Optician Online - CPD Archive

In the third of three articles discussing lens selection, Andrew Keirl continues his overview of the selection of spectacle lens types and designs available for the ‘normal’ power range with a discussion of progressive power lenses, occupational progressive power lenses and enhanced reading lenses. Module C35379, suitable for optometrists and dispensing opticians

Progressive power lenses (PPLs) provide correction at all distances including intermediate distances but for some patients they are not always the lens of choice. PPLs solve the optical and cosmetic disadvantages of bifocal lenses but there are, of course, many bifocal wearers who have tried PPLs but are unable to adapt to the lens or they preferred the larger reading area offered by a bifocal. Tunnacliffe1 gives a helpful summary of wearer PPL selection and suggests that presbyopic patients who are likely to be successful wearers of PPLs include:

Current wearers who are satisfied with a previous pair of PPLs

Young presbyopes who require their first reading addition

Subjects who require an intermediate focus in addition to distance and near corrections

Subjects who do not want dividing lines on their lenses

Subjects who would benefit from the no-jump performance of a PPL

First-time potential PPL wearers who are prepared to gamble on the chance that they will adapt to PPLs.

Patients who are unlikely to succeed with PPLs include:

Previously failed PPL wearers

Patients who require a wide near area and cannot or are not prepared to turn their head for more lateral viewing

Patients who are happy with single vision, bifocal or trifocal lenses

Patients who are not disturbed by or conscious of the segment top in bifocals or trifocals

Patients who are not prepared to gamble on the chance of adapting to PPLs or who are not prepared to wait an indeterminate time for adaptation to occur.

The advantages and disadvantages of PPLs are summarised below:

Advantages of PPLs

Convenience

No visible segment top, giving the appearance of a single-vision lens

Improved cosmesis compared with a bifocal or a trifocal lens

No image jump

Distance, intermediate and near foci in one lens.

Disadvantages of PPLs

Areas of aberrational surface astigmatism resulting in blur

Apparent rocking or moving of objects

Unpredictable period of adaptation.

Narrow near field of view compared with bifocals

Narrow intermediate field of view compared with bifocals.

In the author’s opinion there are five key points that contribute most to wearer satisfaction. These are:

Ease of switching between the distance and near areas

Ease of finding the near area

Comfortable near vision

Clarity of image and absence of peripheral blur

Ease of adaptation.

A simplistic description of a traditional PPL

In very simple terms, a traditional PPL can be thought of as having two spherical surfaces, one for distance and one for near, connected by an aspherical surface. The change in power within the progression corridor is achieved by continuously altering the radii of curvature between the distance and near zones and is achieved by the use of the aspherical surface.

Traditionally, a progressive front surface is obtained by moulding or ‘casting’, which provides the reading addition, resulting in a semi-finished lens. The back surface of this semi-finished lens is worked to give the required distance prescription which of course has no effect on the reading addition. The process of changing the surface power creates unwanted aberrational surface astigmatism either side of the progression corridor, resulting in image blur and restricting the usable width of the intermediate and near portions of the lens to a central ‘corridor’ (Figure 1).

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The second consequence of a progressive surface is that of skew distortion (Figure 2) which occurs because a change in power is accompanied by a change in magnification. Skew distortion is responsible for the ‘swimming effect’ reported by new wearers of PPLs. It should be noted that traditional or ‘conventional’ PPLs with a front progressive surface are appearing less and less in manufacturers’ catalogues.

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Isocylinder lines and mean power plots (contour plots) are methods of describing and comparing the optical performance of PPLs. As mentioned above, a progressive surface displays surface aberrational astigmatism and isocylinder lines are lines joining points where the surface aberrational astigmatism changes by 0.25 or 0.50D. The more lines the greater the variation in surface aberrational astigmatism.

