Computerized Moiré analysis of progressive addition lenses

A computerized image capture and analysis system capable of 256-level intensity resolution was used to digitize and store images as a 512 by 512 pixel array. The system was used to record a standard square-wave grating (0.80 lines/mm) located 1 m from the camera. Subsequently, an ophthalmic lens was interposed between the camera and the grating. Nonzero lens powers induced magnification and thus altered the spatial frequency of the grating image. Subtractive-type Moiré analysis was performed to compare the intensity distributions of the magnified and the unmagnified (reference) gratings. The Moiré patterns created using digital, pixel-by-pixel subtraction of the two images showed graphically the power distributions of the lenses studied. These patterns varied noticeably between different brands of progressive-addition lenses (PAL's) of the same nominal power. Using only commercially available components, this method of analysis was both fast and repeatable, and may be of value to manufacturers and large-volume finishers of progressive-addition lenses.