an appropriate cleaning agent, and use of a high quality ultra-sonic automatic cleaning machine with appropriate fixturing that eliminates the necessity to touch the lens to render it clean.

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. To remove these contaminates, water is moved through specially prepared resin tanks that exchange positively charged ions and negatively charge ions for hydrogen and hydroxyl ions. The end result is pure water with very low electrical conductivity (or high resistance) and few dissolved solids. With this property, we are able to measure or quantify what is “clean water.” For the application of cleaning ophthalmic lenses, clean water is in the range of minimum 1 microSiemen or lower at 20 degrees Celcius. The use of such clean, pure water will prevent a lens becoming more contaminated than it already is from the surfacing and spin coating process. Also, since DI water is missing ions, it will be “hungry,” seeking out and absorbing ions from other sources, like from the lens.

Why use a cleaning agent? Isn’t cleaning with the pure water enough to clean a lens? 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 (30 to 50 degrees Celcius) in combination with DI water can help break the contaminant from the surface of the lens.

What is ultra-sonics and why is it necessary? Isn’t the use of DI water in conjunction with a cleaning agent sufficient? Ultra-sonics are continuous multiple high-frequency bursts of energy. The high-frequency (normally 25 - 40 kHz) 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. This is accomplished with ceramic piezoelectric elements. These high frequency bursts of energy (when applied in a tank of water) create cavitation waves of bubbles in alternating waves of positive and negative pressure. These alternating waves create mechanical forces that clean the lens; like we are doing when we try to clean a lens by hand. Instead, the mechanical force of cleaning behind ultra-sonics does not touch the lens.

There is of course both an upfront and operating cost to having an automated ultra-sonic cleaning machine in your laboratory. The 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 DI tanks, replace filters, and replenish cleaning chemicals can similarly range from several hundred dollars per month, to several thousand dollars per month. However, to keep high quality and most importantly consistent Hard Coating and AR quality, a serious owner/manager should consider the advantages of today’s advancements in cleaning processes and equipment technology.

This article was provided by The Vision Council AR Steering Committee. To promote anti-reflective lenses, the committee provides education and marketing resources to the optical industry. For more information, please visit www.thevisioncouncil.org/ecp.

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May/June LabTalk 2017