Depending on the distribution and amount of surface aberrational astigmatism, PPLs have been described as either hard or soft in design. Some manufacturers have also used the terms firm and ultra-soft to describe their lenses! In hard designs the surface aberrational astigmatism is limited to the nasal and temporal areas of the lens. Hard designs usually have a narrow progression corridor, a relatively wide reading area and a full-width distance portion (Figure 3).

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The closely spaced isocylinder lines of hard designs indicate a rapid rate of change in surface astigmatism. In soft designs the surface astigmatism extends into the distance portion, thus allowing a reduction in its amount and producing a wider progression corridor. However some astigmatic blur is introduced into the distance portion. Soft designs have a narrower reading area compared to hard designs and are said to be easier to adapt to than hard designs (Figure 4).

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The terms ‘hard’ and ‘soft’ are not really ‘scientific’ but are frequently used to differentiate between lens designs and to explain lens performance to patients. It is important to realise that these terms are relative and cause endless confusion! In addition, most PPL manufacturers use a lens power of plano Add +2.00D to construct their contour plots which of course provides a very limited illustration of lens performance as the distance prescription, reading addition, lens design, progression length, pantoscopic tilt, vertex distance and face-form angle will all affect the distribution of the isocylinder lines in a contour plot. The take home message here is use contour plots with care!

‘Hard’ designs

? Provide a relatively wide distance area

? Are good for plus prescriptions

? Are good for longer vertex distances and steep fitting frames

? Are better for eye movers.

‘Soft’ designs

? Provide a relatively wide intermediate area

? Are good for minus prescriptions

? Are good for shorter vertex distances and flat-fitting frames

? Are better for head turners.

PPL markings

To assist practitioners in the fitting, verification and identification of a PPL, a universal marking system is used for PPLs (Figure 5). With reference to Figure 5, A is the distance power checking point, B is the near power checking point; C is the nasal reference point, D the temporal reference point, E the fitting cross and F the prism checking point. The nasal and temporal reference points are often two engraved circles (or other symbols) 34mm apart. The reading addition is engraved under the temporal reference point and the manufacturer’s identifying mark giving information about the specific lens and material is engraved under the nasal reference point.

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Prism thinning

A process known as prism thinning is applied to most PPLs in order to improve cosmesis, particularly the lens thickness at the upper edge of the lens. Approximately 0.6? base-up prism is removed for each dioptre of reading addition, effectively adding equal base-down prism in both lenses equivalent to 2/3 of the reading addition. This thinning prism should always be taken into account when neutralising a progressive lens at the prism checking point. Prism should always be measured at the prism reference point.

The effect of increasing the reading addition

As the reading addition increases, the progression corridor lengthens and the width of the reading area narrows. This needs to be taken into account when dispensing second and subsequent pairs of PPLs where the reading addition will probably have increased. Unless forewarned about, this may give rise to complaint.

Short and long corridor PPLs

Most PPLs are available in at least two corridor lengths usually described as long and short. Historically, short corridor PPLs were introduced to accommodate the trend for shallower frames. However there are other important considerations when choosing a corridor length as the corridor length of a PPL significantly influences optical performance and ultimately wearer satisfaction.

If the corridor is too long for a given frame, reading comfort will be reduced. Reducing the corridor length means that the optics of the PPL design must be ‘compressed’ or ‘squashed’. This means that the rate of change in unwanted surface aberrational astigmatism must increase. This may result in reduced intermediate effectiveness, narrower central viewing zone and increased peripheral astigmatism. In summary:

Short corridor PPLs:

Have smaller intermediate areas

Have smaller distance areas

Are good for moderate to high myopes as less ocular rotation is required

Are good for short vertex distances, an increase in pantoscopic tilt and patients with a head tilt

Are good in cases of anisometropia.

Long corridor PPLs:

Are preferred by hypermetropes

Are good for long vertex distances

Are good for flatter fitting frames

Provide wider intermediate areas

Mean less head tilt when reading.

It is important to note that short progression lengths and high reading additions are not a good combination!

