The positioning of the near-vision zone: a critical factor
Reading is one of the most important near-vision activities. Every person will adopt a distinct body, head and eye posture and will explore the field of near vision with gaze dynamics that are unique to that person. All of this is governed by a complex coordination of eye movements, including a lowering of the gaze, fixations, and saccades, designed to bring the image of the words into the fovea so the text can be read.
To ensure proper visual comfort, the near-vision zone on a progressive lens must be optimally located at the very spot where the wearer directs his or her gaze and explores the lens during reading. That zone demands maximum visual acuity, and its dimensions must correspond to the eye’s exploration of the zone.
In the past, the near-vision zone was positioned on progressive lenses based on average wearer behavior or in accordance with optical parameters such as add power, ametropia and/or reading distance. Today, with the Varilux® X seriesTM lens, it is now possible to personalize the position of the near-vision zone on each lens, based on each wearer’s exact behavior in terms of reading posture and overall near-vision behavior.
« Essilor’s research teams developed a special vision task that can be performed without correction and that accurately reflects the wearer’s reading posture. »
Measuring each wearer’s Near-Vision Behavior (NVB)
Knowing a wearer’s reading posture and gaze dynamics is certainly very helpful, but obtaining that information is no simple matter. The difficulty comes from measuring the wearer’s natural posture, i.e., the posture that he or she would adopt without optical correction... but it’s precisely that correction that presbyopes need in order to read.
To solve that problem, Essilor’s research teams developed a special vision task that can be performed without correction (for uncorrected ametropia ranging from -10.00D to +7.50D in near-vision power) and that accurately reflects the wearer’s reading posture. That task, called «pseudo-reading», consists of observing and tracking a large-size object – i.e. one that does not require visual acuity. The object is blue against a white background and shown on a tablet. The movement of this target across the screen is similar to the average reading behavior of an adult, with fixations of 233 milliseconds and saccades of 6.3 characters.[3 ]
In practice, the wearer grasps the tablet, position it naturally in front of them – as they would with a document they are reading – and follow the target’s horizontal movement with their gaze, line by line. The measurement lasts about 17 to 18 seconds in all, depending on the length of the wearer’s fixations, and the wearer is guided by the target’s movement: he or she can predict the saccade movements that need to be made with the help of gray dots shown on the screen background, which anticipate the target’s movement (Figure 1).
Fig. 1: Measuring Near-Vision Behavior.
It should be noted that this measurement is not the wearer’s absolute behavior, but rather his or her relative behavior in response to the pseudo-reading task. That behavior correlates exactly with the wearer’s actual behavior, as has been shown in validation studies for the measurement protocol conducted on sizable samples of presbyopes. That calculation is then used to establish the actual positions of the wearer’s head and eyes.
In the course of this exercise, the horizontal and vertical head movements are measured in real time so as to determine the wearer’s head posture and gaze position at all times. From this, we obtain four critical data points:
- Three of these datapoints – the downward gaze angle, the lateral offset and the reading distance – describe the wearer’s posture, which is ultimately measured by the average posture during the pseudo-reading task and is called the NVB Point (Figure 2);
Fig. 2: The wearer’s posture data points. The NVB Point: Lowered gaze angle, lateral offset, reading distance
- The fourth datapoint, referred to as the wearer’s NVB Ratio, describes how the wearer adjusts his or her gaze vertically during the entire measurement. That ratio is close to 0 for wearers who have a strong tendency to lower their eyes whenever they move to the next line, and therefore change their head and body posture or the tablet position only slightly; the ratio is close to 1 when wearers maintain a static gaze position, i.e., they have a strong tendency to change their posture and/or the tablet position vertically while reading (Figure 3).
Fig. 3: The wearer’s behavior data points. The NVB (Near-Vision Behavior) Ratio.
Method for Measuring Near-Vision Behavior (NVB)
In order to create a personalized lens, each wearer’s near-vision behavior needs to be measured. This can be done using a tablet, which can be connected to an electronic measuring column or used on its own. The measurement is taken as follows:
1) The wearer’s frame is positioned in a clip that is used to define the frame’s position in space and, by extension, the wearer’s posture and head movements (shown in Figure 1).
2) The wearer’s baseline far-vision position is measured in primary gaze position, using either a measuring column (such as a Visioffice®) or a tablet, by taking two photos – a front picture and a three-quarter view – that can be used to calculate the primary gaze position.
3) The measuring process is demonstrated and explained to the wearer, so that he or she understands the task to be accomplished.
4) The wearer grasps the tablet and gazes at the blue dot in the centre until detected by the camera; then the movement of the target is activated and the gaze position and movements are continuously recorded (Figure 4).
5) The measurement is validated and the near-vision posture and behavior datapoints are saved.
Fig.4: Measuring the wearer’s Near-Vision Behavior during a pseudo-reading task.
The Near-Vision Behavior technology
Once the measurements have been taken, the data used to personalize the lens must be forwarded. The data is sent using a seven-digit alphanumeric code that combines two pieces of information:
- The NVB Point, which is the wearer’s average gaze position during the measurement, representing the wearer’s reading posture;
- The NVB Ratio, which is the distribution of measurements around the NVB Point and represents the wearer’s dynamic near-vision behavior.
The lens can then be personalized in a three-step process:
- The first step is to use the wearer’s data (prescriptions, interpupillary distance, position of the eye’s centre of rotation) and the conditions in which the lenses are worn (shape and size of the frame, lens-eye distance, pantoscopic tilt and wrap angle) combined with the characteristics of the lenses to be produced (front surface, geometry and refractive index).
- The second step is to identify the optimal position of the near-vision zone on the progressive lens, based on the wearer’s posture (as indicated by the NVB Point). Information on ametropia, prismatic effects and binocular vision is taken into account during this stage of the process.
