The Moiré Deflectometer:
New Technology to Measure
Toric Soft Lenses

BY TONY HOUGH, M.B.A., B.A.
SEPT.1996

It's becoming evident in contact lens practice, as in so many areas of business, that you shouldn't try to be a Jack-of-all-trades. In fact, there's already an acknowledged division between those optometric practices that deliver high volume, low cost lenses and those that target the patients who don't fit into the limited parameters of those products.

The key to the latter approach is to identify a market segment that you're comfortable with -- specialty lenses, for example -- and focus your resources on delivering quality and value within that niche.

DELIVERING SPECIALTY LENSES

Toric soft lenses are good examples of specialty lenses in action. There is a good choice of design options for low sphere and cylinder powers. However, offering higher powers and cylinders is still very much a custom, prescription-centered activity because the multiplicity of parameters and design options mitigate against keeping inventory.

Prescribing toric soft lenses for a new patient involves a hit and miss element. Often you must fine-tune the first lenses to obtain satisfactory visual acuity. And it's likely you'll need more than two sets of lenses to fit the average patient successfully. There are two principal reasons for this:

1.) The interaction between the patient's corneal topography and lid configuration and the design characteristics of the lens often cause the lens to stabilize off axis. Manufacturers have data regarding the on-eye dynamics of their designs and should be able to provide the likely orientation characteristics.

2.) Traditional methods for measuring the power-related parameters of toric soft lenses are inexact. Yet, measuring these lenses consistently and accurately is central to the business of delivering them more effectively and more profitably.

MEASURING TORIC SOFT LENSES

Recently, the Contact Lens I working group, a technical committee of the International Standards Organization (ISO), reviewed two instruments used to measure the power-related parameters of toric soft lenses.

At independent sites internationally, the committee determined the powers and axis directions of commercial lenses using a lensometer, which had already been established as adequate to measure the proposed tolerances for single-vision soft lenses. This procedure, called a ring test, was also performed on the Moiré deflectometer, a new instrument developed by Rotlex, Ltd., of Israel, and marketed under the trade name ConTest (Fig. 1).

FIG. 1: THE MOIRÉ DEFLECTOMETER.

The ring test is used to determine the capability of the instrument, set product tolerances and provide guidance to manufacturers and practitioners on how to use the product within the specified tolerances.

The results of the tests demonstrate that the lensometer is relatively imprecise, failing to provide reliable power measurement data for toric lenses in normal office use because of the high number of replicate measurements required. For example, the results show that using the lensometer under controlled conditions requires 19 independent measurements to specify the sphere power of toric soft lenses to within ±0.25D. With the Moiré deflectometer, seven measurements are sufficient to guarantee sphere power to within ±0.25D. If the product tolerance for sphere power were set at ±0.50D, then five measurements would be required with the lensometer compared to two measurements using the deflectometer.

THE MOIRÉ DEFLECTOMETER

In addition to measuring power, the Moiré deflectometer can also provide a visual indication of the optical quality of the lens. The deflectometer is a quantitative instrument for mapping ray deflections of a beam of light passed through or reflected off a test object. The technique is based on the Moiré effect, a phenomenon that causes a fringe pattern to appear when two gratings are placed at a small angle to each other.

The lens is placed in a cuvette of saline as shown in Figure 2.:

The fringe pattern is detected by a CCD camera and is digitized using a frame grabber board. The resulting image is displayed on the personal computer by means of proprietary control software. When the lens is of good optical quality and the deflectometer is correctly focused, straight vertical fringes are apparent (Fig. 3). If the lens is of poor optical quality, the fringes will be wavy or distorted (Fig. 4).

FIG. 3: VERTICAL FRINGES, AS OBSERVED WHEN THE DEFLECTOMETER SYSTEM IS FOCUSED.

FIG. 4: THE QUALITY OF THE FRINGES IS A MEASURE OF THE OPTICAL QUALITY OF THE LENS. THIS IS AN EXAMPLE OF A POOR QUALITY LENS.

The operator can configure the software to show a graphic or numeric quality scale.

Table 1 shows the number of replicate measurements needed to determine the power-related parameters of toric soft lenses to sensible tolerances using both the lensometer and the deflectometer.

As with all test methods, the measurement error is reduced by taking successive readings. For sphere power, the implied product tolerance for successive readings is shown in Figure 5.

ENSURING REPEATABILITY

Statistical analysis is important, but practitioners and product managers want to know "Does this method tell me the same story all the time?" and "Can I use this method to confidently differentiate between consecutive lenses in my product range?" The answers to these questions will determine the ultimate success of the product in the clinic and will also have economic implications for both the lens manufacturer and the practitioner. This aspect of the test method is a measure of its repeatability. The data in Figure 6 was obtained by taking the mean of five sets of 10 replicate measurements of the same lens on both instruments. It shows that the deflectometer is more consistent than the lensometer and the at-a-glance repeatability of the deflectometer is superior.

QUALITY ASSURANCE, THE ULTIMATE GOAL

The measurement of the power-related parameters of toric soft contact lenses has captured much interest as more manufacturers and practitioners compete for this market segment. Fitting toric soft lenses often requires more than one pair of lenses to achieve a successful fit. Often, small off-axis rotations can cause unacceptable deterioration in the patient's visual acuity, so you must be able to detect small differences in the power-related parameters of these relatively complex lenses.

The international testing committee has shown that it's not practical to measure the powers and axis directions of toric soft lenses with the lensometer. Using skilled operators and a well-defined protocol under carefully controlled conditions would provide an improvement, especially in repeatability. Many of the larger manufacturers have now recognized that to deliver a consistent product and have greater confidence in the labeled values, we must have an objective system of measurement such as the deflectometer. The benefits of using such a system, particularly being able to measure the lens in saline and not having to worry about orientation, are considerable.

Although the ConTest instrument is aimed at contact lens manufacturers, it could be a valuable asset for larger practices that promote specialty lenses. Rotlex is developing a practitioner version for in-office use, based on the core technology of the current model, including the ability to measure objectively in the wet state and to assess optical quality. CLS

References are available upon written request to the editors at Contact Lens Spectrum. To receive this information via fax, call (800) 239-4684 and request Document #17. (Be sure to have a fax number ready.)

Tony Hough is a technical expert in the field of contact lenses with 18 years' experience manufacturing lenses and lens producing technology. He is a consultant based in Cambridge, U.K.