Fitting tips

When ordering a PPL the vertical and horizontal positions of the fitting cross must be specified

Place fitting cross on pupil centre not corneal reflex

Always follow guidelines for minimum fitting height

At least 10 mm is usually required above the fitting cross

Frame size and shape can influence the usable distance and near area of a PPL

Use as short a vertex distance as possible

Always fit the frame before taking measurements.

Remember although the fitting height is taken from the pupil centre to the lower rim, don’t forget that the reading area is inset. Make sure that the frame shape in the nasal area allows for this.

Freeform technology

In recent years, freeform has become one of the buzzwords in optometry and dispensing and freeform lens surface generators enable prescription laboratories to cut complex surfaces on an individual basis. However, it is in the author’s opinion, a poorly understood and often misused term.

The term freeform really applies to a surface of a lens as opposed to the lens itself and basically describes a surface that cannot be mathematically defined. Instead the surface is described by its sag measurements at thousands of points using x, y and z co-ordinates. The main advantage of freeform technology is that it gives the lens manufacturer the ability to compensate for the aberrations produced by a lens when working the concave surface of a semi-finished blank. In addition freeform processing enables the fitting characteristics of the wearer to be incorporated into the lens design.

The advantages of freeform technology are:

Better control of aberrations

Personalised lenses as compensation can be provided for the actual fit of the lenses in front of the eyes for example, the appropriate inward decentration, and for the effects of vertex distance (Figure 6), pantoscopic tilt (Figure 7) and face form angle (Figure 8)

Lenses that are custom designed to the wearer’s needs.

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With reference to PPLs, freeform or digital surfacing is currently used in three ways. A conventional front surface PPL lens stocked in semi-finished form will be supplied in a limited number of base curves from which any finished power in the design range can be produced using the appropriate second surface. However, this means that only one power per base curve will have the ideal design, all the others being compromises.

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Freeform surfacing can however be applied to the concave surface of the lens which means the optimum form for each distance prescription can be manufactured, providing the patient with improved off-axis vision in the distance zone of the lens. The software effectively ‘cleans up’ the off-axis image degrading aberrations. Digital freeform surfacing of the back surface of a conventional PPL does not affect the reading addition which of course is provided by the front surface.

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The Hoyalux Trueform series of lenses and the Norville Simage lens are examples of this technique. Secondly, freeform surfacing can be used to produce a surface that incorporates both toroidal and progressive elements on the concave surface or even two different progressive designs, their cylinders and additions all on the same surface (Autor-Pilotor from Norville). A third freeform PPL approach is to split the reading addition between the convex and concave surfaces of the lens.

An example of this methodology is the Hoyalux iD series of lens. In this design, Hoya has incorporated a progressive surface on the convex side that gives rise to a vertical change in power along with a progressive surface on the concave side that gives rise to a horizontal change in power. This approach as represented by the first generation Hoyalux iD influenced the magnification produced by the lens in the peripheral visual field thereby reducing the “swimming effect” reported particularly by new wearers of PPLs.

The second generation Hoyalux iD was the Mystyle, which incorporated personalisation into the lens design. The third generation is the Hoyalux iD Mystyle V+ which, along with extensive personalisation (there are about 65 different variations available), balances the design performance between the right and left lenses to improve binocular performance.

Freeform technology has also allowed lens manufacturers to provide PPLs with multiple corridor lengths as opposed to the traditional ‘long’ and ‘short’. This provides the flexibility to move the reading area up or down to suit the frame selected or to fulfil the patient’s visual requirements. For example, a traditional short corridor PPL selected to give maximum reading area may reduce the width of the intermediate corridor causing problems at the computer.

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Norville offer PPLs with seven different corridor lengths and the Hoyalux iD Mystyle V+ is available in six corridor lengths. Many practices now use online ordering programmes which display an illustration of the lens selected. These are useful to check the positioning of the reading area within the frame before sending the order (Figures 9 and 10). Such devices are also very useful in demonstrating the impact of varying corridor length to both the patient and practitioner.

