Progressive lenses

Progressive lenses, also called progressive addition lenses (PAL), progressive power lenses, graduated lenses and varifocal 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 wearers correction for the other refraction errors. The gradient starts at a minimum, or no addition power, at the top of the lens and reaches a maximum addition power, magnification, at the bottom of the lens. The length of the progressive power gradient on the lens surface is usually between 15 and 20 mm with a final addition power between 1.00 to 2.50 Dioptres for most wearers. The addition value prescribed depends on the level of presbyopia of the patient and is closely realted to of age.

History
The first patent for a PAL was British Patent 15,735, granted to Own 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 next commercial product appears in Italy during the 1950s. Offcine Galileo di Milano developed a PAL they called the "Varifocal" lens, based on a variation on Aves design, the surface has been called an 'elephant trunk', in that it resembles a section from a conical shape with the apex pointed downward and bent backwards. This creates the same optical effect as Aves lens, however requires only one surface (front) to do it.

The first PAL of modern design (the Varilux lens) was developed by Bernard Maitenaz and introduced by the Societe des Lunetiers (Essel) in 1959. Early progressive lenses were relatively crude designs but modern sophisticated progressive lenses have gained much greater patient acceptance and include special designs to cater to many separate types of wearer application, for example lenses may be customized for use with computers, or to offer enlarged near and intermediate view areas. Over the 1980s through to today manufacturers have been able to minimize unwanted aberrations by:

Today the complex surfaces of a progressive lens can be cut and polished on computer controlled machines referred to as 'freeform surfacing', to distinguish this process from the previous technology that required the progressive surfaces of these lenses be cast from moulds.
 * 1) Improvements in mathematical modeling of surfaces allowing greater design control
 * 2) Extensive wearer trialling where lenses are improved by successive approximation to an ideal design based on wearer feedback
 * 3) Improved manufacturing and lens metrology technology.

Advantages and Use
Bearing in mind the lens provides a power that is best suited for distance viewing at the top and near viewing down the bottom of the lens, the wearer can then adjust the additional lens power required for clear vision at different viewing distances by tilting his or her head to sight through the appropriate part of the vertical progression. Further, since near vision tasks such as reading are usually low in the visual field and distant objects higher in the visual field, the lens location of the correct addition power for the viewing distance usually only requires small adjustments to head position. A correctly prescribed and fitted progressive lens can restore vision for the wearer to the level of flexibility that was experienced prior to the clinical onset of presbyopia.

Progressive addition lenses avoid the discontinuities (image-jumps) in the visual field created by bifocal and trifocal lenses and are also more cosmetically attractive to those viewing the wearer. 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 lenses associated with younger wearers [single vision] lenses tend to be free of segments or lines on the surface.

Disadvantages
Distortion: Progressive lenses suffer the disadvantage that the power progression creates regions of aberration away from the optic axis, yielding poor visual resolution (blur). As the lenses combine a range of powers in a single surface there are also geometric distortions to the visual field, some wearers find the visual discomfort caused by these distortions outweigh the benefits of wearing PALs, however acceptance rates are claimed by manufacturers to be in the 90% to 98% range. The level of these aberrations is associated with the addition power, that is the higher the addition the higher the aberrations. Clinicians generally agree that in order to avoid adaptation problems it is best to start wearing progressive lenses as early in the development of presbyopia (around 45 years of age for most people), while the prescribed addition powers are low. The wearer can then adapt to the increases in a series of steps in addition power over a number of years as their presbyopia progresses.

Fitting: Progressive lenses require careful placement relative to the wearers pupil centre for a distance viewing reference position. Incorrect specification of the fitting location can cause problems for the wearer including, narrow fields of view, clear vision in one eye only, on-axis blur and the need to adopt uncomfortable head positions.

Cost: Progressive lenses represent the most advanced form of spectacle based presbyopia correction and so are sold at a significantly increased price premium when compared to other forms of presbyopic correction including bifocal and single-vision reading spectacles.

Other notes

 * For those new to progressive lenses, an accommodation period is required - basically, the brain needs to learn to work with them. This period varies from a few days for some individuals to a couple of weeks. During this period, side effects can include headache and dizziness. It is advised that, when these symptoms set in, the progressives are replaced by normal glasses for the rest of the day.
 * Depth perception and distance estimation can be influenced during the accommodation period.
 * Progressive lenses can produce distortion - lines and surfaces do not appear as straight, this is most likely to appear in the lower half of the lens where the near prescription is blended in. It is noteworthy that all lenses (single vision included) with power can cause the same effect in their periphery, causing straight lines to appear bent.

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