Blinded by the Night
Product Spotlight ‐ ZEISS DriveSafe
Release Date: October 1, 2022
Expiration Date: September 27, 2023
Driving is an intensely visual task that places exceptional demands on our visual system, especially in mesopic driving conditions. Helping patients experience improved visual performance when driving at dusk and night is of great interest to ECPs. I recently gave a night vision course, and the audience was keenly interested in learning about night driving solutions for their patients. As ECPs, we hear about the uncomfortable and distracting effect of poor visibility and oncoming headlights when driving at night. Driving in these conditions is worse with today's brighter blue wavelength emitting headlights. Night driving is more challenging for spectacle wearers and even worse with standard progressive lenses and the bright blue rich headlights of our times.
Upon completion of this course participants will be able to:
- Explain the differences between scotopic, mesopic and photopic vision
- Describe the effect of mesopic illumination levels on vision
- Describe the three reasons night driving is more hazardous
Deborah Kotob, ABOM, is the Director of Education and Training for Jobson Medical Information. Her experience spans more than twenty years in the optical industry. During this time, her roles included optical boutiques owner, optician, optical frame sales, and over ten years in lens manufacturing as a Lens Consultant, Trainer, and LMS content developer. She lectures, trains, conducts webinars on a variety of optical and practice development topics.
This course is approved for one (1) hour of CE credit the American Board of Opticianry (ABO). One hour, Ophthalmic Level 2, Course STWJHI063-2
This is a product spotlight CE supported by an educational grant from ZEISS.
Driving is an intensely visual task that places exceptional demands on our visual system, especially in mesopic driving conditions. Helping patients experience improved visual performance when driving at dusk and night is of great interest to ECPs. I recently gave a night vision course, and the audience was keenly interested in learning about night driving solutions for their patients. As ECPs, we hear about the uncomfortable and distracting effect of poor visibility and oncoming headlights when driving at night. Driving in these conditions is worse with today's brighter blue wavelength emitting headlights. Night driving is more challenging for spectacle wearers and even worse with standard progressive lenses. As a result, many tell us they have stopped driving at night. But this isn't a viable option for most. Come November, we all turn back our clocks by an hour, and the days begin to get shorter until the sun sets earlier than 4:30 p.m. and rises as late as 7:20 a.m. The shorter daylight hours from November to March mean that many of us drive at dusk or in the dark during our work commute. Most eyeglass wearers experience discomfort and anxiety when driving in these challenging light conditions with poor visibility. Their fear is justified: Statistics reveal a much higher risk of fatal accidents in those conditions. Fortunately for the spectacle wearer, lenses can mitigate the discomfort glare from automotive headlights for drivers and improve spatial and temporal vision in mesopic (twilight) illumination levels to make night driving less stressful and safer. This course will introduce DriveSafe, a lens designed to provide optimal vision throughout the day while helping patients address three major night driving vision challenges related to illumination, discomfort glare and complex visual tasks.
Three Vision Challenges: Research confirms that unfavorable light conditions, glare and stressful visual tasks when driving have a substantial impact on the quality of vision driving. The findings indicate the complexity and necessity of designing lenses for safe driving in critical conditions.
I. ILLUMINATION CHALLENGE
A disproportionate number of fatal road injuries occur after nightfall; therefore, drivers are rightfully wary of driving in low-light conditions. As reported in Injury Stats, “While we do only one-quarter of our driving at night, 50 percent of traffic deaths happen at night. It doesn't matter whether the road is familiar or not, driving at night is always more dangerous. More than 42,000 people were killed in car crashes in 2020.” A study in the U.K. revealed that more than half of all fatal accidents occur after dark, although far fewer miles are driven at night. Furthermore, the study found the likelihood of death during an accident is twice as great when the accident happens at night. This study concluded that poor illumination was the principal cause of nighttime road accidents.5 Of the study participants, 50 percent found night driving stressful due to poor lighting and would welcome any system that improves nighttime visibility.6 However, this is a double-edged sword. While higher road illumination enhances the driver's visibility, visibility is diminished from these same high luminance headlights in oncoming traffic. While driving, the visual system must adapt quickly to different light levels. The pupil light response provides the fastest reaction to a change in illumination and significantly affects the performance of spectacle lenses. Minimum pupil size occurs at the highest levels of ambient illumination, at a time when drivers feel safest. The most hazardous driving time is after dusk or in darkness when light levels are low and pupils are large. Three general illumination levels are recognized: the photopic, scotopic and mesopic ranges.
