MEASURING FREEFORM LENSES – PART 1 OF 2

By John Fried and Christian Laurent
The partnership of responsibility between the eye care professional, optical laboratory and lens manufacturer to deliver corrected vision to the eye wear consumer has been well defined over the years. Traditionally, the task of faithfully producing the progressive lens design created by the lens designer rested with the lens manufacturer. optical designs are replicated on semi-finished lens blanks through a carefully controlled molding process that is subject to a stringent process and inspection procedures perfected over the years with feedback from exacting metrology. Through improved metrology better, more consistent lens products have been developed.

With classical progressive addition lenses, the resulting lens design and exact power values afforded by that design depend on the semi-finished blank convex surface. The final lens behavior results from the combination of a simple concave surface and the complex convex surface selected from a limited amount of values. With the advent of digital surfacing and freeform technology used to produce progressive and other complex lens surfaces, the balance of responsibility has shifted toward the optical lab and away from the lens manufacturer. New measurement systems and procedures have been developed to empower the optical lab with the tools needed to meet this new responsibility.

Some New Challenges

The flexibility of digital surfacing and the powerful capability of current lens design software give the possibility of producing personalized lenses. Taking an increased number of patient specific parameters into account like type of use, wearer behavior, frame size and position, and various adapted prescription data, individualized freeform lenses can be optimized. This freedom of design that is customized to the specific needs of the patient can result in an unexpected location of power zones on the lens. In many freeform progressive lens processes, instead of being positioned on the front surface (convex), the reference semi-visible markings used for lens positioning are engraved on the back surface. This way of working requires extra care regarding the parallax effect (concave engraving viewed through the convex surface) during lens positioning for measurement as well as during lens inking on the convex surface for finish blocking and other operations.

The capability to produce extremely accurate lenses in the lab with the new digital surfacing machinery is partially dependent upon accurate measuring systems that provide the means for controlling the process and ensure that the proper lens design and power is produced. Fortunately, in the case of freeform production, the complete accurate lens surface data are available in the lab. The aspects of measuring the results of the digitally surfaced freeform lens can be grouped into three major areas of interest:

1. Measuring the power of the lens accurately and with repeatability. In the case of freeform progressive lenses the power measurement zones include:

a. The prism reference point.

b. The distance vision zone.

c. The near vision zone.

2. Measuring the resultant progressive design on the lens in comparison with the expected lens design intended by the optical designer.

3. Ensuring the quality of the lens surface and validating the absence of unwanted waves and aberrations that may have resulted from equipment or process deficiencies.

Measuring Power

Accurate power measurement is a cornerstone for reliable statistical process control (SPC). It is also fundamental in ensuring manufacturing tolerances, reduction of unnecessary scrap and delivering product that will be accepted by the eye care provider.

It follows that in order to control a process for producing lenses with the needs and capabilities afforded by digital surfacing, a measuring system with accuracies of 0.01 D (diopter) is needed. There are two main measuring instrument design approaches available for measuring power. The first analyzes the deflection of a local and well defined beam of light transmitted through (or off) the lens that is centered over the zone of interest along the power measurement axis of the lens; the second is based upon interpreting the projection or “refraction effect” of a “blanket” of light through the full area of the lens.

This article discusses the defined beam approach as the demonstrated accuracy of such systems can be in line with our goal on achieving a 0.01 D accuracy. In the preferred approach, the lens should be positioned so the power zone is centered over the instrument’s target light beam. The inclination of the lens should be so the instrument axis is aligned along the normal to the concave lens surface at the measuring point. The reference power values should be calculated at least for one of the possible measurement configurations described by the standards (ISO 8980) and defining the light beam direction—focus on axis or infinity on axis (‘FOA’ or ‘IOA’)—and the projected light wavelength (e-line or d-line frequency).

As mentioned previously a personalized, progressive, freeform design could result in a variety of near view placement options in order to address the unique frame, prescription and patient interface. The traditional inset and drop may not be valid for a given prescription and frame combination even with a particular brand of progressive. This means that the capability of placing the measuring device within the proper power zone of interest is imperative. Power measuring instruments utilizing lens positioning assistance systems that reference the micro-engravings (permanent semi-visible positioning reference marks), used in concert with a direct data exchange with the host computer, will provide the operator with the right combination of information so the proper power zone on the lens can be evaluated.

This approach also takes the parallax effect (concave engraving viewed through the convex surface) into account for the lens placement. For measurement systems without positioning assistance based on the micro engravings, the lens must be inked on the convex surface with the far view and near view measurement area markings located at the exact position for the individualized product, prior to the measurement. In that case, the positioning depends very much on the inking operator as well as estimation and skill of the measurement operator. Moreover, when measurement devices are not connected to the host computer the measurement results can not be automatically used for further analysis (SPC).

Since a key requirement in accurate, reproducible measurements is the lack of influence the operator may have on the measurement, an instrument and process that is designed to eliminate that influence will result in an objective, reliable and consistent measurement.

Of course, the best conditions will be obtained with fully automatic systems equipped with accurate automatic or robotic positioning stations and highly accurate measurement systems. Such systems lead to reliable operations and enable the possibility for multiple (2 way) data exchanges with the host. The requirements for measuring progressive lenses produced with a digitally surfaced freeform process are similar to that for standard progressive lenses.

The possibility for variability of near zone placement and power values with freeform as well as the possibility of errors in the manufacturing process simply make the requirement of accurately and consistently taking the measurements on digitally surfaced lenses all the more important.

In Part 2, determining the faithful reproduction of the lens design and ensuring control of the digital surfacing process is discussed. This article was written by John T. Fried, president and Christian Laurent, director of research and development, at A&R Machinery, producers of automated inspection and blocking equipment.

CURRENT ISSUE


August/September LabTalk 2017