Progressive Lenses

摘要

Progressive lenses, or to use the normally accepted terminology, progressive addition lenses or PAL's, have now superseded bifocals and trifocals as the lens of first choice for presbyopes. This is evident from the lens designs listed by the various manufacturers for the major economic countries, although, due to cost considerations, it may not be correct for the less well-developed economies. There is now a bewildering number of designs and types now available to the Rx house and dispenser - in virtually all lens materials - apart from 1.9 index glass. The reasons for this rise in popularity are not too hard to fathom. They have many advantages over conventional 'multifocals', although they do have disadvantages too. The design of a progressive lens differs from that of the multifocal, which has a set number of power areas (normally two or three) where the focal power is fixed. These fixed powers are normally supplied for distance and near, or reading use (bifocals), or in the case of a trifocal as three power areas - distance, intermediate and near. In a progressive design, there is no division between the different power areas of the lens, which blend into each other seamlessly.