Black magic or precision?

The production of spectacle lenses is an art in itself. Especially, the cleaning and coating process in particular should be carefully thought out and implemented to ensure minimal breakage and the highest quality spectacle lenses. This article provides an insight into the technologies and practices involved in these essential stages of the lens manufacturing process – from perfect cleaning methods to nanotechnology in AR-coating.

Today’s lenses undergo a multifaceted and highly precise series of process operations through a range of differing equipment before reaching the hands – or eyes – of consumers.

Among these, cleaning and coating are not merely steps in a linear chain but critical operations. They determine the overall performance, longevity, optical clarity, durability, and customer satisfaction associated with the final product and ever-growing user expectations. Today’s high-precision cleaning and coating technologies are able to eliminate microscopic contaminants or to apply ultra-thin, multifunctional coatings. However, in order to achieve the best performance, the crucial steps must be carried out with absolute precision and using good equipment.

Perfectly prepared: pre-cleaning steps 

This precision already begins with cleaning, which is carried out in several steps rather than just one. Before spectacle lenses enter the precision cleaning stage, they are usually subjected to a pre-cleaning process designed to remove coarse debris such as swarf, dust, or leftover polishing compounds. These initial cleaning actions are vital, as they prevent larger contaminants from clogging downstream filtration systems or disrupting ultrasonic cleaning operations. Operators can either wipe the lenses with alcohol-based solutions that help to dissolve and remove greasy smudges or fingerprint marks. Or they use mild detergent baths, either through automated stations or cleaning by hand, to break down stubborn contaminants. 

Dual action: spray and brush cleaning

Automated spray and brush cleaning systems are effective in the pre-cleaning stage for reaching lens areas that may be difficult to clean using ultrasonic cavitation alone. As those methods utilize mechanical action.

These systems typically use rotating soft-bristle brushes to gently scrub the surface of the lenses while simultaneously applying high-pressure jets of deionized water. This dual-action mechanism effectively removes residual particulates after the polishing process and ensures that the lens surface is completely clean prior to coating departments.

Cleaning – a ‘black magic’ and the ­foundation for success

Often considered a ‘black magic’ compared to the other processes within the chain of lens manufacturing, cleaning is an essential step. And it is the foundation for success for many growing high-value add-on coating treatments.

Contaminants such as oils, polishing compounds, or airborne particles can inhibit coating adhesion, resulting in optical distortions or premature wear and delamination. Consequently, the quality and methodology of the cleaning process have a direct impact on the effectiveness of every subsequent operation.

To achieve flawless results, lens cleaning involves multiple stages that incorporate precision chemistry, mechanical action, and fluid dynamics. And they are often tailored to a wide variety of lens materials and different indices.

These cleaning stages are typically carried out using highly automated cleaning and coating machinery in controlled environments to ensure consistency. These high-grade cleaning systems are designed not just to clean but to protect delicate lens materials and ultimately prepare them for the following coating and curing steps .

Chemistry, water quality, and filtration 

The effectiveness of lens cleaning hinges heavily on three key factors: the chemistry of the cleaning agents, the purity and type of water being used for each stage, and the quality of the filtration systems.

Each of these elements plays a synergistic role in ensuring that the lens surface is completely free of contaminants and adequately dried for coating processes. The chemical solutions used should be tailored to remove specific contaminants without interacting adversely with the lens substrate. For instance, aqueous-based surfactants are particularly effective for eliminating hydrophilic particles, while solvent-based solutions are better suited for dislodging oily residues.

These formulations are frequently developed through extensive research and development to strike the optimal balance between cleaning efficacy and material compatibility, particularly in the case of high-index or specialty lenses. Deionized (DI) water is indispensable in the final cleaning process. Unlike standard tap water, which contains dissolved ions such as calcium and magnesium (but can be useful for rinsing stages after chemical ones), DI water has been stripped of these impurities to prevent mineral deposits from accumulating on the lens surface.

These deposits can significantly degrade visual clarity and interfere with the proper adhesion of subsequent coatings. Therefore, ultra-pure water must be used to ensure that each rinse phase leaves no trace contaminants behind.

The filtration systems embedded within these cleaning setups are equally critical. Precision filters, often with low micron ratings, are utilized at key stages of the cleaning process to capture the smallest particulates. These should be routinely monitored through both standard operational preventative procedures and, if applicable, the equipment’s onboard systems for monitoring pressure drops, saturation, and/or conductivity levels, replacing these filters at defined intervals to maintain peak operational efficiency and highest quality yields. 

The gold standard – ultrasonic cleaning 

Widely considered the gold standard for removing microscopic contaminants from lens surfaces without physical abrasion prior to coating applications. This cleaning technique employs high-frequency sound waves, typically around 40kHz – 100kHz, which are propagated through a liquid chemistry medium to create microscopic cavitation bubbles. As these bubbles implode near the surface of the lenses, they generate powerful yet localized micro-jets of energy that dislodge contaminants effectively and uniformly. Ultrasonic cleaning systems are usually designed with a series of tanks, each fulfilling a specific function.

The initial pre-wash tanks loosen and suspend the bulk of the contaminants. This is followed by the primary ultrasonic bath, where the cavitation effect performs the detailed cleaning at a micro level.

Subsequent chemical and rinse tanks ensure that all residual solutions and particles are thoroughly washed away.

Automation and control allow managing multiple batches of lenses. Those technologies also play a crucial role in maintaining tight control over parameters such as bath temperature, exposure time, and ultrasonic power to ensure consistent and reproducible results across all lens batches. 

Deionized water rinsing and filtration 

The rinsing of lenses in final stages using deionized water is one of the most crucial steps in the lens cleaning process. This stage ensures that all traces of cleaning agents, surfactants, and microscopic debris are removed from the lens surface before the application of any coating.