Photopic vision is cone cell-mediated color vision in high illumination levels (daylight, 1 lx and higher) with peak wavelength sensitivity at 555 nm. Scotopic vision is rod cell-mediated dark-adapted vision with no color sensitivity in very low illumination levels (0.01 lx or lower such as during a moonless night sky) and peak wavelength sensitivity at blue green 505 to 510 nm. We don't use scotopic vision for night driving because some light is always available while driving at night. Instead, we use our mesopic vision in low light conditions such as twilight or dusk. Mesopic vision falls between the photopic and scotopic ranges with illumination levels between 0.01 and about 1 lx.
For night driving, we use rods and cones. Most night and twilight driving are illuminated at the mesopic level, and the most common driver complaints about poor illumination happen at this level. Acuity, color sensitivity and reaction times are reduced at the mesopic level compared to photopic levels. The pupil light response produces the smallest pupils at photopic light levels (miosis) and the widest pupil at scotopic levels (mydriasis). But pupil dilation is still quite significant at mesopic levels, and the dilation occurs at the cost of increased aberrations and decreased acuity. With a larger pupil size, an increase in the eye's high-order aberrations (HOA) reduces the retinal image's contrast. It changes the effective refractive error so that a different set of correcting dioptric powers are needed than the ones measured when the pupil is small.15
Rod photoreceptors provide scotopic vision. Since there is only one kind of rod, scotopic vision is only brightness without the sensation of color. The rods have poor acuity and are slow to respond, providing poor input to reaction time. Furthermore, achieving the best vision in scotopic light levels requires adapting for at least 20 minutes. Typical automotive headlights cast an illuminance of about 0.3 lx at a range of 150 m in the U.S. and 0.4 lx at a range of 50 m in Europe (in relation to different standards and norms), approaching the photopic range.13, 14
However, a driver's peripheral vision may be challenged to detect poorly illuminated hazards outside the illuminated patch provided by a car's headlights, even in mesopic versus scotopic illumination levels.
PROBLEM: VISIBILITY AND CONTRAST
At mesopic illumination levels, acuity and contrast sensitivity decrease intrinsically at the retina and the brain's visual pathways. In addition, mesopic illumination levels cause the pupil to dilate, allowing more light to transmit to the retina and reducing retinal image contrast. In the presence of fog and rain, the contrast of objects outside the car is also reduced by light scattered from water droplets in the air. When the visual system's physiological response is reduced by low light, and object contrast is further reduced by atmospheric effects, it is imperative that the optical performance of spectacle lenses be as good as possible. Yet the off-axis aberrations of single vision lenses and the intrinsic second-order aberrations of progressive lenses can further interact with the enlarged pupil to degrade image quality.
SOLUTION: LUMINANCE DESIGN® TECHNOLOGY
The traditional way of designing a lens is by following a “chief ray” at any point of interest on a lens, determining the curvatures of the lens at the points where the chief ray intersects the lens surfaces and calculating the change in dioptric powers according to the angles at which the chief ray strikes the surfaces. The traditional calculation assumes that the pupil has only a location, not a diameter. ZEISS introduced Luminance Design 2.0 technology to overcome that limitation in all SmartLife progressive lens designs. The new method of lens computation calculates dioptric powers using the entire beam of light that passes through the pupil opening, whose size is an age-dependent factor. SmartLife Lenses are general-purpose progressive lens designs optimized for the expected frequency and lighting level of various daily tasks. With DriveSafe lenses, the larger pupil sizes expected in mesopic conditions are factors included in the calculations and design optimization. Pupil size enlarges under mesopic light levels allowing a wider beam to transmit to the retina. Pupils constrict under brighter daylight conditions, only allowing a narrower beam through to the retina. With a progressive lens designed using traditional methods, the eye with a larger mesopic-size pupil will “sample” a larger part of the blurry transition along the border of areas of peripheral astigmatism, resulting in constricted viewing zones and reduced contrast. When looking through an area near the edge of a zone that is supposed to be clear, the effect is reduced contrast and smeared vision, resulting in decreased clarity. By compensating the progressive surface using Luminance Design, those errors are corrected in DriveSafe lenses, resulting in improved contrast and acuity. Although single vision wearers do not have to concern themselves with progressive viewing zones, single vision lenses, like progressive lenses, suffer from off-axis aberrations that decrease optical quality as the eye looks away from the center of the lens. Pupil size directly influences these aberrations, so the optimization of the single vision design using Luminance Design Technology also includes a dilated pupil in the optimization calculations.