- The third step is to enhance the progression profile based on the wearer’s gaze dynamics, in light of the NVB Ratio. The objective is to adjust the size and shape of the progressive lenses’ near-vision zone in accordance with the wearer’s vertical exploration of that zone.
Figure 5 shows an example of an optimal near-vision zone position and shape on a Varilux® X seriesTM lens, based on a wearer’s near-vision behavior. We see how, as a result of this optimization, the position of the near-vision zone on the lens (the point where the add power is at 100%, shown by a blue cross) has been modified, and the size of the so-called arm’s-length vision zone (from 85% to 60% of the add power) has been adjusted.
Fig. 5: Example of a near-vision zone on a Varilux® X seriesTM progressive lens that optimally reflects the wearer’s near-vision behavior.
The exact values for progression length and inset for the near-vision zone can only be defined and forwarded to the optician once this lens calculation has been performed.
Mapping near-vision behavior
In order to show these findings in simple graphic form, Essilor’s Research & Development specialists designed a way to map each wearer’s results on a graph of possible behaviors (see Figure 6). On this graph:
- the horizontal axis shows the wearer’s average posture while reading, expressed as a downward gaze angle (from 12 to 30 degrees);
- the vertical axis shows near-vision behavior, i.e., the dispersion of the gaze direction (between 0 and 1).
Fig. 6: Mapping of Near-Vision Behavior.
Thus, a wearer who adopts a sharp downward gaze while reading and primarily uses his eyes to explore his near-vision vertically will fall at the bottom-right portion of the graph. By contrast, a wearer who lowers eyes only slightly to read and primarily changes posture or moves the tablet while reading will fall at the upper left of the graph. Every kind of behavior between these two extremes can be located on the graph.
Moreover, this mapping process includes a colour code; there is a significant effect on the optical design of the lens only if the colour codes for two measurements can be differentiated by the eye. This offers an immediate way to verify that the measurements are reproducible.
Thanks to multiple measurements performed on numerous presbyopes, we were able to show that each wearer’s behavior is reproducible and represents an appropriate datapoint for customization, since it is both specific to each individual and differentiating.
Once the measurement has been taken, the optician provides Essilor with the alphanumeric code for the wearer, so that a corresponding custom-designed Varilux® X seriesTM lens can be manufactured. We should note that the alphanumeric code generated for each measurement is encrypted and can be deciphered only by Essilor’s computer systems. Two codes that are very different may represent two very similar vision behaviors; conversely, two codes that are similar may correspond to very different behavior patterns. Whatever the case, each code contains all the information needed to manufacture a lens that corresponds very accurately to the wearer’s needs.
Some simple illustrations for explaining a near-vision behavior profile to wearers
A very simple communication tool has been designed to give wearers an easy explanation of the principle behind the NVB customization process. It consists of four very simple illustrated scales that show the four aspects of wearer behavior: lowered gaze, reading distance, lateral offset and vertical movements during reading. These illustrated graphics appear on the tablet screen after the measurements have been taken; cursors automatically indicate where the wearer’s behavior falls on the scales (Figure 7). This gives opticians an easy way to tell each customer about his or her specific near-vision behavior and explain how that behavior will be taken into account in designing the customer’s personalized progressive lens. Because while the near-vision behavior measurement is important, what’s even more important is its value to the wearer!
Fig. 7: Illustrations of the wearer’s near-vision behavior profile: 1) Lowered gaze, 2) Reading distance, 3) Lateral offset, 4) Vertical movements.
An innovation hailed by wearers
To attest to the performance of the new Varilux® X seriesTM lens with NVB personalization, Essilor conducted a multi-site study with a large sample group of wearers to evaluate the lens’s overall performance and key benefits.
The wearers were asked to score their vision quality under a range of circumstances on a scale of 1 to 10. Figure 8 presents the results of this study, showing that wearers enjoy the quality of their vision in all circumstances, to an impressive degree.
Fig. 8: Vision quality as assessed by the wearers of Varilux® X seriesTM lenses with NVB option (percentage of wearers with vision clarity or average clarity and width rated from 7 to 10 on a 10-point scale).
What’s more, wearers expressed a clear preference for the NVB version of the Varilux® X seriesTM lens over the traditional version without NVB customization. For example:
- The transition from far vision to near vision was easy for 94% of wearers and was very easy for 84% of wearers with the Varilux® X seriesTM lens with the NVB option, compared to 86% and 76% respectively for the traditional version of the lens (Figure 9).
- The adaptation to their Varilux® X seriesTM lenses with the NVB option was completed in less than one day for 82% of wearers and even less than 1 hour for 71% of wearers, compared to 75% and 61% respectively for the traditional lenses (Figure 10).
Fig. 9: % of wearers who experienced an easy (rating from 7 to 10) or very easy (rating from 8 to 10) transition from distance to near vision.
Fig. 10: % of wearers who experienced a quick or very quick adaptation.
These results demonstrate the extent to which measuring the posture and near-vision behavior of wearers can further improve their comfort and satisfaction with their progressive lenses.
Varilux® X seriesTM lenses that are personalized for the wearer’s Near-Vision Behavior are pushing the boundaries of a progressive lens to provide even more benefits than their conventional counterpart, including an extended field of near vision. This customization based on near-vision behavior represents the first time that the power progression on a progressive lens has been tailored to the wearer’s precise use of the lens in near vision. The wearer’s posture, lowered gaze, lateral adjustment of the reading plane and vertical movements during near-vision exploration can now be taken into account when designing his or her progressive lenses. That’s a considerable advance, hailed by wearers, and promises even greater visual comfort for presbyopes who wear progressive lenses. •
Note from the authors: this article is an adaptation from the White Paper published by Essilor’s R&D.