Correlation analysis of the optics of progressive addition lenses

To investigate the relations between selected key optical parameters and the sizes of the clear viewing areas of progressive addition lenses (PALs).

The optics of 28 PALs (plano with +2.00 D add) currently on the market were measured with a Rotlex Class Plus lens analyzer. Horizontal cross sections were analyzed in 1 mm vertical steps with respect to the fitting cross. Distance, intermediate, and near viewing zone widths and areas were calculated from the measurements. The maximum amount of unwanted astigmatism, minimum zone width (0.50 DC limit), and maximum power rate in the corridor were also recorded for each lens. Correlation coefficients were determined for all relations.

Each of the three viewing zone areas had a significant negative relation with the other (r of -0.4 to -0.8), indicating design tradeoff. Maximum power rate was significantly related to minimum zone width (r = -0.695), which was significantly related to maximum astigmatism (r = -0.616), but there was not a significant relation between maximum power rate and maximum astigmatism. Higher power rates and narrower minimum zones were significantly related to smaller intermediate and larger near zones (r = 0.4 to 0.9). Maximum astigmatism was related to distance zone width (r = 0.42) and to intermediate zone size (r = -0.4 to -0.56), but not significantly related to near viewing zone. Power rate and astigmatism each vary relatively uniformly across each lens.

The fundamental relation appears to be between power rate and zone width, each of which is highly related to sizes of the intermediate and near viewing zones. The maximum amount of astigmatism is related to zone width, but not to maximum power rate. The amount of astigmatism is unrelated to the size of the near zone. The pattern of correlations between the optical and viewing zone parameters help identify the underlying optical relations of PALs.