MESOPIC PUPILLARY DIAMETERS
After careful consideration of the frequency and duration of various tasks and light levels via an illuminance weight factor, ZEISS established a varying pupil diameter based on the wearer's age for the optimization of SmartLife progressive lenses. Because the most troublesome driving conditions arise under mesopic conditions, ZEISS recalculated the frequency and duration of driving tasks at a lower light level for DriveSafe lenses. This resulted in the choice of a 4.3 mm pupil diameter for the DriveSafe progressive lens and 5.0 mm for the DriveSafe single vision lens. The slightly smaller pupil size used in the progressive lens Luminance Design calculations is a consequence of age-related miosis, in which pupil size declines throughout middle age. Because DriveSafe progressive lenses are primarily for presbyopes falling within the range of middle age, the database of pupil sizes according to light level contained correspondingly smaller values than the database used for the single vision design.
II. GLARE CHALLENGE
To illuminate the view in low light conditions, automotive lighting suppliers developed highintensity discharge (HID/Xenon) and LED headlights which are considerably brighter and Illuminate the road much better than their halogen predecessors. However, while drivers appreciated the heightened illumination and visibility, the increased brightness of these oncoming headlights causes disability glare (reduced ability to see objects close to the direction of the light source). Furthermore, newer LID and LED headlights emit more blue light than older halogen lights. This color shift increases the frequency and severity of the glare sensation experienced when looking toward a bright light.7 Older drivers often experience increased lenticular light scatter or have cataracts and therefore report more severe glare symptoms than younger drivers.8 Older drivers also have a slower glare recovery speed after being dazzled by bright oncoming headlights and lower contrast sensitivity.
Light is vital to sight, all life on our planet and our physical and emotional well-being. Light's spectral properties, exposure period, intensity and spatial distribution influence our circadian rhythm and cognitive capacities. But when illuminance increases suddenly, adaptation lags, and glare is the result. This problem is especially acute when background luminance is low, especially at night, and when storm clouds darken skies. The problem can be intensified by reflections from wet pavement that act like a mirror for overhead roadway lighting and headlights. There are different types of glare. It is the discomfort glare sensation that ZEISS research and wearer studies have confirmed can be alleviated with DriveSafe lenses.
PROBLEM: DISCOMFORT GLARE
According to Jan Theeuwes et al., in their Relation Between Glare and Driving Performance study, “The effect of glare on target detection performance on dark road stretches is large, and even relatively low intensities of 690 cd per headlamp (intensities that are typically considered to cause only discomfort and not the impairment of vision) cause a severe performance decrement...glare illuminance levels within the range that are generally agreed to cause only discomfort in practice, also causes a drop in object detection performance.” They go on to say that: “An important consideration is that a glare source that causes discomfort may result in a behavioral adaptation to reduce the discomfort of the glaring source. This behavioral adaptation, such as looking away from the glare source or fixating more than usual to the right side of the road, may lead to worse object detection performance. This poorer detection performance is not a result of the glare source causing a luminous veil over the scene (as is the case with disability glare) but because of a strategic adaptation to cope with the discomforting glare source.”
Discomfort glare is a subjective phenomenon caused by the presence of one or more bright light sources in the field of view with highly different illumination levels requiring a recovery or adaptation period by the visual system. Glare recovery adaptation takes longer for aged eyes; discomfort glare is a greater problem for older drivers.18, 19 Discomfort glare is worse in the presence of oncoming Xenon/HID or LED headlights compared to older halogen types. These new lamps have higher levels of blue light emissions, and studies have shown a relationship between higher amounts of blue light in the glare source and the degree of discomfort glare experienced.20 Discomfort glare is not only uncomfortable but also distracting, and distraction leads to unsafe driving conditions. Although any headlight may cause discomfort glare, the tight beam and bluish spectral shift make these modern light sources a greater risk for discomfort glare.13 The DuraVision DriveSafe antireflective coating has been designed for a light transmission spectrum that optimizes performance to ameliorate discomfort glare in the presence of HID and LED headlights. The maximum peak of the spectral intensity of a white light lies at 440 nm in the blue end of the visible light spectrum. On the other hand, the maximum sensitivity of the visual system under mesopic light conditions lies between the photopic peak of about 550 nm and the scotopic peak near 510 nm.21 The transmission of DuraVision DriveSafe is at a maximum for the mesopic range but decreases significantly for shorter wavelengths that are most likely to cause discomfort glare.
