Vision is the most complex and efficient of all human senses. The visual system comprises all the organs and processes that lead to the interpretation of images, from reception of light through to cortical processing based on the signals received.
Both the optical section, from cornea to photoreceptors, and the cortical section, from retinal image to conscious perception, is specific to each individual: different eye anatomies can correspond to the same ametropia; two people can have different perceptions for the same image on the retina. Everyone who wears ophthalmic lenses, in addition to the individual characteristics of his visual system, uses their own spectacles in a specific way.
Practitioners are well aware of these inter-individual differences: their patient‘s medical history enables them to understand their requirements and lifestyle both of which can have major impacts on the assessment of future spectacles. This knowledge enables ECPs to direct their patients towards the most adequate solution. To assist opticians and optometrists with the adaptation of lenses to their patients‘ specific requirements and improve perceived performance, manufacturers have been offering personalized lenses for over ten years.
Personalization has been made possible through the use of different technologies: increasingly elaborate measuring instruments, the concept of individualised lenses and the Digital Surfacing process, which means that each lens can be made precisely and individually. Software has been developed to calculate the complex surfaces on the front and back of the lens based on ever increasing numbers of parameters measured on the wearer. Individual digital surfacing means that the exact desired lens can be obtained.
There are various types of personalization currently available on the market. Some parameters characterise the positioning of the lenses in front of the eyes, others address the eye‘s anatomy and optics and a third category describes the wearer‘s physiology and posture.
The importance of lens wearing conditions has been known to opticians for a long time, which is why this method of personalization was historically the first to come into being, at the end of the nineties.
The lens‘ optical efficiency is calculated in the references linked to the wearer‘s eye, centred on the Eye Rotation Centre (or ERC), the only point that remains immobile when the eyeball rotates in its orbit. To model this efficiency, the lens has to be positioned exactly within this reference, in terms of both distance and tilt.
Tilt is generally described by means of two angles: the pantoscopic angle (between the plane of the lens and the vertical plane, when the wearer is in a primary gaze position), and the face form angle, defined as the angle between the plane of each lens and the plane of the frame. The first method used to define distances that appeared on the market consisted of measuring, on an image, the vertex distance (which separates the lens from the summit of the cornea), and then calculating, using an anatomic model of the eye, an approximate position of the ERC.
The direct measurement method, used by Essilor since 2009, is based on the measurement of several visual axes, the intersection of which determines the ERC. Each visual axis is measured using an image, on which the gaze point and the corneal reflection can be positioned exactly. (Fig.1)
Fig. 1: The ERC is the intersection of gaze axes.
These lens wearing conditions have a direct impact on its efficiency:
- at the control points wearer power is different from the power measured using the frontofocometer, which explains the double-labelling of the lenses
- on the complete design: the power and astigmatism for each gaze direction are modified by wearing conditions. Also, when the ERC is known, eye directions correspond exactly to those actually used
by the wearer.
This effect is present with all types of lenses, single-vision and progressive alike. (Fig. 2)
Fig. 2: Effect on optical efficiency when the ERC is taken into account: Left: lens with personalised ERC - Right: standard lens.
Since the year 2000, the deployment of aberrometers has meant that higher order aberrations (HOA) of the eye can be used as a personalization parameter.
Their role in visual efficiency is still today the subject of research, which is looking at, for example, their distribution, their shape and their stability.
The measurement is taken by selecting a multitude of light beams, each deviated by a specific part of the eye. It is the measurement of the beam deviation for each direction that is used to recalculate the entire wavefront that is characteristic of the eye. This can be modified by the eye‘s seeing conditions (proximity, gaze direction, ambient light, etc.).
Aberrations provide additional information to modelling of the eye that can be used to modify the calculation of the lens‘ optical characteristics. They are measured in far vision, and used by some manufacturers in addition to the subjective prescription.
Other designers use the aberrometric measurement for near vision in their progressive lenses.
However, it is not possible in ophthalmic lenses to compensate exactly for ocular HOA for all gaze directions.
In order to integrate lens usage into their design, a personalization method that appeared only recently covers the wearer‘s posture and behavioural parameters.
The wearer‘s natural lowering of the head when in a reading position is measured by the differential between posture for far vision and for near vision, by means of identification in real time of the head‘s
position. Comfortable reading distance can be measured by means of a tablet held by the wearer. The device defines the distance that separates the eyes from the tablet.
These two parameters help to position near vision zones, with the head lowered and in lateral position (progression length and inset).
Visuomotor strategy characterises the wearer‘s propensity to perform wide-ranging movements of the eyes or head. The measured eye-head coefficient characterises the movements that the wearer makes when the visual stimulus appears, which is produced by light sources placed on either side of the straight-ahead position. An eye-head coefficient close to 0 characterises a visionaut person ("eye mover"), whereas a coefficient close to 1 characterises a cephalonaut person ("head mover"). This coefficient is used by a series of lenses on the market: for a visionaut person, a design with wider fields will be calculated, where the enlarging effect is given priority, whereas for a cephalonaut one will seek to minimise swimming effects since the head is highly mobile.
The dominant eye has very recently also joined the list of personalization criteria: its measurement is close to the classic optometric measurement. The wearer looks at a target through a hole and the straight line formed by the target and the hole passes through the dominant eye. This measurement provides an extremely important physiological measurement. In association with the reading comfort
distance referred to earlier, it is taken into account to make the binocular calculation for lenses in the Varilux S4D® product. Experiments have shown that optical efficiency provided to the dominant
eye plays a major role in reaction time to peripheral visual stimulation: this characteristic is used to maximise binocular efficiency of the lenses. (Fig 3)
Fig. 3: The personalization parameters used in the design of Varilux S4D®.
As we have just seen, consideration of new criteria, specific to each individual patient, is now a market reality, involving everyone in ophthalmic optics:
ECPs include in their sales processes a complete measurement system that has to be robust, precise and as representative as possible of real life situations.
Communication systems between the ECP and manufacturers must evolve in order to transfer new data.
Lens manufacturers use individual lens calculation methods and Digital Surfacing to achieve the desired precision in the lens manufacturing process.
Finally, as is the case for standard lenses, the mounting of lenses into frames, and the adjustment and stability of the equipment in wear are fundamental in order to offer our patients the best possible efficiency and improved comfort.
Personalization parameters, which are increasingly present in our business thanks to the combined progress made in terms of in-store measurements, the processing of „wearer“ data and optical design and
manufacturing processes, are fundamental to lens efficiency. The measurement of new parameters relevant to individual wearer perception will add to the knowledge that ECPs have of their wearers,
enabling them to offer increasingly innovative designs and new visual benefits.