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Testing of the fracture of spectacle lenses

SOLA International

Moderators: Stephen Lucas (University of South Australia) and Jim Hill (University of Wollongong)

The Company

SOLA began in 1956, with nine optical technicians experimenting in a garage in Adelaide, South Australia. Their goal was to cast spectacle lenses from recently discovered plastic materials. After a few early successes, those pioneers founded SOLA (or Scientific Optical Laboratories of Australia, as it was first called) in 1960. SOLA's first overseas subsidiary opened in Japan, in 1968, and in ensuing years, operations were added throughout Asia, Europe and the Americas. In 1979, SOLA was acquired by Pilkington plc, and in 1988 the corporate headquarters moved from Australia to Menlo Park, California. The company's global expansion continued during the 1980's, with new manufacturing operations opening in Venezuela, Taiwan and China. In 1993, SOLA was purchased by AEA Investors Inc., and in March 1995 the company was listed on the New York Stock Exchange. Further double-digit sales growth came from new products, international expansion and through the acqusitions of US lensmakers Neolens and American Optical

SOLA's regional structure now includes North America, Europe, Asia, Australia, South America and an international Sunlens Division. Across the world over 100 million people go about their daily lives wearing SOLA lenses. The company now operates major research and development centres in Adelaide, South Australia and Petaluma, California; supported by a specialist process engineering team at a plant in Wexford, Ireland. From SOLA's earliest years, lens technology has continued to evolve. The result is thinner, lighter lens forms, innovative new designs and high performance coatings that combine exceptional cosmetics with optical excellence. A measure of SOLA's success in technical development and manufacturing is the fact that over 60% of sales come from new, value-added products.

Background

To satisfy FDA safety requirements for sale in the USA, all spectacle lenses must comply with the "drop-ball impact test". This test simply involves dropping a steel ball of diameter 5/8" onto the convex surface of the lens from a height of 50". The lens is held in place on a silicon ring on a supporting plate, with an additional load ring placed above to keep it in position. The steel ball can be guided, and must impact the lens near its center. The central region of the lens deflects as the ball strikes, and cracks may form, which could lead to the fracture of the lens. The lens passes the test if it doesn't break.

There is a similar European ISO/CEN test for spectacle lenses, which involves a "static load", rather than an impact. In this case a force of 100 Newtons is applied to the convex side of the lens, through a steel ball. The load is applied for 10 seconds, and then removed. The lens passes if it does not break.

There are several factors that could contribute to whether a lens passes the above tests:

• Nature of the lens material: the material properties of the lens determine how it will deform under a load, and can also effect the way in which cracks propagate through the material. Chemical treatments to the lens surface could modify its susceptibility to cracks forming and propagating.
• Coatings to the back surface: most lenses are coated with a hard glassy material to enhance scratch resistance, or to produce modified optical characteristics, such as reduced glare. These coatings are often much more brittle than the bulk lens material, and are likely crack initiation sites. However, these layers are typically only a few microns thick, and so will not have a significant effect on the overall deformation of the lens.
• Lens power and curvature: minus powered lenses are the ones in which impact problems are most likely to occur. In these lenses, the radius of curvature of the front of the lens is larger than the back, leading to lenses which are thinnest at the center (we can assume here that lenses are axisymmetric). It is the ratio of these radii of curvature that define the power of the lens. The perennial search for thinner lighter lenses lead to choosing the flattest possible curves and the thinnest possible centers, both of which will make the lens more likely to fail an impact test.

The Problem

Over many years, SOLA has amassed a large amount of empirical information on materials, coatings, lens forms, and their effect on impact performance, which could be made available for MISG contributors to this problem. However, there has been little modelling or theoretical analysis on the problem of lens testing under load. This is true throughout the lens industry. The problem is to attempt to understand more fully the problem of when and how lenses fracture, so that in the future less empirical testing is required to find an appropriate lens form that will satisfy the lens fracture criteria.

There are many potential approaches to this problem, some of which would be useful to SOLA:

• In the case of the static load test, can a model be found that relates lens form and material properties to the deflection of the lens? If this is also related to stress within the lens, lens fracture initiation sites could also be predicted, as well as at what load fracture occurs. (Will a classical infinitesimal elasticity model be appropriate, is a finite elasticity model more suitable, or can we assume the thin plate equations?)
• Can a similar model be found for the impact test, where the ball hits the lens and produces a varying force on the lens over a period of time?
• Can a model be found to relate material properties and lens geometry to the mechanism of crack propagation through uncoated lenses?
• Can a model be found to describe crack propagation through multiple layers on the lens surface, each of which has different physical properties?
• Can energy balance equations for the lens be used to predict deformation and/or fracture?
• How sensitive is the test to the position of the impact (losing the axisymmetric nature of the problem)?

The Solution

SOLA would be happy with any theoretical advances in the theory of lens fracture, and there are a number of possible approaches to the above questions. Statistical testing of the empirical data may point out some previously hidden simple relationships between the physical properties and fracture. The theory of elasticity can be applied in a number of ways to attempt to solve the static problem, both analytically and numerically (the axisymmetric geometry of the lenses is not particularly complex). Dynamical studies would be appropriate to investigate the behaviour of ball impact on an elastic material. Finally, the theory of fracture mechanics would be of use in analysing the form of crack propagation through the lens. Of course, a variety of other approaches could be developed during the workshop.