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Optimising Single Vision Lenses for Today’s Patient

2 CPD in Australia | TBA in New Zealand | 3 May 2017

 

By Nicola Peaper

Discussions with optometry students recently highlighted the fact that two decades ago a practice could differentiate itself by using high index and aspheric lenses, as they were reasonably new to the market. When asked where they thought 1.5 index ranked as a percentage of the single vision market 26 years on, they guessed 20 per cent. The students were shocked to discover that in fact they make up between 60 and 65 per cent. Asphericity is not usually chosen but is often a result of moving up an index, and as such, over 90 per cent are ordered without correct measurements.

In 1990 there were clear optical benefits for patients to move to an aspheric single vision lens. In 2017 the development of digitally surfaced, optimised single vision lenses has again improved the single vision offering. This article will examine the different aberrations of the eye and corrective lens, and how different technologies are used to control and minimise these. It will also explore the use of these technologies in conjunction with specialised single vision categories such as sports (high curve) lenses and modified single vision that have a boost for prolonged near work and the relatively new driving category.

Aberrations of the Eye

When assessing the best performance of a spectacle lens it is important to understand the final level of correction that is necessary. The focusing system of the eye is subject to various aberrations and a spectacle lens is worn to correct those aberrations. The corrective spectacle lens is, in turn, subject to aberration and it is the role of the designer to keep that aberration to a minimum and to produce a ‘best form’ spectacle lens.

Until relatively recently it was only possible to measure lower order aberrations of the eye (LOA), those of defocus (myopia and hyperopia) and regular astigmatism that can be resolved with a cyl power at a particular axis. More recently it became possible to measure irregular astigmatism of the cornea with corneal topography and this could then be corrected for with a rigid contact lens. If, however, the irregular astigmatism was lenticular, it was hard to measure and little could be done about it.

In 1961, the higher-order aberrations (HOA) of the human eye were first measured by Smirnov, who went on to predict that customised lenses would be made to compensate for the HOA of individual eyes.1 It is now possible to measure both LOA and HOA with wavefront technology to give a complete optical status of the eye. Wavefront technology looks at the deviation of a wavefront by an optical system and this can then be expressed using Zernike polynomials that can be shown graphically as Figure 1.

Figure 1. Aberrations of the eye expressed as Zernike polynomials

 

The LOA and HOA can now be classified as:

  • Second order (n=2) representing defocus and regular astigmatism (LOA)
  • Third order (n=3) representing coma and trefoil (HOA)
  • Fourth order (n=4) representing tetrafoil, secondary astigmatism and spherical aberration (HOA).

The third and fourth order aberrations represent irregular astigmatism. Those most commonly associated with reduction in clarity of vision are coma (Figure 2) and spherical aberration (Figure 3).

Figure 2. Representation of Coma 
Figure 3. Representation of spherical aberration
       

Spherical aberration is caused by a change in the refraction of light rays as they pass through the edges of the lens or cornea. This change in refraction is caused by the change in shape of the cornea and lens towards the periphery and also changes in the refractive index of the lens. It is increased the further away from the optical axis you are. Hence a larger pupil allowing more peripheral rays through will increase the effect of spherical aberration (Figure 4). The focus of light, instead of being on the retina, is spread in front of and behind the retina causing a blur circle and contributing to haloes around light sources (Figure 5).

Figure 4. Effect of spherical aberration

 

Figure 5. The effect of HOA on night vision

 

Coma is caused by a wavefront arriving at the cornea or lens at an oblique angle. As with spherical aberration, the light is not focused to a point but is spread to produce a comet like image. Again, as with spherical aberration, Coma is increased by a larger pupil. Coma impacts on night vision causing comet shaped haloes and distortions around light sources (Figure 5).

With the advent of wavefront technology, it is now possible to assess aberrations from both the crystalline lens (internal optics) and the cornea and calculate the effect of both on the quality of images on the retina. In 2001 Pablo Artal et al2  concluded that, in a research group of young, healthy subjects, there was a significant negative correlation between corneal and internal optics aberrations. In other words aberrations of the cornea were cancelled out by equal but opposite aberrations of the lens. This supported the well-known fact in clinical practice that astigmatism of the lens tends to compensate for corneal astigmatism. However, what is now known is that this compensation also takes place for higher order aberrations. A large amount of the spherical aberration of the cornea is cancelled by the internal optics. The magnitude of coma like aberrations of the cornea is also significantly reduced by the internal optics.