Compare: A ZEISS study including 50 subjects compared DuraVision DriveSafe coating efficacy against glare with DuraVision Platinum and one other AR coating. The three AR coatings' effect on traffic glare was tested and evaluated. The study results confirmed that the parameters of contrast sensitivity threshold, spontaneous eye blink rate (SEBR) and eye closure (squinting) align with the literature findings. When asked which lens was most comfortable for driving, DuraVision DriveSafe was preferred by more than 2 to 1 over the other coatings. DuraVision DriveSafe is anti-static to repel dust and offers superior scratch resistance and easy cleaning.
III. THE CHALLENGE OF COMPLEX VISUAL TASKS
Driving, day or night, presents a rapidly changing set of complex visual and attention requirements. Rapid shifts in the distance viewed to anticipate future turns and stops compete for our visual attention, including maintaining peripheral awareness of spatial location traffic flow and detecting potential threats from other drivers or road hazards.9 Further challenging our visual attention is the need to continuously check rear and side mirrors while rapidly identifying key information presented in multiple visual displays on the instrumentation panel, both straight ahead and to the side. In the name of safety, automobile manufacturers continue to increase the number of information sources in their cars, adding features such as proximity warnings and blind spot detection lights on side mirrors. The complexity of this suite of tasks forces frequent eye and head movement with concomitant changes in gaze direction, fixation locus and accommodation.10
The effect of increased driver visual attentional load is longer reaction times which was confirmed with presbyopic wearers of progressive addition lenses when greater eye and head movement were observed.11 In stressful conditions of poor visibility, reaction times increase, and the time spent changing fixation becomes even more critical. Research also has shown that even a small amount of night myopia (between -0.50 and -1.00 D) in the presence of a subcritical glare level of 0.4 lx decreases contrast sensitivity at night more than glaring LED headlights of 1. lx when vision is fully corrected.12
For single vision lenses, but even more for progressive addition lenses, it is vital to map the distribution of optical properties to the spatial and temporal composition of the environment and tasks. Drivers face conflicting requirements that compete for attention. The road view, the peripheral view, the instrument panel, and the side and rear-view mirrors must all be considered. This competing set of tasks requires frequent eye and head movement, gaze direction, fixation and accommodation changes. As analyzed by ZEISS, driving demands led to the development of driving designs for both single vision and progressive lenses.
PROBLEM: ACCOMMODATIVE/ CONVERGENCE STRESS AND DYNAMIC VISION
The dynamics of the vision process while driving include changes in gaze direction, convergence and accommodation. The dynamics of cognition follow the dynamics of vision, but the cognitive modifications relate to the focus of attention for a given task. The focus of attention and the visual dynamics are powerful influences on driving safety.22 Each dynamic task element has unique requirements. For example, the view down the road requires parallel lines of sight for the two eyes, i.e., there is no convergence and therefore, no accommodation. Ideally, a spectacle lens for this purpose should have a very wide field of clear far vision.
Similarly, the view through rear-view mirrors requires no convergence or accommodation, but the field of view is small. On the other hand, locating the mirror during a fast saccadic eye movement is critical so that as little time as possible is lost in the effort. This requires that spectacle lenses minimize spatial distortions and present little or no blur in areas typically used to look through the mirrors. Again, this requirement can only be understood according to the amount of head movement used together with the eye rotation angle that defines the final coordinates of the gaze.
On the other hand, viewing an instrument panel requires both convergence and accommodation in most drivers (the exception is very advanced presbyopes requiring high addition powers, who rely entirely on their lenses for refractive dioptric power). Therefore, one must be able to locate the object pre-selected by a change of attention. Research reveals that experienced drivers focused more on trajectory planning than on fixating down the road.23 For example, to check speed, a driver must plan to look at the speed indicator, then find it through a change of gaze angle accompanied by convergence and accommodation. Looking for the temperature controls on the center panel requires a different planned visual trajectory and accurate, fast localization. If the progressive lens is not designed optimally, these changes of gaze and convergence may be followed by a corrective change in head position to achieve the best focus. Preferably, a spectacle lens enables this to occur efficiently on the first attempt by providing a clear, wide intermediate field of view that does not present visual obstacles to effective spatial localization.
