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Introducing PAL

Progressive addition lenses, or multifocal lenses, are prescription lenses used to correct presbyopia instead of bifocals and reading glasses.

They are characterized 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 and 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.

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 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 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.

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.

Disadvantages:

Initially progressive lenses suffered from the disadvantage of the power progression creating regions of aberration away from the optical axis and yielding poor visual resolution. This aberration varies in relation to the quality of the lens.

However, with the latest generation of progressive lenses manufacturers claim acceptance rates of 90% - 98%.

Clinicians generally agree that in order to avoid adaptation problems it is best to start wearing progressive lenses early in the development of presbyopia (around 40 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.