Interestingly, research of corneal aberrations among subjects aged between 20 and 70 years shows that the cornea becomes more spherical with age causing an increase in spherical aberration. Coma and other HOA also increased with age. The conclusion being that the balance between corneal and lenticular aberrations decreases with age and this contributes to a reduction in retinal image quality with age.3 This suggests that measuring and compensating for HOA becomes increasingly important with age.

Comparing the eye to a lens system, where each subsequent lens is designed to correct the aberration of the previous, a spectacle lens now needs to correct the resultant LOA and HOA of the combination of crystalline lens and cornea. As, until recently, the only measurement possible was that of the LOA of defocus and astigmatism, lens design concentrated on keeping the sphere and cyl power constant across the lens surface. With the advent of aberrometers both LOA and HOA can be assessed and compensations introduced into spectacle lenses. HOA needs to be assessed with a larger pupil size for distance tasks than for near tasks. This takes into account the dilation and constriction of the pupil for different tasks and the fact that HOA has the greatest effect in low light levels.

Spectacle Lens Aberration and Design

When a spectacle lens is mounted in front of the eye in such a position that the optical axis of the lens coincides with the visual axis, then the form of the lens does not matter. The eye is then viewing through the optical centre (OC) of the lens and the image formed by the lens is not afflicted with any defects or aberrations that might affect its sharpness or shape.5 As the eye rotates across the lens, the off axis performance is affected by various aberrations that reduce image shape and clarity. The most significant aberrations for the spectacle wearer are:

  • Transverse chromatic aberration
  • Distortion
  • Oblique astigmatism

Transverse Chromatic Aberration

As Transverse Chromatic Aberration (TCA) is related to the Abbe number of a lens, it is a product of the material chosen and is not specifically affected by lens design.

TCA causes coloured fringes to be seen around an image and/or a reduction in clarity away from the centre of the lens.

As the eye moves away from the OC of a lens, prism will be experienced in the amount of:

Prism = distance from OC (cm) x power of the lens (D).

A prism will refract different wavelengths of light by different amounts based on the Abbe value of the lens.

TCA = Prism / Abbe number (Figure 6).

Figure 6. Transverse chromatic aberration

 

From this the amount of TCA increases:

  • With lower Abbe numbers.
  • With higher lens powers, as the amount of prism experienced increases rapidly as the eye moves away from the optical centre.

Assuming a threshold for TCA to cause symptoms is 0.1, then the distance from OC when TCA may be noticeable is represented in Table 1.

Table 1. Distance from OC that TCA may cause symptoms

 

This suggests that a high Abbe number should be always be used but, as patients will vary in their sensitivity to TCA, this should be balanced against reducing the lens thickness.

Oblique Astigmatism

If a ray of light is incident at 90 degrees to a spherical lens, at the optical centre (OC) the light will be focused at one point and there will be no distortion of the image. Consider a spherical lens of +8.00D. If placed flat on the measuring plate of a vertometer, the light ray passes through undistorted to a point focus.

If the lens is now slid horizontally keeping it flat, the light will hit obliquely, appearing as an oval causing the lens to act as though it is astigmatic. The ray of light will focus at two different distances, causing blur (Figure 7).

Figure 7. Oblique astigmatism; light incident at an oblique angle focused at two points

 

The different foci can be plotted graphically as a representation of power at different points moving away from the OC of the lens (Figure 8).

Figure 8. Spherical Lens designs

 

Looking at the graph for the +5.50 D lens:

  • At the OC the power is +5.50D.
  • At 15mm from OC the power is +5.50 / +0.25.
  • At 25mm from OC the power is approximately +5.50 / +0.75D.

If this is a 70mm blank then at the temporal edge of the lens there will be over 1.00D of cyl present.

If this lens were to be produced as best form without any oblique astigmatism, the two lines would be superimposed along the zero axis.

There is a similar but larger change in cyl power on the +8.00D lens. However, since this now falls outside the realms of best form, the spherical power also has significant changes.

