So far, this series has concentrated on progressive lenses offering vision correction from distance to near. This article will consider modern lens designs targeting correction where the visual demands are more centred around intermediate and near work.
Degressive power lenses
When an emmetrope wears a pair of +1.00 D lenses, the effect is to render the wearer 1.00 D myopic, with an artificial far point at 100cm in front of the lens (figure 1). Objects which lie further than 100cm from the lens appear blurred, and images take on the typical characteristics of pincushion distortion.
Figure 1: Range of vision through near vision spectacles
Of course, it is for this reason that subjects who are given a first pair of reading glasses are reminded, before they put them on for the first time, that the lenses are designed to focus their eyes between arm’s length and their normal near vision viewing distance. If they try to look across the room, or at the television, objects will be out of focus. The distance from the lens to the artificial far point is found, in centimetres, by dividing the near addition into 100. The artificial far point distance for various near additions is illustrated in figure 2 and it will be realised that not only is distance vision sacrificed as the near addition increases but, with adds over about +1.50 D, intermediate vision also suffers.
Figure 2: Ranges of vision for various near additions (ignoring depth of field effects)
These figures ignore depth of field effects which will slightly increase the ranges of clear vision. Typically, for a pupil diameter of 4mm, the depth of field is about ±0.25 D1 which would extend the range of vision by some 0.25 D at each end of the scale. As the reading addition increases with advancing presbyopia, intermediate vision also lies beyond the artificial far point provided by the near addition.
It was to offer extended ranges of vision for intermediate and near vision that degressive lenses were introduced. 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.
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 (figure 3). Certainly, the subject would not have a clear view of a colleague sitting three metres away, nor of a clock on the wall at the opposite end of the office.
Figure 3: Typical work station
One solution for office wear, that 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 4a.
Figure 4: Astigmatism associated with degressive power lenses
This quantification is given only to emphasise 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 centre 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 4a is of a so-called ‘hard’ design with no attempt to redistribute astigmatism in the distance portion of the lens. This progressive power 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 (figure 4b). If the design was a ‘soft’ design where the astigmatism was 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.
A further advantage of degressive power lenses is their use generally improves posture for the wearer, reducing neck and shoulder strain, when worn for long periods in front of the computer. For example, a subject who wears only single vision lenses for near vision, who must see both the computer screen and any material on the desk, will tend to lean forwards to bring the screen and the material within their near vision range. This may result in back and neck strain.
Progressive lens wearers on the other hand, may need to raise their heads, to view the lower part of the screen through the intermediate zone of the lens and any small detail on a paper to which they need to refer, or indeed, the keyboard itself, resulting in strain to the neck and shoulder muscles. Several lens manufacturers give advice on posture to support their promotion of degressive lenses.
There are several occupational progressive designs available today. More and more practitioners are recognising the value of occupational pairs of spectacles and suggesting to clients who would benefit from such a correction that they should keep a pair for the office. Some examples are given in the table 1. Most manufacturers suggest 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. Although designed primarily for intermediate and near vision use (some manufacturers refer to this as ‘indoor’ or ‘room’ vision and thus these zones of the lens are prioritised in their design), some lenses also incorporate a small area at the top of the lens for distance vision and although not strictly degressive power lenses they have been included in table 1. It is easy to understand why most of these trade names were chosen.
Table 1
Typical iso-astigmatism and mean power plots for different degressive lens designs are shown in the accompanying figures. Note the long corridor lengths for these designs. Needless to say, since there is no distance area for most degressive lens designs, they are totally unsuitable for driving.
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 centre of the uncut lens, should be fitted 3 to 5mm below the pupil centre (figure 5).
Figure 5: Power law for the Sola Access Lens +3.00 D for NV with -1.25 degression power
The power shift zone is about 10 to 12mm deep and it is recommended the near prescription is checked some 9mm below the geometric centre of the lens. Normally, the 0.75 D degression is supplied on near additions up to +1.50 D and the 1.25 degression on additions over +1.50 D but the practitioner can obtain either degression power for any near addition, if required. Iso-astigmatism and mean power plots for the Sola Access design with a +3.00 D near prescription and -1.25 D degression power are shown in figure 6.
Figure 6: Iso-astigmatism and iso-mean power plots for Sola Access design, +3.00, Add -1.25
Degressive designs, intended for indoor use have now been introduced by most of the major lens manufacturers. In the 1990s, Essilor introduced their Readers, which were intended for providing early presbyopes, or even subjects approaching presbyopia, with a small near addition to relieve accommodation, which when prolonged, might cause fatigue. Two designs were available, Orma Readers D which had a 12mm long progressive zone and was of relatively soft design, and Orma Readers S which was a harder design, whose corridor length increased from 6 to 12mm as the near addition increased. The upper portion of the lens was afocal and adds were available between +1.00 and +2.00 D. Interestingly, despite their progressive nature, they were described as ‘advanced single vision lenses’.
These two lenses were later replaced by the Interview design, introduced for use at the workstation, for desk use, or at a VDU. Two designs are available, with fixed additions across the progression zone of either +0.80 D, or +1.30 D, the full addition being obtained at a point 18° below the major reference point. Originally this was designed to be positioned in front of the centre of the pupil (figure 7). Later mounting instructions suggested that the lower reference line be mounted in line with the lower lid and the vertical line mounted at the pupil centres.
