Originally Posted by
Darryl Meister
Unfortunately, it is impossible to measure accurately the base curve of any non-spherical lens (including aspherics and progressives) using a conventional lens clock. The curvature of the surface of such a lens changes across the width of the lens clock pins, which are about 19 mm apart. Consequently, the lens clock (calibrated to show measurements from a spherical surface) is "fooled" into thinking the surface is a spherical surface of value X.XX D instead of an aspheric surface of value Y.YY D. The more aspheric the design is, the more the lens clock will be off.
When using the lens clock, it is important to ensure that your lens clock is perpendicular to the lens surface. You may also want to take both horizontal and vertical measurements, and then average them out, in case the lens is warped at all. Finally, since the lens clock is calibrated for a reference or tooling index of 1.530, you will need to either compare the measurement to the 1.530-based value from the manufacturer (called the true curve) or convert the value over to the actual power of the surface using the equation:
Actual Power = (Actual Index - 1) / 0.530 * Measured Power
Best regards,
Darryl
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