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Working distances

Consideration of the patient’s required working distances is probably the most important task in dispensing for the presbyope. Before the onset of presbyopia, the eye exerts an ever-increasing amount of accommodation to view objects that lie closer and closer to the eye. On reaching presbyopia, a near vision correction is required to assist the eye in focusing upon near objects.

A single-vision reading correction will render the patient artificially myopic, as distance vision will be blurred. This in turn reduces the range of clear vision (ROCV) and as the reading addition increases, this reduction becomes more noticeable. The ROCV is the distance from the patient’s artificial far point to his/her artificial near point. The artificial far point tells us how far a patient can see with his/her spectacles (unaccommodated eye) while the artificial near point tells us how close a patient can see with his/her spectacles (fully accommodated eye).

For a reading correction, the position of the artificial far point (Art FP) depends on the near addition and is given by the simple expression:

When using the above expression the position of the artificial far point is given in centimetres. For example, if the Add is +1.00D, Art FP = 100 cm. If the Add is +2.00D the Art FP = 50 cm and so on. The determination of the position of the artificial near point is rather more complicated and depends on the reading addition, the patient’s amplitude of accommodation and depth of field. Table 3 shows maximum working distances for various additions and ages.

An addition that is too strong for the expected/required working distance is a common problem, particularly in the workplace. In order to improve (increase) the range of clear vision, a weaker addition or a more sophisticated lens is required. Such lenses are known as enhanced reading lenses or occupational progressive power lenses.

There are now many lenses currently on the market suitable for occupational/vocational use. They all have different characteristics, but are designed to provide enhanced intermediate and near vision compared to a standard single vision lens. They are very useful for avoiding and solving working distance problems.

The need for an intermediate correction

It is well known that today’s presbyopic patients face more demands for intermediate vision than before. Many face the challenges of technology at work and even when retired they still need an intermediate correction for things they do every day. Whether looking up information on the web, checking email, playing cards, cooking or simply trying to see items on shelves at a supermarket, the modern presbyope is active and immersed in a complex visual environment.

Although bifocals work well for reading at close distances, they provide no correction for intermediate distances, particularly if the patient is an established presbyope with a higher reading addition. Bifocal wearers are therefore potentially missing an important part of today’s demanding middle distance visual environment. PPL wearers who spend long periods of time at a VDU may also benefit from a lens which provides an increased field of view for near and intermediate distances along with a more comfortable head position and posture.

The early presbyope has sufficient accommodation in reserve to be able to focus intermediate distances through the upper, distance portion of a bifocal lens. However, for most patients over the age of 50, and certainly by the age of 55, the amplitude of accommodation is such that this is not possible and the patient experiences intermediate blur.

Bifocal wearing patients in the 50 to 55 age group typically complain that objects at intermediate distances are too close to view through the distance portion of the lens and too far away to view through the reading portion of the lens. The solution to this problem is of course some form of intermediate correction.

Enhanced reading lenses and occupational PPLs

This group of lenses do not in the main provide useful distance vision and should not be used for driving. However, some provide room distance vision and are suitable for wearing indoors at home or walking around an office. Some manufacturers describe these lenses as dynamic reading lenses which provide comfortable vision over the entire reading range and at intermediate distances. Such lenses are suitable for computer workstations and leisure activities, which involve intermediate vision and can be prescribed:

? To experienced wearers of reading spectacles to improve medium distance range

? To first-time presbyopes

? As a second pair to traditional PPL wearers who require a larger near and intermediate field

? To presbyopes with specific vocational or occupational requirements

? To contact lens wearers as an overcorrection for near vision.

