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Tuesday / July 16.
HomemilensesChoosing the Right Polarised Lenses

Choosing the Right Polarised Lenses

Polarised lenses are gaining in popularity and are being offered by an increasing number of lens manufacturers. But with such a range to choose from, which polarised lenses are right for you?

Many of the tests described here have never been published before, and much can be learnt about what makes a good polarised lens. It is my hope that not only the test results, but also the description of the tests will help you, the users of polarised lenses, to make informed decisions for your customers on what polarised lenses are best for them.

Polarising Film Location

Precisely locating the polarising film from the front surface of the lens is one of the most difficult tasks in lens casting. The goal is to position the film within a narrow tolerance; ideally between .4mm and .8mm.

It is important for lens processing that the polarising film is reproducibly positioned near the front surface of the lens. To confirm this, cross-sections are sliced from lenses, and the distance of the polariser from the front surface is measured at multiple locations across the width of the lens by using a calibrated scale and optical magnification.

Polarisation efficiency is a particular problem when polarised lenses are produced in much lighter colours such as light blue, green or yellow

Polarisation Coefficient

Darker coloured polarised lenses provide excellent polarisation, with a coefficient above 99 per cent. Polarisation efficiency is a particular problem when polarised lenses are produced in much lighter colours such as light blue, green or yellow. These lenses often have polarised efficiency so low that they cannot really be considered polarised lenses at all. Looking at a glare demonstrator through less efficient polarised lenses you can see that virtually none of the glare is cut out.

Polarising Axis Alignment

Industry standards allow a 3° degree tolerance for axis alignment. Beyond that, the wearer may experience visual discomfort. In Graph two, ‘Polarising Axis Alignment’, shows all products tested fall within the industry tolerance, and 95 per cent of all Hoya NuPolar tested product falls within 1° degree of perfect alignment.

For most effective blocking of glare, the polarising absorption axis should be aligned as closely to horizontal as possible. Lenses are typically marked with this alignment axis to assist proper positioning in the frame. Misalignment of the axis by more than 3° may be noticeable to the wearer, and ISO 8980-3 requires that the marks and the axis differ by no more than +/- 3°.

Unwanted Cylinder

During the casting process of any hard resin lens, enormous thermal-chemical forces are at work and hard resin materials shrink about 11 per cent. This creates tremendous stresses and strains on every plastic lens produced. With a hard resin polarised plastic lens, these forces become even more significant. Not only do you have a thin film stuck within this thermal-chemical reaction, you also have a much thinner layer of hard resin in front of the film than behind it. In addition, the film itself is created by “stretching” a polyvinyl alcohol sheet to line up the iodine crystal and make the film polarised.

This stretching process creates additional stresses and strains which all interact in the complex casting process. How well this entire polarised casting process is controlled and monitored often shows up on the front surface as cylinder.

Hoya’s NuPolar cylinder is consistently low at every test point, and never exceeds more that 0.03 diopter. Product G also does a good job of controlling cylinder. However, Products H, I, and J do not. Product J, with nearly 0.07 diopters of cylinder, would not be acceptable in Younger’s testing.

If the front lens surface is warped from its designed spherical or aspheric surface, optical measurements of the uncut lens blank will show unwanted cylinder. This excess cylinder can compound optical errors in the finished lens, and therefore should be kept to a minimum. Lenses were checked for unwanted surface cylinder by measuring reflected light (using a focovision instrument) at five locations distributed across the area of the lens.

Polarising Film Adhesion

Years ago, the biggest complaint about polarised lenses was that they separated or delaminated. When this failure occurs, it is really due to the fact that the film did not completely chemically “bond” itself to the hard resin material. Theoretically, the stronger this bond, the less likely a delamination will occur. The good news is, most lenses have greatly improved their adhesion in the past few years and can survive through subsequent laboratory and finishing processes.

Keith M. Cross is a registered Dispensing Optician with a license to practice in the State of Massachusetts. He is Board Certified by the American Board of Opticianry; a Fellow of the National Academy of Opticianry and an Honored Fellow of the Opticians Association of America. He owned and operated Northeast Lens Corporation, a full service Rx lab (including surfacing, edging / glazing and AR coating) in Massachusetts (USA) for over 10 yrs, which he sold to Carl Zeiss Vision in 2005. After working for CZV for a two-year transition period, he joined Younger Optics in his current role as the Senior Director of Rx Technologies where his global responsibilities include Rx product development, Rx processing and technical support.