Progressive lens

File:20100105 MultifocalGlass.jpg

View through a progressive lens at some distance. In normal use, a much smaller section of the glass is used, so that the distortion is much smaller.

See also: Multifocal Diffractive Lens for Laser Applications

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 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.

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 axis 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 world's first commercially available PAL (Ultrifo) sold by "Gowlland of Montreal". This was based on an arrangement of aspherical surfaces.

Irving Rips at Younger Optics developed the first commercially viable blended lens in 1955 called the Younger Seamless Bifocal.

The Varilux & Carl Zeiss lenses were 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. The breakthrough for the adaptation and the comfort of the progressive lens occurred in 1972 with the market introduction of Varilux 2. Bernard Maitenaz discovered the importance of the design periphery for the peripheral and dynamic vision. So while for Varilux the surface structure was close to the characteristics of the bifocal lens, with an upper aberration-free half of the surface for far vision and a rather large "segment" for clear near vision, Varilux 2 was distinguished by a totally aspheric design.

Early progressive lenses were relatively crude designs. Right and left were identical variable power lenses with distance and reading power centers in the upper and lower part of the lens, respectively. The glazing was made to accommodate that the wearer changes eye position from distance viewing to reading. The point of reading is about 14 mm below and 2 mm to the nasal side in comparison to distance viewing. By tilting the reading power towards the nasal side in perfect symmetry, appropriate reading power was given to the wearer.

The symmetric design, however, was difficult to accept for patients, because the eyes in general work asymmetrically. When you look right, your right eye view distal and left nasal. Modern sophisticated progressive lenses are designed asymmetrically for 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.

The typical progressive lens is produced from a so-called semi-finished lens. The semi-finished lens is molded with an asymmetrical power pattern on the front. On the back side a custom surfacing is made to adjust the power for each patient. This method, however, problematic especially for astigmatic prescriptions. The reason being that the semi-finished front pattern is designed for a spherical prescription. Freeform designs are tailored to each prescription and do not have this problem.

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, thus explaining the difference in price. In short, the price is based on the technology used and the year the lens came to market.

Advantages and use

• The additional lens power required for clear vision at different viewing distances is accounted for by the natural position of the eye during the majority of day to day tasks, and can be further adjusted by tilting the 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, because 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. Because 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' because 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 low-power and high-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

Peripheral Distortion: Progressive lenses suffer the disadvantage of the power progression creating regions of astigmatic aberration away from the optic axis, yielding poor visual resolution. The area of aberration varies in relation to the lens power change gradient. As the lenses combine a range of powers in a single surface there are also geometric distortions to the visual field. Aberration and distortion increase together with the gradient of power change. In order to mitigate, either higher lenses with slower changes or lower addition powers can be used. Sometimes the gradient is not constant, with abrupt changes sacrificing the mid-power "corridor". An alternative solution is to leave the far correction out ("office" glasses) leading to a less abrupt change in power (gradient).

Some wearers find the visual discomfort caused by these distortions outweigh the benefits of wearing PALs; this is known as progressive non-tolerance. However, 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 and the distortions are lower. 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. On the downside, using the minimum possible correction requires frequent and costly lens changes as presbyopia advances.

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 generally dispensed at a higher price than bifocal and single-vision reading spectacles due to the increased manufacture and professional service costs. Some researches have been conducted recently to reduce the fabrication cost by precision injection molding.

When selecting a progressive lens design, an eyecare practitioner will usually ask some lifestyle questions, which coupled with prescription restrictions or recommendations and cost can effectively establish suitability for 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.

Miscellaneous

• For those new to progressive lenses, an accommodation period is often required because the brain needs to learn to adapt to them. This period varies from a few hours for some individuals up to around two weeks. During this period, side effects can include headache and dizziness. It is advised that, when these symptoms set in, the progressive lenses be removed for a short period and replaced after symptoms have subsided. Returning to an older prescription or different type of lens design (bifocal, trifocal) only serves to increase the adaptation period to the progressive lenses.
• Depth perception and distance estimation can be influenced during the adaptation period.