Until now binocular vision has not been taken completely into consideration in any progressive lens. And yet respecting the various aspects of binocular vision, whatever the prescriptions for both eyes,
is necessary for optimal visual perception. Further, the dominant eye plays a highly specific role in the ocular pair. Taking this parameter into account enables us to improve the wearer’s binocular visual performance still further. For the very first time 4D Technology^TM enables these various parameters to be taken into account in the calculation of progressive lenses.

BINOCULAR VISION: THE PHYSIOLOGICAL BASIS

Human beings have two eyes separated by a short distance in space, such that a large part of the visual field is perceived simultaneously from different points of view. The two retinas transmit disparate monocular images to the cortex via visual pathways (fig. 1).

Fig. 1: Cortical mechanism of binocular vision.

The two images must be of good quality and have a high degree of resemblance (fig. 2) so that the brain can analyse their similarities and differences in order to merge them into a unique, three dimensional perception. This process is known as binocular vision. Binocular vision not only enables one to see but also improves visual performance compared to monocular performance; this is thanks to the phenomenon of binocular summation [1, 2].

Fig. 2: Binocular summation a function of the differences of optical quality between the two eyes (Castro et al. 2009).

Requirements for good binocular vision 

According to Castro et al. [3], binocular summation is optimal when both eyes have the same level of optical quality. The authors have used the Strehl ratio as an indicator of the eye’s optical quality, and show that there is a statistically significant correlation between binocular summation and difference from the Strehl ratio between the two eyes (fig. 2).
Thus, when the two eyes have the same level of optical quality, binocular summation is higher, whatever the age of the patient.

Cyclopean eye and binocular visual direction

Like the Cyclops in Greek mythology, although our two eyes each receive and analyse an image of the surrounding world, in binocular vision we see a single image from a virtual point of view known as the Cyclopean eye.

Hering’s demonstration is a good illustration of this phenomenon (fig. 3). A person looks through a hole pierced through a glass panel such that the house is seen only by the right eye and the tree by the left eye. When the two eyes are opened the house and the tree appear to be superimposed in the same egocentric direction coming from a point located approximately in the median plane of the head [4]. This reference point, known as the Cyclopean eye, is where the binocular visual direction originates. This is a well known concept in the analysis of binocular vision.

Fig. 3: Hering’s demonstration (Howard & Rogers 2002).

Oculomotor coordination and dominant eye

The dominant eye plays a specific role in the ocular pair; it is preferred by the visual system in the performance of motor tasks and behaves like a dominant guide for the other eye [5]. Other results [6, 7] suggest that visual information from this eye is given priority by the visual system.

An experiment carried out at the Essilor International R&D department confirms the preponderant role of the dominant eye during dynamic visual tasks [8]. The aim of this experiment was to compare the impact of a monocular optical defect on binocular visual performance during a dynamic task of visual detection (fig. 4). Half of the subjects involved in the experiment had a dominant right eye, the other half had a dominant left eye. The results show that an optical defect in the dominant eye significantly increases the response time of the subjects (p<0.05), which is not the case when an equivalent optical defect is placed on the other eye.

Fig. 4: Experiment with the Essilor R&D Virtual Reality Simulator – The subject had to indicate, using a joypad, the successive directions of Snellen’s E (4 directions possible) displayed for 1500ms at different positions in the binocular field of vision.

Between each display of E, the subject had to stare at a central cross placed in the primary direction of the eyes. An additional optical defect was placed alternately on the dominant eye or on the other eye. For each of the targets displayed the response time (joypad response – target display t) was recorded.

There are numerous tests to define the dominant eye. Porac and Coren [9] distinguish two types of tests: those for which monocular aiming is conscious and those for which it is unconscious. The authors’ preference is for the second type of tests. This is the case of the “Hole in the Card Test” where the subject has to stare, with both eyes open, through a perforated card at a target facing him.
A protocol as well as a specific mechanism have been developed and integrated into the Visioffice in order to enable opticians to make this measurement in their shops (fig. 5). The customer holds the tablet in both hands and looks at the Visioffice fixing target through the tablet viewer. The new software integrated into Visioffice automatically identifies the person’s dominant eye. Thus, using the new
4D tablet, an optician can quickly identify his customer’s dominant eye.