Figure 7: Power law for Essilor Interview 080 lens
The power degression of the lens is either -0.80 D from the reading zone to the intermediate zone or -1.30D, and it is the relatively small change in surface power, combined with the low level of surface astigmatism, that provides such a wide and stable field of clear vision across the lens (figure 8). Adaptation to such a soft design should be as easy as with single-vision lenses, and the wide field of vision means that wearers need only scan from side to side across the lens, rather than having to turn their heads, as would be necessary with the usual progressive designs.
Figure 8: Iso-astigmatism and iso-mean power plots for Essilor Interview 080 lens, +2.00, Add -0.80
Iso-astigmatism and iso-mean power plots for the Essilor Interview 080 design are shown in figure 8. When checking the prescription, the back vertex power measured at the power checking circle should correspond to the ordered near vision prescription. It was suggested that the advantages of this occupational design could be demonstrated quite quickly to prospective wearers simply by holding up -0.75 D lenses in the case of the 080 design or -1.25 D lenses in the case of the 130 design, in front of the subject’s reading correction. The advantages of the increased range of vision should be immediately apparent. It was recommended that the mounting reference line is fitted so that it is tangential with the lower edge of the iris. The lens is ordered in terms of the subject’s near prescription.
Later, and in order to emphasise the advantages of wearing degressive power lenses at the computer, Essilor introduced the Computer SV lenses in two different forms, the Computer 2V and Computer 3V. The Computer 2V offered a wide area of clear vision for both intermediate and near viewing distances which would provide full coverage of both the screen and the keyboard. It was suggested this design would be suitable for a subject who worked at the computer but did not normally wear an intermediate correction. It was recommended that this design should be fitted so the reference dot coincided with each near monocular centration distance and with the reference line coinciding with horizontal centre line of the frame. The lens should be ordered by giving the full near vision correction and the monocular near centration distances and verified upon return from the laboratory by confirming the near vision prescription at the marked checking circle.
The Computer 3V lens also included a small semi-distance area at the top of the lens in addition to wide intermediate and near vision fields and was suggested as being suitable for subjects who worked in an open office environment or at a reception desk and required reasonable vision at room distances. On ordering, it was recommended that the lens be considered in the same way as a normal progressive power lens, the full distance prescription and near addition being provided together with monocular fitting heights and centration distances measured from the pupil centres in distance vision.
Recognising the advantages offered by degressive power lenses with very low near additions, Essilor introduced an Anti-Fatigue lens which was designed as a distance vision lens with a progressive power of just +0.60 D. It was thought upon as a single vision lens with some relief for pre-presbyopes who spent long hours using near vision. Although strictly not a degressive power lens, it was recommended that it should be ordered by stating the full distance prescription and the fitting cross mounted at the centre of the pupil and by supplying monocular fitting heights and centration distances.
A similar philosophy has been applied to the relatively new Essilor Eyezen design which offers not only a low near addition to relieve accommodation but also absorption of ultraviolet radiation and the shorter blue light wavelengths which are emitted by the screen. Eyezen is available with three different additions, Eyezen Initial whose addition is +0.40 D and is suggested to be prescribed to those in the age group 20 to 34. Eyezen Active whose addition is +0.60 D and is suggested to be prescribed to those in the age group 35 to 44 and Eyezen Focus whose addition is +0.85 D and is suggested may be prescribed to those in the age group 45 to 50 instead of some other form of correction for their presbyopia.
The newest degressive design from Essilor is the Varilux Digitime which is described as a ‘mid-distance progressive lens’ designed to be worn when a full distance correction is not required. Available in three versions, Digitime Near which offers a wide near vision zone at the bottom of the lens and a range of clear vision out to 50cm. Digitime Mid offers a wider intermediate zone and a range of vision out to 100cm and Digitime Room has a wide extended zone out to 220cm at the top of the lens which would enable a TV screen to be watched comfortably if it lay within this distance from the viewer.
Hoya Lens offer several aspheric degressive power lens designs including their Addpower 60 and Tact series together with the Hoyalux iD WorkStyle lenses. The Addpower 60 is described as an asymmetric single vision lens with a degressive power of -0.75 D offering improved depth and width correction when compared to traditional single vision lenses for near. It is recommended the lens is ordered by its near vision prescription and monocular near centration distances. The fitting cross should be mounted on the horizontal centre line of the frame which should lie approximately 3mm below the pupil centre. The monocular fitting heights should be specified in any case. A minimum vertical lens size of 24mm should also be provided to ensure the lens contains the full degressive power.
The Tact design is available in two different forms, Tact 200 whose degression power is designed to offer an intermediate range out to 200cm and Tact 400 with a greater degression power allowing an extension of the intermediate range out to 400cm. Tact 200 should be ordered by increasing the distance prescription by +0.50 D and reducing the near addition by 0.50 so that the eventual near vision prescription is of the prescribed value. Tact 400 should be ordered in the usual way by the distance prescription and the full addition. In both designs the fitting cross should be mounted in front of the centre of the pupil.
