AR Residual Color

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All AR lenses reflect a small amount of color, typically referred to as residual color. Many AR lenses have a green residual color, while others have a blue, gold or yellow-green residual color.

This phenomenon is associated with the reflectance curve of a broadband AR coating across the visible spectrum of light. AR has a peak reflectance at some point on the visible light spectrum (400–760 nm), and it is the color associated with that peak that is reflected most. AR lenses with a green residual color tend to have peak reflectance in the 520 - 550nm range, which is also the easiest color to produce consistently. Some manufacturers move the curve peak closer to the blue end of the spectrum, while others move the peak towards yellow. Most AR producers choose their AR design and resulting peak color reflection based on customer preference or visual performance goals. Eye sensitivity during daytime or twilight has also been cited as criteria for residual color choice.

The ability to control manufacturing consistency is also a factor. Peak reflectance can drift slightly from day to day or even within the same day if the process is not monitored carefully. Environmental changes in production such as vacuum chamber cleanliness or even changes in humidity are factors that can lead to inconsistent residual color. Lenses produced at different times can have unmatched residual colors, resulting in a patient receiving two lenses with different residual colors. This can also occur if a stock AR lens is used in conjunction with an AR lens custom made in the laboratory if the in house process is not matched with the reflectance color of the stock lens.

Rainbow Fringe or Newton Fringes

Why do we see a rainbow effect on certain lenses?

The effect occurs when there is an inconsistency of a lacquer on a substrate with a different index. In this example, the effect is caused by the variation of lacquer thickness, or scratch resistant coating, on a lens substrate (figure 1). The lacquer not only provides scratch resistant properties, but it also acts as a stable foundation for AR.

Reasons of thickness variation

Dip coating naturally gives a wedge effect due to gravity. An exaggerated example of this would be dipping a butter knife into a jar of honey then pulling it out. Observe how the honey drips (runs) off of the knife, it flows in small ripples not a perfectly smooth sheet. Irregularity in the pull out mechanism motion and vibrations in the lacquer vessel are also contributing factors.

Spin coating suffers different irregularity problems. The lacquer is spread on the lens surface by means of centrifugal force and high-speed rotation, resulting in concentric circular ripples in the lacquer coating.

Rainbow fringes become more apparent when the difference in index between the substrate and the lacquer increases. It also becomes more apparent after AR coating because the rainbow fringe is typically obscured by the surface reflections on non-AR coated lenses.

The rainbow fringe effect can be minimized by:

Applying a thicker lacquer layer–A thicker lacquer layer reduces the optical effect of the ripples. The downside of a thick lacquer layer is it takes longer to cure and may have a negative impact on the lacquer’s scratch resistance and adhesion. Using index matched lacquers–The closer the index of the lacquer is to the substrate the less noticeable the rainbow fringe. The down sides of index-matched lacquers are increased cost and technical know how. You will need two to three different indexed lacquers for the various lens indexes.

Various index lacquers will all have a different residual color if coated with the same AR design.

All manufacturing processes require checks and balances. Whether you process AR in house or use lenses manufactured off site, it is important to monitor your process and the lenses you select for you customer. AR is one of the final steps in producing high quality eyewear and is a critical factor on how your quality is judged by your customer. Being critical can only lead to increased business and satisfied customers.

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Labtalk-November/December 2017