To understand these requirements better, ZEISS commissioned a study by the Research Institute of Automotive Engineering and Vehicle Engines Stuttgart (FKFS)24 using advanced full-motion driving simulators. The simulators included head and eye tracking systems to observe a driver's visual behavior. In addition, a real-world course was developed for further testing. Data from 44 subjects were recorded, totaling more than 33 hours of net driving time. The study found that drivers focus on the street ahead and distant moving objects about 97 percent of the time, look at the dashboard 2 percent of the time and alternate viewing dynamically between several rear-view mirrors 1 percent of the time. If elapsed time were the only consideration in lens design, it would seem obvious to design driving lenses only for distance vision. But the situation is complicated because it is during those moments of quick changes in task and attention that increased reaction time might lead to an accident. In the visual dynamics of driving, head and eye movements are coordinated. Progressive lens wearers must move their heads more than single vision wearers to locate clear zones free of aberration or with the correct addition power for a given task. The study by FKFS found that progressive lens wearers make more horizontal head movements to keep their gaze in clear viewing zones while looking at different regions of interest (ROI). Furthermore, progressive lens wearers tend to hold their heads more upright and point their heads straight ahead, indicating that a reduced distance field of vision in progressive lenses affects head position. A further finding is that the closest object viewed on the instrument panel is approximately 75 cm away from the driver's eye, indicating that the near zone of a progressive lens, designed for a much closer distance, is virtually unused while driving,
SOLUTION: DRIVESAFE DESIGN TECHNOLOGY
The DriveSafe design provides excellent visual dynamics, extra width and clarity for distance vision. The periphery is optimized for distance visual acuity in the single vision lens. Increasing the width of the distance zone in the progressive design allows easier side mirror views. The start of the progressive design is shifted upward slightly relative to the fitting cross; this helps relieve some of the stiff, unmoving head posture observed in the FKFS simulator studies. The extra width of the distance zone helps smooth the onset of addition power, and the design's longer corridor decreases the slope of power increase. Important because the power change rate is directly related to the surface and marginal astigmatism level in a PAL. When taken together, the span of the distance zone is increased, and the intermediate zone is expanded in all directions, even slightly upward. At the same time, the somewhat smaller near zone is offset by the longer corridor combined with an upward shift, providing sufficient near vision performance for daily life activities, including driving—the successful fusion of the design results in a 14 percent increase in far-field distance vision zones.
These design characteristics lead to a broader view of the road and easy access to side mirrors.
In addition, they support faster and easier switching between the dashboard instruments and other driving tasks. DriveSafe designs enhance comfort and reduce stress for driving at night. And the bonus is that both the single vision and progressive DriveSafe lens designs are well suited for all-day use in all kinds of activities.
SOLUTIONS FOR ENHANCED COMFORT AND SAFETY
ZEISS DriveSafe lenses address three major visual challenges of driving:
- Reduced visibility in low light conditions.
- Discomfort glare induced by modern high-intensity headlights.
- Visual stress due to complexity of dynamic vision while driving.
Luminance Design Technology preserves wide and clear viewing zones even with pupils enlarged during low-light driving conditions. The DuraVision DriveSafe coating relieves the problem of discomfort glare caused by modern headlights like LED and Xenon/HID. DriveSafe lens designs are engineered for increased comfort and reduced stress during demanding visual driving tasks. All three work together to enhance safety, especially during hazardous driving conditions created by low light levels, fog or rain. Clinical trials25 compared the effectiveness and acceptance of DriveSafe lenses. Factors assessed included overall satisfaction while driving, driving in the dark and twilight, dynamic vision in near, intermediate and far vision, perception of colors and dazzle from headlights. Subjects rated DriveSafe high in all categories.
Additionally, subjects rated DriveSafe lenses high for general-purpose activities when working in the office or home. ZEISS DriveSafe lenses are designed for enhanced driving while enabling a full range of daily life activities. Patients are eager to find help to improve their vision while driving, and ECPs are keen to help. With DriveSafe, ECPs can help make driving at night safer and more comfortable for patients.
Sources provided upon request.