A perfect fit


By Sharon Ann Holgate A CONTRAPTION based on a modified CD player could make contact lenses more comfortable to wear. Scientists at the Cooperative Research Centre for Eye Research and Technology in Sydney, Australia, have used the reading head of a CD player to make a novel optical profile measurement device called a “profilometer”. In a pilot study reported in Measurement Science and Technology (vol 9, p 1259), rigid lenses from three different manufacturers were measured. They all showed marked deviations from the shape expected. In a follow-up clinical study, “poor edge shape was shown to correlate with unsatisfactory fitting and low comfort”, says the instrument’s developer Klaus Ehrmann. Optical profilometers are not new, but because they rely on detecting reflected light, they can scan only relatively flat surfaces as curves deflect the light out of the path of the instruments’ fixed sensors. But in the new profilometer, the CD laser head is mounted on a motorised, rotating stage. Feedback from the CD head’s “tracking error” signal—which normally stops the head skipping data tracks—is used to rotate the head until it is perpendicular to the lens surface. This ensures that no data is lost, whatever the surface shape. The head is scanned across the surface, and at each data point, the angle through which it must be rotated corresponds to the slope of the lens profile. The lens can also be rotated, allowing scanning in successive planes. The device can measure the surface shape of a contact lens to an accuracy of 2 micrometres. And because it uses a laser beam as a probe, it does not damage the lens. So for the first time, complete three-dimensional representations of lens shapes can be built up. Graeme Young, director of Visioncare Research of Farnham, Surrey, which tests eye products, cites poor comfort as the most common reason for people giving up rigid contact lenses. “This instrument is therefore a welcome tool in helping to develop more comfortable lenses,
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