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.

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 3.00 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 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 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 (that 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 lenses may be customized for use with computers, 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 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.

Disadvantages

Distortion: Progressive lenses suffer the disadvantage of the power progression creating regions of aberration away from the optic axis, yielding poor visual resolution (blur), which varies in relation to the quality of the lens. As the lenses combine a range of powers in a single surface there are also geometric distortions to the visual field, which increase with the addition power. Some wearers find the visual discomfort caused by these distortions outweigh the benefits of wearing PALs. 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 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.

Peripheral Vision Loss: Because of the narrow vertical range of undistorted vision, there is an inherent loss of peripheral vision with progressive lenses, as with all prescription spectacle lenses. This loss of peripheral vision and the resulting side vision distortion may result in the wearer not being able to see more than 25% of a computer display without having to move the wearer's head from side to side to clearly see and comprehend the monitor's content, however for wearers who use computers regularly and for prolongued periods of time, many optical practitioners recommend a progressive lens tailored specifically to the intermediate and near prescription for computer and reading use.

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 narrow fields of view, clear vision in one eye only, on-axis blur, and the need to adopt uncomfortable head positions.

Availability: Progressive lenses use highly complex proprietary curves in combination with the patient's basic bifocal prescription and measurements. All lens-curve types—there are hundreds—are not available from all opticians. A patient may be able to get what is prescribed from few local opticians, or only from the prescribing optometrist, however most optical establishments offer a variety of multifocal lens qualities.

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.

Because of the relatively narrow vertical band of prescriptively accurate optics characteristic of progressive lenses, wearers may have some difficulty becoming used to the coordination of horizontal head movement necessary to retain focus when reading across a computer screen or a paper page. This is particularly true when the reading add power is strong (+2.00 or above). The experience has been described as "moving your head back and forth as if you are watching a tennis match". This can be mitigated by using high quality lenses, and by having your glasses made and fitted by a competent optician who is experienced in fitting progressive lenses. It is also beneficial to avoid the shallow eyeglass frames (i.e. the frames with very small vertical dimension) and choose a frame that is somewhat larger instead.

There is no such thing as a perfect progressive lens. Selection involves multiple tradeoffs among near, mid, and far vision quality and in factors which empirically affect visual ease-of-use.

Miscellanea

• For those new to progressive lenses, sometimes an accommodation period is required because the brain needs to learn to work with them. This period varies from a few hours 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 lenses are 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.
• Progressive lenses are good enough for most focal lengths, but not ideal for computer users. Regular progressive lenses have a small area in the lens for computer distance. Also you need to tilt your head back to bring the computer in to focus. This can lead to neck and back pain as well as eyestrain.

References

Optometry Glossary

pl:Soczewka progresywna