Current progressive lens performances are limited due to compromise
Top end progressive lenses currently offer excellent quality of vision. However, some wearers say that their vision is still restricted. Two phenomena can alter the quality of visual perception.
Firstly, objects can appear blurred when the eyes look to the side.
Secondly, objects appear distorted and when the head is moved objects seen through the various areas of the lens appear to move within the surroundings, and change shape. This phenomenon is known as the "swimming effect" , , , , .
Vision is not therefore always optimised when progressive lenses are worn, and more particularly during adaptation phases , .
These phenomena are caused by the distribution of the inevitable optical aberrations generated by the power of the lens. These aberrations limit the width of the clear fields of vision and oblige the wearer to change the way he naturally carries his head, for example when reading. In order to extend the clear fields of vision, a manufacturer can push the aberrations away from the useful areas of the lens, but this then causes major variations in power, which increase the swimming effect sensation.
Lens performance is currently the result of this compromise, which each manufacturer can manage as they so wish.
Measurements have been carried out in the laboratory to evaluate the performance of the main top end progressive lenses currently on the market (Fig.1). The performances of lenses in group G1 have been
defined in order to help to reduce swimming effects to the detriment of wide fields of vision. The performance levels of lenses in Group G2 have, on the contrary, been defined to offer wide fields of vision, to the detriment of reducing the sensation of swimming effects.
Fig. 1: Laboratory measurement of Varilux Physio 2.0 and main top end competitors' lenses
The best balance between "width of clear fields of vision" and "limited swimming effects" has now been found with the Varilux® Physio 2.0 lens, whose excellent performance is recognised by all, from the prescription writer through to wearer. (Fig. 1).
Varilux® STM series breaks away from the compromise inherent to the progressive lenses currently on the market
With 11 new pending patent applications which protect core products technologies, Varilux® STM series is a major scientific innovation which pushes back the performance limits of progressive lenses.
Lens structure entirely designed to reduce swimming effects to a minimum
A new optimisation method has been developed to generate an original lens structure
Power variations linked to the progressive function generate prismatic deviation variations in every zone on the lens. Prismatic deviation directly affects the shape and position of perceived objects .
When the wearer moves, the prismatic deviation variation thus leads to variations in the perception of objects placed in the wearer's surroundings. He then suffers from swimming effects (Fig. 2).
Fig. 2: Variation in the prismatic deviation when the wearer moves his head. Due to the prismatic deviation, object A appears to come from B.
It is well known that prismatic deviation D increases in absolute terms if optical power P increases as illustrated by Prentice's law (Fig. 3a).
It is also known that in the case of a prism, prismatic deviation D can be varied by modifying angle A of the prism or the angle of incidence i of the light ray (Fig. 3b). Which means varying the relative position of both sides of the prism or its orientation.
Fig. 3: Optical parameters influencing prismatic deviation
But in a lens the prismatic deviation of a ray of light at a given point depends not only on the optical power, the relative position of both sides and the incidences of the rays on the surfaces, but also on the value of the curvature on the front and back sides of the lens.
It is therefore possible to get the prismatic deviation to vary whilst retaining optical power by modifying the camber of one point on the lens, the camber being defined as the half-sum of the front and back side curvature at the point in question .
Thus, the variation of the lens camber can mange the variations in prismatic deviations separately from the variation in power.
The Varilux® STM series lens has been optimised in order to even out the prismatic deviation variations across every area of the lens, whilst maintaining the variation of power necessary between near and distance vision. During the optimisation process the lens' structure has been considered like a set of small elements set alongside each other, thus offering new levels of freedom in lens design. Each of these elements has been calculated so as to increase optical power between distance and near vision zones, whilst reducing the lens camber.
Varilux® STM series: a manufacturing challenge
The variations in the lens camber, and therefore these variations of curvature on both sides lead to a progressive lens made up of two specially complex surfaces. In the past such variations in curvature have never been used in the design of an ophthalmic lens.
This new level of complexity, never before achieved, has therefore required the development of new production means. The industrial process is based on an essential stage which takes account of the precise measurements of the lens position in the blockage system, in order to ensure perfect tooling with the diamond tip. It uses all the skills of current Digital Surfacing and is considerably more precise in terms of the alignment of both surfaces of the lens.
The performance level of the Varilux® STM series lens is therefore conditioned by the use of this new process, which is known as S Digital Surfacing. This new lens structure and the optimisation process involved are known as NanoptixTM.
The benefits of this new lens structure were observed during the tests we performed in a virtual reality simulator.
Thanks to the unique virtual reality simulator developed by R&D at Essilor (Fig.4), we are able to reproduce the prismatic effects of ophthalmic lenses and thus simulate dynamic effect linked to movement .
