Researchers have conducted a detailed analysis of progressive addition lenses (PALs), commonly known as multifocal lenses, from leading manufacturers Essilor, Hoya, and Zeiss. The study investigated how the orientation of astigmatism correction within these lenses affects their optical performance.
The team tested various PAL designs, all with the same reading power addition of +2.00 diopters, but differing in the axis of their cylinder correction – one set corrected astigmatism at 90 degrees and another at 180 degrees. The lenses were designed to correct for distance vision prescriptions typical for individuals with myopia, or nearsightedness, particularly reflecting common refractive errors found in Hong Kong Chinese populations.
For each manufacturer, two popular PAL models with different corridor lengths, the area of progressive power change, were included in the testing, resulting in a total of 12 lenses per brand for examination. All lenses were made with a refractive index of 1.6.
Using a specialized instrument called the Rotlex Free Form Verifier, which operates on the principle of Moire deflectometry, the researchers precisely measured the refractive power profile across the surface of each lens. This involved projecting light through the lens and analyzing the resulting patterns to map the lens’s focusing power. To ensure accuracy, they input specific lens parameters into the measurement software, including the refractive index, center thickness, and front surface curvature.
The researchers developed sophisticated methods to analyze the measurement data, creating error maps that visualized the difference between the intended, or target, prescription and the actual refractive power delivered by the lens at various points. These error maps assessed spherical defocus, astigmatism, and overall optical blur across the lens surface.
To quantify the area on the lens providing clear vision, the study defined a metric called “area of clear vision.” This refers to the region of the lens where the difference between the actual and target prescription was within a tight tolerance of ±0.25 diopters, reflecting the threshold for noticeable blurring. By analyzing this clear vision area, the researchers aimed to compare the optical performance of different PAL designs and understand the impact of astigmatism axis orientation on visual clarity provided by these lenses.
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