Distortion

Distortion comes from the fact that the power of a spherical lens increases towards the periphery and so, on a grid seen through a high plus lens, each subsequent line further away from the optical centre is larger. This produces a pincushion image. With a high minus lens the lines are minified producing a barrel distortion (Figure 9). In each of the images, the blue line is shorter than the red line. When other aberrations are controlled using base curve, distortion tends to reduce at the same time.

Figure 9. Barrel and pincushion distortion

 

Best Form Lenses

If a lens power is considered to be the sum of the front and back curves then, in the past, lens design involved varying the two curves to reduce aberration in the central 30 degrees around the OC. When a lens is produced with the minimal amount of aberration it is referred to as ‘best form’. Tscherning showed this could be plotted graphically producing Tschernings ellipses (Figure 10).

Figure 10. Tschernings ellipse for 1.5 index material

 

It can be seen that aberration free lenses can be produced on two curves; Wollaston and Oswalt. In practice the shallower, Oswalt, curve is used.

It can also be seen that ‘best form’ has power limitations, particularly for the hyperope over +7.25D.

To produce best form lenses between the powers of -22.00D and +7.25 requires a large number of base curves. Interestingly when looking at stock spherical 1.5 index lenses, many manufacturers employ only five base curves to produce lenses in the range -5.00D to +4.00D. Depending upon the number of blanks of different base curves stocked by a manufacturer, the same can be said of grind spherical lenses where again, a low number of base curves may be employed for lenses across the whole power range.

Asphericity

Using purely spherical curves is limiting when resolving aberration. The first step away from this was to produce a moulded aspheric front surface to a lens and grind spherical curves on the back. The change in curvature across the front surface of the lens creates surface astigmatism that reduces aberration caused by a ray of light with an oblique angle of incidence.

Referring back to a +8.00D lens on a vertometer, now as the lens is moved off axis the ray of light appears to remain more spherical. As the asphericity is designed to reduce the aberration of specific powers, it is essential that the base curve interval is small, increasing the number of blanks required to cover a full range of powers.

Comparing a graphical representation of the foci of aspheric lens design (see Figure 11):

  • At the OC the power is +5.50D.
  • At 15mm from OC the power there is still approximately 0.25D of cyl but the resultant power is still close to +5.50D.
  • At 25mm from OC the power is approximately +5.25 / +0.50D. Again the resultant spherical power is close to +5.50D.

The aberration comes in much more slowly than with the equivalent spherical design.

The aspheric +8.00D is also more stable than the spherical, giving lower aberration towards the edge of the lens.

Figure 11. Aspheric lens designs 
     

While producing an aspheric design will reduce aberration, there is also a weight and cosmetic advantage due to the flatter base curves. The cosmetic advantage is of increasing importance with moderate to high plus powers. Looking at Tschernings ellipse, any lens over +3.00D has a best form base curve of over 10.00D. So to produce best form, the base curve will cause unsightly magnification and a lens that will not fit into a frame without bowing out. In practice a +3.00D spherical lens is produced on a 7.26D base curve. Indeed spherical lens design has often been a compromise between good optics and good aesthetics.

By comparison a +3.00D aspheric lens is produced on an equivalent base of 4.00D. This will produce low aberration in a more cosmetically acceptable form.

Similarly, best form for a spherical +6.00D lens is 14.00D, causing magnification similar to a goldfish bowl! As a compromise, spherically it will be produced on an 8.75D base, which will still have a high level of magnification. In aspheric form it can be produced on a 6.5D base giving both good optics and a good cosmetic result.

With both plus and minus lenses, using a flatter base curve will produce a lens with less thickness, mostly because the sag of the flatter base curve is lower (Figure 12). This will reduce the weight of the lens.

Figure 12. Lower sag on aspheric lens to give a reduction in edge thickness

 

Front surface aspherics of this type are considered to have one axis of asphericity and as such will only compensate for the aberration caused by the spherical part of a spectacle script. To correct astigmatic scripts it is possible to produce a lens with different aspheric curves along the principle meridians on the front surface. While better, this will still leave areas off axis with levels of aberration.

