A theoretical and experimental investigation of design and slow tool servo machining of freeform progressive addition lenses (PALs) for optometric applications

摘要

Different from the traditional single or bifocal lens, freeform progressive addition lens (PAL) comprises of a far view zone, a near zone and a progressive zone between the far and near zones. Freeform PAL provides a good solution to correct presbyopia and other disorders of accommodation, which is more widely adopted in the present optometric applications. However, there is still a lack of a standard procedure for the design of a PAL. Besides, the traditional manufacturing of freeform PAL is usually done by the approaches of grinding and polishing one by one, which is not only expensive but also time-consuming. This paper presents the research of a new approach for design and manufacturing technology for freeform PAL by ultra-precision machining approach. Extended polynomial is employed for the representation of freeform surface, which is used for the optimization of optical performance by some commercial optical software such as Zemax. Ultra-precision machining technology based on single-point diamond turning with slow slide servo (3S) machining is used to produce the mould inserts for precision moulding of the freeform PAL. Machining mechanics of 3S is discussed. A freeform PAL with an aspherical surface as front side and a freeform surface as back side has been designed. The precision mould insert of the designed freeform PAL was produced by ultra-precision machining process, and a plastic freeform PAL made of polycarbonate was also produced by precision moulding. Preliminary results obtained show that the new approach for design and fabrication is helpful to advance the design procedure optimization and mass production of PAL in optometry.