Fig. 5: Measuring the dominant eye with Visioffice.

OPHTHALMIC LENSES: BINOCULAR OPTIMISATION

Progressive lenses disturb the quality of natural left and right retinal images by the presence of optical aberrations which are inevitable and are generated by the optical strength variation in the periphery of the lens. They thus affect natural binocular vision. 
This can lead to difficulties in the merging of images and the perception of relief, as well as a reduction in binocular visual fields. 
Taking account of binocular vision in the design of lenses has been indicated for many years. For example, some methods allow for correct positioning of vision zones in order to follow the wearer’s natural convergence. However, current calculation methods are based on a monocular approach that does not entirely satisfy wearers’ binocular requirements. When both eyes look simultaneously at the same object, the current performances of left and right lenses can be different for coupled eye directions, particularly in the case where the two eyes do not have the same far vision prescription.
In fact, today, lenses are calculated separately and performances are optimised lens by lens without any consideration for the pair of lenses mounted in the spectacles: the concept is said to be monocular.

A system of monocular coordinates describes an erroneous environment from a wearer point of view since the directions of the eyes are not coupled, which leads to poor theoretical positioning of the objects for each of the two eyes (fig. 6).

Fig. 6: Monocular references – A monocular reference is defined for each eye based on its centre of rotation (CRO) with angular expression of the directions of the eye (α_Right, β_Right) et (α_Left, β_Left). For each eye the radius from the CRO, passing through the centre of the pupil, is deviated by the lens and associates a point O of the object space, modelled by a 3D environment, with its images formed on the retina.

Since each eye is considered separately from the other, a monocular eye direction (α_Right, β_Right) et (α_Left, β_Left) does not correspond to the same object point for each eye (O_Right and O_Left). The gap between these two points in the object space O_Right and O_Left is greater the bigger the difference between the prescriptions for the two eyes.

The result is an inexact calculation of the optical characteristics of each of the lenses (strength, astigmatism, gradients, etc.).

Binocular marker and Cyclopean eye

To have a true representation of the environment, it is necessary to establish a system of binocular coordinates, originating in the Cyclopean eye (fig. 7). We can then define the relationship between each
object in the environment observed by both eyes simultaneously and the corresponding pair of left and right eye directions. The eye directions for each pair are said to be “equivalent” or “coupled” - (_{R R} and (_{L L} – because they are used simultaneously by the wearer to look at the same object. They correspond to a virtual binocular eye direction (_{Bino, Bino}) linked to the Cyclopean eye.

Fig. 7: Binocular references and binocular performances of monocular concept lenses.

The optical performances of each lens can then be analysed according to these new binocular references. On the right of figure 7 are shown the astigmatism performances of two lenses calculated by monocular concept. The binocular performances of the two lenses are different.
Thus, the right and left images, which form on the corresponding retinas, have a different optical quality. Merging of the images is not optimal. The result is also narrower binocular fields of vision.

4D Technology^TM: binocular calculation that takes account of the dominant eye

Binocular calculation used in 4D Technology^TM consists of adjusting performances between the two lenses such that the optical qualities of the left and right retinal images corresponding to the coupled eye directions are similar whilst ensuring optimal vision for the dominant eye (fig. 8).

Fig. 8: 4D Technology binocular calculation.

To achieve this, a unique new binocular optical design is defined for the two lenses in the pair. It is built up, amongst other things, from the prescriptions for the two eyes and information about the dominant eye. For each binocular visual direction, a range of binocular criteria is thus defined and expresses the binocular physiological requirements of wearers. Definition of this new binocular design is based on the know-how we have acquired from the evaluation tests we continually carry out on wearers.
Each lens is them optimised to ensure that optical performances achieve the binocular optical design thus defined.
The method, which takes the dominant eye into account in the definition of performance and ensures right left performance symmetry in the binocular coordinates systems, means that optimal binocular performance can be achieved whilst improving speed of detection in wider binocular fields of vision. 

CONCLUSION

Binocular calculation implemented in 4D Technology^TM favours the dominant eye with the aim of preserving its role whilst minimizing the optical quality differences between the two eyes. The wearer retains good image qualities for both eyes and binocular vision is improved as well as dynamic vision performance.