Like the Tact design, the Hoyalux iD WorkStyle is also available as WorkStyle 200 and WorkStyle 400 but employs Hoya’s iD free form design technology to provide a progressive lens with a small area for distance vision near the top of the lens but the intermediate and near portions prioritised to provide wide intermediate and near fields for office use. Sensibly, Hoya emphasises these designs are not suitable for driving. The WorkStyle designs should be ordered in the same way as normal progressive power lenses, by the distance prescription and near addition, and monocular fitting heights and near centration distances should also be provided. Hoya also ask that the near working distance and the position of the trial addition lens in the trial frame or phoropter should be stated so these parameters can also be taken into account in the design.
An early degressive design from Rodenstock was called the Cosmolit P (in some markets known as the Cosmolit Office) and was a shallow-base, low-power aspheric design, described as a variable power lens for near vision. It was also recommended for use for VDU users, or those who require greater flexibility for near vision than is provided by a simple, single-vision lens. The viewing range that the design offers when prescribed as suggested by Rodenstock, is from some 80 to 100cm in the intermediate field, down to approximately 25cm in the near field.
The lens offered wide viewing fields at intermediate and near, with minimum distortion in lateral regions of the lens. Mechanically, the lens was thin and light due to its aspheric form and the incorporation of thinning prism. There was a built-in variation of either 1.00 or 1.75 D depending upon the power that is ordered. The power profile for the Cosmolit P design is illustrated in figure 9. The reading prescription is effective at the near reference point, BN which lies 16mm below the geometrical centre of the lens, G, and it is the near vision prescription which should be ordered, determined for a working distance of 40cm.
Figure 9: Power law for Rodenstock Cosmolit P (Cosmolit Office) lens
When checking the prescription, the back vertex power measured at the near reference point is the near prescription. The power variation which is provided is indicated on the lens by a micro-engraving. The lens is marked 10 in the case of the 1.00 D power variation or 17 in the case of the 1.75 D power variation. For near vision powers up to +1.75 D, the power variation is 1.00 D and for the near vision power range +2.00 to +2.50 D, the power variation is 1.75 D.
Rodenstock’s Nexyma degressive power lens, which has now replaced the Cosmolit P lens, is available in two forms, Nexyma 40 which offers a wide near portion in the lower half of the lens and the Nexyma 80 design which has a wider intermediate area at the expense of the near zone. The Nexyma 40 has a degression power of -1.00 D (figure 10) and the Nexyma 80 offers two degression powers, -0.80 D (80A) for near additions 1.25 to 1.75 D and -1.50 D (80B) for near additions 2.00 to 2.75 D. The near reference point for the Nexyma 40 design lies 7mm below the horizontal centre line of the uncut and 14mm below the horizontal centre line in the case of the Nexyma 80 designs. The power law shown in figure 10 indicates the power change starts some 8mm above the horizontal centre line of the uncut.
Figure 10: Power law for Rodenstock Nexyma 40 lens with 1.00 D degression
The latest computer lenses from Rodenstock include the Ergo Near Comfort range, which claim to allow up to 40% better vision at near and for intermediate and still enable clear vision over the intermediate range which varies with the specific degression power which has been chosen for each of the three different designs, Ergo Book, Ergo PC and Ergo Room. Ergo Book covers the close and intermediate range out to about 90cm, Ergo PC covers the range out to about 120cm and Ergo Room out to 5m from the wearer.
In addition, each design can be obtained in three different forms, the top of the range, Impression Ergo FS 2, which allows all characteristics of the wearers visual behaviour to be incorporated in the design, Multigressive Ergo 2 and Progressiv Ergo 2. Although no distance portion is provided, Rodenstock ask that these lenses are ordered in the same way as progressive power lenses with the distance prescription and near addition. The design engine computes the intermediate power and the final change to provide the full near addition.
Zeiss also introduced a degressive lens which was made available in either 1.60 index glass, Gradal RD, or CR 39 material, Clarlet Gradal RD. It was a progressive lens specifically designed for use as an indoor lens. Whatever distance prescription was ordered, Zeiss added +0.50D of the total reading addition to the distance portion and the rest of the addition is then provided in progressive addition form. Thus, if the prescription +3.00 add +2.00 is required, Gradal RD is supplied with a power of +3.50 in the upper portion of the lens and an addition of +1.50 then takes place down to the near reference point where the power becomes +5.00D (figure 11).
Figure 11: Iso-astigmatism plot and power law for Zeiss Gradal RD design, +2.00, add 2.00
This expedient ensures there are wide intermediate and near fields. Zeiss claimed the whole width of the lens offers comfortable vision, without the narrowing of field that is encountered with normal progressive power lenses. The elliptical shaped uncut, produced using the Optima surfaced-to-shape technique, ensured that the lens was as thin and light in weight as possible.
Professor Mo Jalie is a visiting professor at Ulster University and author of the new edition of Principles of Ophthalmic Lenses.
References
1 Rabbetts RB (2007). Clinical Visual Optics, 4th edition, Elsevier (London). Pages 304-305.