Computer design and digital surfacing can now optimise across the lens surface giving the equivalent of ‘multiple axes of asphericity’ to improve the optics of a lens in all directions of gaze (Figure 13a,b & c).

Figure 13. Approximation of areas with residual aberration, shown in dark blue

 

Compensating and Optimising

With computer design and digital lens production, various compensations can be built into lens designs:

  • Position of wear can obviously have an effect on the oblique astigmatism that a patient experiences as wrap and pantoscopic tilt (PT) effect the angle of incidence of light. With digital surfacing, instead of using average values for PT, wrap and Back Vertex Distance (BVD) actual measured values can be used to produce a lens customised for the frame it is fitted in to (Figure 13d.)
  • As the eyes scan across the lens they will rotate in accordance with Listings Law. The eye rotation can be calculated and the cyl axis altered in accordance at every point around the lens. Again this will give exceptional clarity of vision in the periphery of the lens for any prescription with a degree of astigmatism.
  • Optimisation of power. Unlike conventionally produced lenses, which offer optimum correction at the OC, the optimisation of computer designed, digitally surfaced lenses is carried out at every vision point across the lens.
  • HOA control. As previously stated HOA represents irregular astigmatism and therefore cannot be completely corrected by a spectacle lens. However, coma (Figure 2) and spherical aberration (Figure 3) have an influence on the best sphero-cylindrical correction and compensation for them can now be included in lens calculations.

While it is possible to have an aberrometer measure the HOA of the  individual eye with different pupil diameters, and use the results to calculate the best sphero-cylindrical correction for each point on the lens, this is not yet  commonplace.

In lieu of this, lens manufacturers have now built physiological models based on averages of HOA and pupil size to form the basis of optimisation of a spectacle lens. This is available more commonly in multifocal lenses but is equally important in single vision corrections, especially when worn for driving in low light levels.

Prescribing Specialised Single Vision Lenses

When considering using a low aberration multi aspheric lens, the concern of the practitioner is always that the patient will not appreciate the improvement in visual performance. For this reason, recommended limits on the powers of prescriptions that should be used are often asked for. However, every patient is different and levels of tolerance vary.

If we consider the types of visual tasks that we require correction for, even the ‘simple single vision’ reader when used on an Excel spreadsheet on a laptop needs to be low aberration. It should also be remembered that the thickness and weight reduction of digitally surfaced aspheric lenses is noticeable from low powers.

Resolving patient symptoms and problems is a more important factor when considering lens choice.

Anti-Strain Category

This is a category of modified single vision lenses. These lenses have varying ‘boosts’ of plus power in the lower half of the lens presented in long corridors of around 18mm. As these lenses are intended for use with digital devices, often with desktop and multiple screens, an optimised digitally surfaced lens will perform far better than a simple spherical version.

Although intended for the pre-presbyope with digital eyestrain, these lenses can equally be used for the presbyope using multiple screens. The script ordered should be the intermediate power to focus the outer screens and, depending on the add, the appropriate ‘boost’ chosen. Used in this way these lenses will give clear vision with smaller head and eye movements for multiple screen use.

Driving Options

This is a relatively new category of lenses designed to reduce visual problems frequently encountered while driving. Wide fields of useful vision, good binocular vision and a reduction in glare and distortion of lights are all required when driving. When referring to fields of vision, it is becoming increasingly apparent that current testing methods employing flashes of light do not give a full picture of the facts. What is more important is the speed at which a driver identifies and then reacts to something within his visual field. A lens designed to be low in aberration may increase the speed of reaction and also provide image stability, allowing for more natural and, therefore, quicker head/eye movements when looking in wing mirrors or looking at a GPS and instruments.

Another frequently reported problem is haloes around lights and, in the pre-presbyope, night myopia. Prescribing a lens that has HOA compensation included in the design may reduce or eliminate these problems.

Glare from oncoming headlights and reflections from the back surface and inside the lens can be addressed with the use of a multicoat. Modern headlight bulbs, particularly the Xenon, tend to have a high blue content and so reduction of the blue end of the spectrum will help. Blue multicoats will reflect a level of light at night and so a better solution may be to include a filter tint with a standard multicoat to remove a level of blue light.

