Achievements of modern technology

Freeform technology brought the possibility, not only of optimizing the performance for any individual prescription, but also of varying the design characteristics of the lens by incorporating more fitting data relating to the facial characteristics of the wearer. That is, the lens could be optimized and personalized.

What is the difference between optimization and personalization?

An optimized lens is one which has been designed to eliminate, or at least, minimize certain stated defects in its image forming properties. The most significant aberration for the spectacle lens is oblique astigmatism and if we were to make a plus lens in plano-convex form, the increase in astigmatism would be quite rapid, reaching over 2.00 D in the case of a +4.00 D lens made in plano-convex form.

By the beginning of the 20^th century, it was realized that by bending the lens into a curved form, it would be possible to eliminate oblique astigmatism and this became the standard form for most best form spectacle lens series. 

In the case of progressive power lenses, the inevitable Minkwitz astigmatism arises and the best that the designer can do is to ensure that all prescriptions perform close to the design specification for each individual power.

What is required, is for the lens to perform in the way which the designer intended and as the Kelch Patent of 1995 and the Hof Patent of the year 2000 described, this can be achieved using freeform technology (Fig. 1).

Personalized lenses

A personalized lens is one which, whether single vision or progressive in power, can be designed in real time for an individual wearer, by inputting the actual wearing conditions of the lenses. In addition to the monocular centration distances and fitting point heights, which have always been required in progressive lens fitting, the actual distance of the eye’s center of rotation, the vertex distance, the pantoscopic and face form angles, the individual’s working distance and details of the lens form can be incorporated in the lens before manufacture.

The mechanical details of the lens and the frame can also be personalized for the individual wearer, by inputting the dimensions of the lenses. Typically, this includes the A and B dimensions of the lens, the effective diameter required for the shape, and if the lens is measured by a frame tracer, the data can be linked to the lens design software to obtain the optimum surfaced-to-shape lens of minimum thickness and weight (Fig. 2). Clearly, the flagship designs from the major lens manufacturers can be described as personalized, optimized designs.

Prioritizing zones

Many of the latest developments in progressive power lens design relate to prioritizing the zones of the lens which are used most by different wearers, all made possible by the use of freeform technology. 

Freeform production enables the design to prioritize the zone of the lens which the wearer uses most frequently, by ensuring that the Minkwitz astigmatism has been minimized for the zone that the wearer uses most, that is, the distance zone, the intermediate zone or near zone of the lens.

The patent assigned to Rodenstock^[1], suggests some typical uses for different wearer requirements (Fig. 3). Eventually, three different designs were marketed, under the names Active, which has an extra large distance portion, Allround, which gives equal priority to distance, intermediate and near, and Expert, which offers a wide intermediate zone and subsequently a wider near vision zone. Complete personalization and choice of design type is offered in their flagship design, Impression Freesign 3 Individual.

Hoyalux iD Mystyle introduced a choice of the basic surface design depending upon the wearing characteristics of the subject. By means of the MyStyle iDentifier, the design profile and most effective corridor length, 11mm, 14mm or 16mm is selected for the subject depending upon the answers to a series of questions prompted by the iDentifier. The three basic designs range from the hardest design known as Clear which offers an extra wide deformation-free distance part with a clear and free field of vision; a balanced design known as Balance, whose structure offers a more dynamic lifestyle, in which focused vision and stability at all angles and in all situations is required through to the softest design, known as Open, whose design is soft so that, instead of placing high demands on specific vision areas, the focus is on optical balance, a soft progression structure and a wide reading area (Fig. 4). Each design is made with three different corridor lengths, 11mm, 14mm and 16mm.

Carl Zeiss Vision use the term IndividualFit Technology to describe their designs personalized for lifestyle prioritization for their lenses intended mainly for distance vision use, or near vision use, or balanced for distance and near. Those prioritized for distance vision are often referred to as designed specifically for driving – so-called driver lenses – which usually incorporate a coating that eliminates reflections, Zeiss DriveSafe Progressive lenses claim to offer an up to 14% larger distance-viewing zone and up to 43% larger intermediate zone.

Essilor’s progressive lens designs in the 21^st century, have incorporated the best features of their previous designs. Beginning with Varilux Ipseo whose design took into account the wearer’s Head/eye behaviour, then Varilux Physio which considered control of higher-order aberrations, the Varilux S-Series which incorporated the progression on both front and back surfaces, and introduced control of differential prismatic effect, and developments in freeform surfacing technology, the Varilux X-Series with its Extend technology by redesigning the power law to enable an increase in the volume of vision provided to the wearer culminating in today’s flagship design, Varilux XR, providing an even greater field of useful vision (Fig. 5). Each generation of progressive lens design incorporating the best features of earlier versions of the lens.

The double-progressive surface Varilux S-Series introduced control of differential prismatic effect using a technique they described as Synchroneyes.

If the prescription for the R and L eyes is the same the vertical prismatic effect at corresponding points used in binocular vision will be the same.

However, if the prescriptions differ, as would occur in anisometropia, binocular balance would be lost. The process described as “Synchroneyes” restores binocular balance at corresponding points on the pair of lenses a feature which could not be obtained with designs whose progressive surfaces were fixed as with earlier progressive lenses.