Modern final rinsing stages incorporate multi-stage configurations, often with cascading water tanks that allow for progressively cleaner rinses. These systems are also equipped with continuous water purification units that maintain the purity of the DI water supply.

Inline sensors are employed to measure the conductivity of the water in real time, ensuring that the rinse quality meets stringent specifications.

Advanced filtration systems are integrated to keep the rinse water free from particulate contamination, further safeguarding the quality and consistency of the final product. By coupling the ultra-pure rinse action of the DI water with a controlled extraction from the liquid, the potential for water staining can be greatly reduced before progressing to the drying stage.

Drying processes like hot-air, vacuum or infrared

After rinsing, it is imperative to dry the lenses completely before they proceed to the coating stages. Residual moisture can lead to defects such as water spots, delamination, or inconsistent coating adhesion.

One common method is the use of HEPA-filtered hot air-drying systems, which provide controlled airflow and temperature to gently evaporate any remaining moisture.

In applications where lenses are sensitive to heat, vacuum drying may be employed. This technique lowers the boiling point of water, enabling low-temperature drying that is both efficient and non-damaging to the substrate.

Another effective drying method is infrared (IR) drying, which accelerates moisture evaporation by selectively heating water molecules without significantly raising the temperature of the lens material itself. These drying processes also act as pre-cure steps, stabilizing the lens surface to ensure optimal conditions for subsequent coating applications. 

Coating technologies for enhanced performance 

Once lenses are thoroughly cleaned and dried, they are ready to receive a variety of functional coatings. These coatings serve multiple roles, from enhancing optical clarity (AR coatings) to improving mechanical durability and resistance to environmental factors (hard coatings).

The most common types of coatings are hard coatings for scratch resistance, and anti-reflective coatings for visual clarity, mirrored coatings, hydrophobic and oleophobic layers for ease of maintenance, and UV-blocking layers for wearers eye protection. 

Hard coatings for scratch resistance

Hard coatings are designed to form a transparent, durable protective barrier on the lens surface that resists scratches from everyday handling and wear. These coatings are typically applied using either dip-coating or spin-coating techniques.

In dip-coating on modern equipment, lenses are batched in volumes, immersed in a polymer-based coating solution, and then withdrawn at a controlled speed, allowing the coating to spread evenly and dry without streaking. The final cure is normally achieved through a thermal curing stage after manual or automated inspection.

Spin-coating is more commonly used for smaller lens volumes. Usually, it is singularly processed and involves placing a small amount of coating material on the lens surface, which is then rapidly spun to distribute the coating across the concave surface of the lens. The coating material is usually a UV-curable polymer resin that hardens upon exposure to ultraviolet light. The final properties of the hard coating, such as its hardness, adhesion, and optical transparency, depend not only on the resin formulation but also on the precision of the application and curing process. 

Anti-reflective coatings 

Anti-reflection (AR) coatings are essential for reducing glare, protecting the eye, and increasing market value. AR coatings enhance light transmission and minimize unwanted reflections that can interfere with vision.

These coatings are typically applied using well adopted vacuum deposition techniques for best results within a clean room environment to prevent contamination. Common deposition methods include electron beam evaporation, ion-assisted deposition, and magnetron sputtering. Each of these techniques enables the application of nanometers-scale layers of dielectric materials, such as magnesium fluoride or titanium oxide, in highly controlled sequences (the elements form what is known as a stack).

The resulting multilayer structure is finely tuned to cancel out specific wavelengths of reflected light, thereby enhancing both the visual and aesthetic performance of the lenses. 

Hydrophobic and oleophobic coatings 

As a final protective layer, hydrophobic and oleophobic coatings are applied to repel water, oils, and smudges. This makes lenses easier to clean and improves long-term user satisfaction. These coatings are often applied using plasma-enhanced chemical vapor deposition (PECVD) or spin-on techniques, followed by thermal curing to fix the coating to the lens surface. Advanced formulations are now available that provide long-lasting durability, resisting degradation from ultraviolet exposure and temperature fluctuations. These top-coat layers not only enhance performance but also extend the useful life of the lenses. 

Clean room environments for coatings

Because even a single dust particle can compromise the integrity of a high-performance coating, the application of any such coatings should occur in clean room environments that adhere to rigorous cleanliness standards, typically ISO Class 7 or better.

These environments should be strictly controlled for airborne particulate levels, humidity, temperature, and airflow dynamics. Operators working in clean rooms should follow behavior protocols to prevent any unnecessary contamination.

HVAC (heating, ventilation and air conditioning) systems equipped with HEPA or ULPA filters continuously cycle and purify the air to maintain the required environmental conditions. These measures are essential to ensuring the flawless application of coatings and the consistent production of high-quality optical lenses. 

Sustainability in cleaning and coating processes 

Modern lens manufacturing increasingly emphasizes sustainability. Many equipment designs now incorporate energy-efficient features and components. Chemical formulations are also being developed with reduced environmental impact in mind.

Water recycling is becoming a growing standard practice, achieved through multi-stage filtration and UV sterilization. Low-VOC (volatile organic compound) coatings are being adopted to minimize atmospheric emissions.

While smart energy management systems are being implemented to monitor and optimize power consumption across production lines. These efforts help manufacturers comply with environmental regulations. Reducing waste during cleaning/rinsing by harvesting water at either plant or machine level and increasing the active life of coating materials all work towards improving sustainability during operational lifetime too. 

Conclusion 

Cutting-edge systems, advanced chemical engineering, and tightly controlled environments enable any lens manufacturing lab to deliver coated products that meet the very highest standards of optical clarity, durability, and user satisfaction.

As consumer expectations rise and new optical technologies and entrants into the market emerge, the demand for refined cleaning and coating processes will only continue to intensify.