| Progressive spectacle lenses, also called progressive addition lenses (PAL),
progressive power lenses, graduated prescription lenses, and varifocal or
multifocal lenses, are corrective lenses used in eyeglasses to correct
presbyopia and other disorders of accommodation. They are characterised by a
gradient of increasing lens power, added to the wearer's correction for the
other refractive errors. The gradient starts at the wearer's distance
prescription, at the top of the lens and reaches a maximum addition power, or
the full reading addition, at the bottom of the lens. The length of the
progressive power gradient on the lens surface depends on the refractive index
of the lens, with a final addition power between 0.75 to 3.50 dioptres for most
wearers. The addition value prescribed depends on the level of presbyopia of the
patient and is closely related to age and to a lesser extent, existing
prescription.
History
The first patent for a PAL was British Patent 15,735, granted to Owen Aves with a 1907 priority date. Aves' patent included the progressive lens design and the manufacturing process. However this was unlike modern PALs. It consisted of a conical back surface and a cylindrical front with opposing axes in order to
create a power progression. This design was never commercialized.
While there were several intermediate steps (H. Newbold appears to have designed a similar lens to Aves around 1913), there is evidence to suggest that Duke Elder in 1922 developed the worlds first commercially available PAL (Ultrifo) sold by "Gowlland of Montreal". This was based on an arrangement of aspherical surfaces.
The Varilux lens was the first PAL of modern design. It was developed by Bernard Maitenaz, patented in 1953, and introduced by the Société des Lunetiers (which later became part of Essilor) in 1959.
Early progressive lenses were relatively crude designs but modern sophisticated progressive lenses have gained greater patient acceptance and include special designs to cater to many separate types of wearer application: for example progressive addition lenses may be designed with distance to intermediate or
intermediate to near prescriptions specifically for use as an occupational lens, or to offer enlarged near and intermediate view areas. Since the 1980s, manufacturers have been able to minimize unwanted aberrations by:
Improvements in mathematical modeling of surfaces, allowing greater design control
Extensive wearer trials
Improved manufacturing and lens metrologic technology
Today the complex surfaces of a progressive lens can be cut and polished on computer-controlled machines, allowing 'freeform surfacing', as opposed to the earlier casting process.
Advantages and use
Wearers can adjust the additional lens power required for clear vision at different viewing distances by tilting their head to sight through the appropriate part of the vertical progression;
The lens location of the correct addition power for the viewing distance usually only requires small adjustments to head position, since near vision tasks such as reading are usually low in the visual field and distant objects higher in the visual field.
Progressive addition lenses avoid the discontinuities (image-jumps) in the visual field created by the majority of bifocal and trifocal lenses and are more cosmetically attractive. Since bifocal and related designs are associated with 'old age', proponents have suggested the lack of segments on the lens surface of
a progressive lens appears more 'youthful' since the single vision lenses associated with younger wearers are free of segments or lines on the surface.
Progressive lenses can use the optimization principle, which is based on linearization and represents the lens surface using special spline functions. The lens’ power progressively increases between the high-power and low-power region of the lens. This specialized lens surface provides an accurate power
distribution for both near and distant vision and reduces the number of optical aberrations. |
Article Details