Seven years ago, Essilor introduced their X-Series design whose modified power law was designed to extend the range of intermediate and near vision. Xtend™ technology involved a redesign of the power law between the intermediate and near vision zones as shown in figure 5, which compares the shape of the power law of previous generations of Varilux designs with the power law adopted in the X and XR series. This, apparently minor change, extends the range of vision provided to the wearer by some 16% and hence, the volume of vision obtained by the wearer.

The latest progressive lens from Essilor, the Varilux XR-Series, is described as the first eye-responsive progressive lens. It incorporates all the design features of the Varilux X-Series, the design being assisted by artificial intelligence in the form of an “avatar” whose characteristics, duplicate those of a real-life wearer, to reproduce the wearing conditions of the individual. 

The latest generations of flagship progressive power lenses incorporate all the technologies which experience, and clever freeform design can bring. Provided that the lens fitting has been properly undertaken, patient rejection of progressive lenses is now a thing of the past. Taking full advantage of personalization of the design offers spectacle wearers bespoke lenses, which they can truly believe have been designed specifically for them.

Degressive power lenses

In 1996, Sola Optical introduced a design intended for use only for intermediate and near vision, a concept which had become known in America as an occupational progressive lens. Such designs are ideal for use at the desk and for use with the computer. The Access Reading Lens enabled clear vision from the near point out to at least arm’s length, and beyond, under certain circumstances. The lower part of the lens is made to the prescribed power for near vision and the power in the upper portion is either 0.75 D or 1.25 D weaker for intermediate use. There is no other choice in the power reduction which has come to be called the degression power, and progressive lenses of this type, of which several have now been introduced, are known as degressive lenses. Sola recommended that the major reference point, which lies at the geometrical center of the uncut lens, should be fitted 3mm to 5mm below the pupil center (Fig. 6).

It was to offer extended ranges of vision for intermediate and near vision that degressive lenses were introduced. The Minkwitz condition shows that the width of clear vision in the progression zone of a lens is dependent upon the power of the near addition and the length of the progressive corridor. The lower the near addition and the longer the progression zone, the wider becomes the aberration-free corridor between the upper and lower portions of the lens. 

The Minkwitz condition reminds us that the width of clear vision in the progression zone of a lens is dependent upon the power of the near addition and the length of the progressive corridor. The lower the near addition and the longer the progression zone, the wider becomes the aberration-free corridor between the upper and lower portions of the lens.

Modus operandi

The modus operandi of a degressive power (or occupational progressive) lens is as follows. Suppose an office worker who is a presbyope (assumed here to be emmetropic) needs a near addition of +2.00 D. If this is worn in the form of a pair of +2.00 D single vision lenses, the artificial far point would lie 50cm in front of the lenses and it is probable that, not only most of the subject’s desk top, but also the screen of their computer monitor would lie in the blurred region beyond the artificial far point. Certainly, the subject would not have a clear view of a colleague sitting three meters away, nor of the clock on the wall at the opposite end of the office.

One solution for office wear, which would provide a full range of clear vision, is a progressive power lens made to the prescription 0.00 Add +2.00. Typically, this lens would have a progression length of 18mm and would provide a range of clear vision from infinity down to the subject’s near point.

The Minkwitz condition implies that such a design would provide a corridor width of some 4.5mm, before unwanted astigmatism on either side of the meridian line attained a value of 0.50 D as shown in figure 7(a).

This quantification is given only to emphasize the significance of the power of the reading addition and the progression length on the optical performance of the corridor. It is well known that, in practice, most progressive lens wearers are unaware of the effects of the aberrations near the center of their intermediate zones. For most people, the visual system seems to adapt very readily to the off-axis effects of modern progressive lenses, especially when the addition is low.

Note that the progressive lens depicted in figure 7(a) is of a so-called “hard” design with no attempt to redistribute astigmatism in the distance portion of the lens. This progressive lens would provide a range of vision from infinity through intermediate zones down to the near point at about 25cm from the lens.

A second solution for use in the office would be to use a degressive power lens whose specification is +2.00 Add -1.00. (Note that this is equivalent to the specification, +1.00, Add +1.00 D). Such a lens would have a corridor length of about 28mm and since its addition is only 1.00 D the corridor width would be 14mm before unwanted astigmatism on either side of the meridian line attained a value of 0.50 D (Fig. 7(b)).

If the design was a “soft” design where the astigmatism is redistributed in the distance area the wearer would obtain a wide central field with very low aberration which would make the lens very easy to wear. The artificial far point with this design would lie at 100cm and the subject would obtain a range of clear vision extending from 100cm down to 25cm from the lens. This range would adequately cover the work area and provide a more useful middle-distance field than the original +2.00 D reading lenses.

There are several occupational progressive designs available today, the better known are given in the accompanying table. Most suppliers suggest that these lenses are ordered by stating the required near power together with the degression power, which is minus the near addition which the lens provides.

Needless to say, since there is no distance area for degressive lens designs, they are totally unsuitable for driving.

References:
1. US Patent 6685316 (2004) Baumbach P. et al, Method of Manufacturing Progressive Ophthalmic Lenses.