The lower area of enhanced reading lenses display a stable reading power, which gradually decreases towards the top of the lens. This decrease in power or ‘degression’ results in an increased range of clear vision resulting from an extension of the patient’s artificial far point. Examples of such lenses are discussed below. Most of these manufacturers produce lenses in two designs, one for the early presbyope with a low power reduction and one for the older or ‘experienced’ presbyope with a greater power reduction. Low power reduction designs are ideal for first-time wearers of reading spectacles, younger presbyopes and also contact lens wearers. Lenses with a greater power reduction are more suited to experienced wearers of reading spectacles and can be offered as a vocational lens to wearers of ‘general purpose’ PPLs.

Occupational PPLs tend to provide a much wider field of view for intermediate distances when compared to an enhanced reading lens and also provide longer working distances. These are ideal for home and office use and also for certain ‘difficult’ occupations such as plumbers, electricians, decorators, opticians and optometrists. A small selection of the vast array of currently available lenses will now be discussed.

Essilor InterView

This enhanced reading lens has two degressions, 0.80D and 1.30D. The power ordered reduces by 0.80D and 1.30D respectively. The 0.80D degression is suitable for early presbyopes and the 1.30D design for experienced presbyopes. When using InterView practitioners should order the full near prescription. The lens is fitted exactly like a bifocal lens with the mounting reference line positioned level with lower limbus (Figure 9). In addition, monocular near centration distances should be specified.

Sola Access

This enhanced reading lens is available in degressions of -0.75D and -1.25D. The full reading correction should be ordered and the fitting points placed 3mm to 5mm below the pupil centres with the eyes in the primary position. The monocular near centration distances should be specified. In other words, Access is fitted in a similar way to Interview.

Zeiss Officelens

Officelens is a range of freeform vocational lenses that replaced the Clarlet Business and Gradal RD. When designing this series of lenses Zeiss opted for an ‘individual’ approach in determining the wearer’s maximum intermediate distance (MID). The Officelens Plus and Officelens Superb designs are available with two MIDs. Lens type ‘near’ is an enhanced reading lens that provides clear vision from a reading distance to a fixed MID of 200cm. Lens type ‘room’ is an indoor PPL which provides clear vision from a reading distance to a fixed MID of 400cm. Officelens Individual has a personalised MID which can be customised to the wearer’s needs in a range from the individual reading distance up to 400cm.

All Officelenses are fitted like a traditional PPL. Officelens Plus has a minimum fitting height of 18mm and Officelens Plus Short has a minimum fitting height of 14mm. As these lenses are primarily for reading, it is recommended that the short version is used whenever possible. Officelens Superb and Officelens Individual have variable fitting heights from 14mm to 20mm. Officelens Individual can be customised to take into account vertex distance, pantoscopic angle, face form angle and working distances. All Zeiss Officelenses are fitted like a traditional PPL.

Hoya Addpower

This lens is an aspherical enhanced reading lens made in CR39. It is available in a single degression of 0.75D from the full near prescription.

The monocular near centration distances should be specified and the fitting points should be placed on the horizontal centre line of a previously adjusted frame, approximately 3mm below the pupil centre.

Other enhanced reading lenses include Essilor Computer 2V, BBGR Extenso, Nikon Online, Rodenstock Nexyma, and Norville Versatile Office (with four degressive options). Occupational PPLs are available in both traditional and freeform designs and include Hoya Hoyalux Tact and Hoyalux iD Workstyle, Essilor Computer 3V, Rodenstock Ergo, Kodak Software and Nikon Home & Office.

Summary

This article has only provided a very brief overview of the many lens designs and materials available. As eye care professionals, a good working knowledge of the available lens types, designs and their characteristics is an essential part of our role. Not only must we remain familiar with the more traditional lens designs and materials available but also keep up to date with the ever changing range of new products and technologies that make our profession so diverse and so interesting to work in. ?

References

1 Tunnacliffe A H (2003) Essentials of Dispensing, ABDO, London, p 48.

C35379 Essential course in dispensing - part 9