Sports / High Wrap Lenses

Sports generally demand the high vision stability that low aberration lenses provide. Reducing the aberration encountered as the eyes move across the lens allows easier fusion and so improved binocular vision. High wrap frames force unsuitable base curves to be used but if the degree of wrap is given, then optimisation within script limits will still give clarity of vision across the lens into the periphery. The script limitations are caused by the differing angles between the lens and the direction of gaze during lateral eye movements (Figure 14).

Figure 14. The same angles between lens and direction of gaze with normal spectacle frames compared with different angles with sports curved frames

 

As the wrap increases, the distribution of aberrations becomes more asymmetric both nasally and temporally. As a result, magnification and distortion will differ, causing different visual impressions.

Fitting Techniques

Optimisation of all single vision lenses starts from the OC. This means they should be fitted so that the optical axis passes through the centre of rotation of the eye. For this the OC needs to be dropped by 1mm for every 2 degrees of pantoscopic tilt. There are various measuring devices on the market that will measure an aspheric height. If a device is not available then the height should be measured with the patients head tilted so the frame is at right angles to the floor.

Monocular PD should be taken, and, in the case of high wrap lens designs, the PD should be corrected for the prismatic effect of the lens.

With so many options, we can reach beyond the idea that a single vision lens provides broadly similar optics dependent on material and possibly an optional coating. It is preferable to consider multiple adjustments that can improve the optical performance for the patient. When dispensing a single vision lens, we should of course consider the index and the anti-reflective coating, but we should also consider; the best form, asphericity, higher order aberrations, capabilities of the surfacing technique and the fitting measurements. All of these can be combined to provide a significant optimum visual experience suited to purpose; everyday wear, sports, digital use or driving. 

Higher-order Aberrations and Night Myopia

Night myopia was first described in the 18th century and particularly noted in the Second World War when the navy employed light signals. It effects pre-presbyopes and reduces as presbyopia approaches. Many causes have been suggested but it is generally accepted that the eye accommodates in very low light levels, causing myopia of up to -1.00D. While the need for a separate, more myopic script for night driving has been dismissed, it is generally agreed that accurate correction of normal refractive errors is more important for night driving than during the day due to the greater blur associated with the larger night-time pupil.

In a study, Norberto López-Gil et al.4 found that the visual stimulus affected the degree of night myopia. A point light source, more commonly encountered at night, caused a greater degree of night myopia than a chart of black letters on a white background. The study stated that “optical modelling suggested this difference in refractive state is due to spherical aberration”.

 
 
 

 
Nicola Peaper qualified as an optometrist in the UK in 1985 and practiced in private and corporate practice in the UK for 20 years. In 2001/2 she was employed as Ophthalmic Advisor to Kensington, Chelsea & Westminster Health Authority. Since moving to Australia in 2005 she has worked in optical laboratories advising on procedures and quality. In roles as State and National training manager she has gained extensive experience in presenting the technology behind, and the prescribing and fitting of, ophthalmic lenses. Nicola Peaper is currently Professional Services Manager for Rodenstock Australia. 


References
1. Naoyuki Maeda MD, Clinical applications of wavefront aberrometry – a review. Clinical and Experimental Ophthalmology 2009; 37: 118–129
2. Pablo Artal, Antonio Guirao, Esther Berrio, David R Williams. Compensation of corneal aberrations by the internal optics in the human eye. Journal of Vision (2001) 1, 1-8
3. Antonio Guirao, Manuel Redondo, and Pablo Artal. Optical aberrations of the human cornea as a function of age. Journal of the Optical Society of America A Vol. 17, Issue 10, pp. 1697-1702 (2000)
4. Norberto López-Gil; Sofia C. Peixoto-de-Matos; Larry N. Thibos; José Manuel González-Méijome. Journal of Vision May 2012, Vol.12, 4. doi:10.1167/12.5.4 Shedding light on night myopia
5. Mo Jalie (2008). Ophthalmic Lenses and Dispensing (3rd ed.) Butterworth Heinemann. p. 29 

' Computer design and digital surfacing can now optimise across the lens surface giving the equivalent of ‘multiple axes of asphericity’ to improve the optics of a lens in all directions... '