Study on power law along meridian line for progressive addition lenses

The surface curvature of the progressive addition lenses varies gradually from the distance-viewing area to the near-viewing area. The curvature of the principal meridional curve varies progressively from point to point to provide a predetermined dioptric focal power at each point according to a predetermined power law. Several kinds of power laws along meridian line including linear combination and polynomials, are illustrated. The details for determining the coefficient and order of the polynomial are also introduced. Based on different power laws along meridian lines, the results of the computer evaluation are given out. Progressive addition lenses corresponding to different forms of meridional power laws have been manufactured with Satisloh VFT-compact machine. The lenses manufactured have been measured using the Class Plus lens analyzer to provide sphere, cylinder, and axis values across the surface of the lens, and distortion distributions are also presented. The results of measurement indicate that the performances of progressive addition lenses are consistent with those of the computer evaluation. These results are shown that the lenses have more wide distance-viewing or near-viewing areas and short corridor but with higher level of astigmatism in the peripheral areas while the curvature along the median line is constant in the distance zone and the rate of meridional dioptric variation is quicker, on the contrary, the lenses have more narrow distance-viewing or near-viewing areas and long corridor. The comparison and analysis prove that determining the power law along the meridian line is an important task in designs for progressive addition lenses.