Fig. 4: Virtual reality simulator (left) and prismatic effect of a simulated ophthalmic lens
We carried out an experiment to show the benefits of this new lens structure compared with a traditional structure, that is to say a lens whose progressive function is on the front side . A grid with an object distorted by the prismatic effects of the lenses evaluated is projected in front of the wearer. The latter expresses his preference from amongst a batch of lenses with different geometrical properties, but identical power and astigmatism.
In this experiment, 73% of wearer choices preferred the new lens.
These results confirm that the variation of the lens camber enables considerable modification to prismatic deviations, which are perceived by the wearer, this is the case independently of power and astigmatism variations, in other words, without any modification made to the fields of vision.
Innovative binocular optimisation to increase clear fields of vision
Good binocular vision and the associated wide binocular fields of vision are conditioned by retinal images that are similar for both eyes , , .
When the eyes look simultaneously at the same object, the actual performances of the right and left lenses may be different for joint vision directions, particularly in cases where the 2 eyes do not have the same prescription. Indeed, today lenses are calculated separately and performance levels are optimised lens by lens, without consideration for the lens couple formed by the spectacles. Thus, a right lens of power +1 and a left lens of power +2 for distance vision do not have the same performance levels with regard to the distributions of power and astigmatism defect aberrations, linked to the progressive function in every area of the lens.
The binocular optimisation method developed for the Varilux® STM series lens is used to achieve optimal binocular equilibrium between the performance levels of the two lenses, whatever the prescription couple. Thus, right and left lenses will have similar optical aberration distributions whatever the differences in distance vision power between these two lenses.
On the other hand, for a given power, this distribution will be different depending on whether the right lens is combined with a left lens with a distance vision power of +1.50 (Fig. 5 Case A) or if this right lens of the same prescription is associated with a left lens with a distance vision power of +2 (Fig. 5 Case B).
Fig. 5: Binocular calculation system (left) and performance levels for each lens in the binocular system (right)
This new method of calculation guarantees a level of quality for foveal images that are identical in both eyes. Binocular fields of vision are therefore extended.
This new optimisation method is known as SynchronEyes.
C/ Wearers like the Varilux® STM series lens
As is always the case, the proof of progress can only come from wearer tests carried out amongst a large sample of wearers. Tests are carried out according to a rigorous methodology, in real life wearing conditions, during which the wearer continues his normal activities.
To assess the performance of Varilux® STM series, we created a multicentre, worldwide comparative wearer test. The test was carried out according to a cross-over, randomised, double blind experiment plan, in 3 separate centres. It was carried out according to a rigorous scientific protocol approved by Professor José Sahel's1 team at INSERM research centre 968, at the Pierre et Marie Curie University in Paris.
97 wearers were recruited, distributed equally according to their ametropia and addition.
Wearers evaluated Varilux® STM series in comparison with Varilux® Physio 2.0, the market benchmark in performance terms. Interviews and specific questionnaires were used to collect both objective and subjective evaluations after a 15-day wearing period for each type of lens.
All the test evaluations show the superiority of Varilux® STM series over Varilux® Physio 2.0 (Fig. 6)
Fig. 6: Average grades given to each set of correction
The overall satisfaction level is very high with Varilux® STM series. The difference compared with Varilux® Physio 2.0 is also highly significant from a statistical point of view.
Similarly, evaluations of all visual criteria are in favour of Varilux® STM series.
Moreover, the remarkable quality of Varilux® STM series is confirmed by the intensity of difference perceived when the choice was made between the two sets of lenses. Over 50% of the wearers who chose Varilux® STM series reported a big to very big difference, which is an exceptional result.
Finally, the test shows that wearers adapted to the Varilux® STM series lenses particularly quickly. Over 60% of wearers adapted immediately or within just a few minutes.
Wearers perceive the benefits brought by the innovations
The percentages of very positive evaluations of the Varilux® STM series lens, demonstrated in grades of between 15 and 20 on a scale of 20 points, are particularly high compared to Varilux® Physio 2.0 for questions linked to dynamic vision. Also, Varilux® STM series offers wearers extended fields of vision at all distances compared with Varilux® Physio 2.0. The indicators Figure 7 represent, for each useful distance both the evaluation in terms of vision quality and the width of perceived field of vision. Over 50% of wearers perceived a big to very big difference compared with Varilux® Physio 2.0 when using these evaluation criteria.
Fig. 7: Evaluation of the benefits of innovations implemented in Varilux S Series lenses
The combination of an innovative lens geometry with a new method of binocular calculation means that the Varilux® STM series lens pushes up the performance levels of progressive lenses (Fig. 8).
Fig. 8: Varilux S Series pushes up the performance levels of progressive lenses
The work done for Varilux® STM series has resulted in two new calculation methods, NanoptixTM to minimise "swimming" effects and SynchronEyes to extend fields of vision.
These innovative design methods with their unique manufacturing processes now open up the way for the integration of new degrees of freedom in lens calculation. In the future their performance levels could be improved still further.