Progressive Addition Lenses

Progressive 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 design of the lens, with a final addition power between 0.75 and 3.50 dioptres. The addition value prescribed depends on the level of presbyopia of the patient. In general the older the patient, the higher the addition. They are also known as multifocal lenses, progressive addition lenses (PAL), varifocal lenses, progressive power lenses, graduated prescription lenses, or progressive spectacle lenses.

Advantage 

• Compared to single vision lenses, progressive lenses provide the correction required for a presbyopic patient to see clearly at all viewing distances, typically adjusted by tilting the head slightly — or else by moving the object that is being viewed.

• Progressive addition lenses avoid the discontinuities (image-jumps) sometimes found with bifocal and trifocal lenses

• Some people find them more cosmetically attractive. Because bifocals and related designs are associated with old age, proponents have suggested the lack of visible lines makes a progressive lens appear similar to the single vision lenses worn before the onset of presbyopia

Disadvantages 

Peripheral Distortion: 

• Progressive lenses suffer regions of aberrations and geometric distortions in the periphery, leading to poor vision when turning the eyes down and to the sides. Different designs of progressive lenses have more or less of this distortion.

Fitting: 

• Progressive lenses require careful placement relative to the wearer's pupil centre for a distance-viewing reference position. Incorrect specification of the fitting location can cause problems for the wearer including (depending on the design of the lens) narrow fields of view, clear vision in one eye only, on-axis blur, and the need to alter the natural head position in order to see clearly.

Cost: 

• Progressive lenses are more expensive than bifocal and single-vision lenses due to higher manufacturing and fitting costs. Some research has been conducted to reduce the fabrication cost by precision injection molding.

When recommending a progressive lens design, an eyecare practitioner will usually ask the customer some questions about their lifestyle, which coupled with prescription restrictions or recommendations and cost can establish the suitability of various models of progressive lens. Different lenses have different glazing restrictions, lens material availabilities, maximum and minimum fitting heights, prescription ranges and as such the variation in quality between higher and lower end varifocal lenses is considerable.