L&T: RxPertise

Feb
2013

Eyes on the Road:

Helping Drivers See Their Best


By Palmer R. Cook, OD

Operating a motor vehicle is a rite of passage through much of our society. We all have seen significant and previously undiagnosed refractive problems that are finally revealed by the vision screening that’s part of the teenager’s first-time application for a driver’s license. Certainly at age 16 or so, applicants who have vision problems are aware that they do not see as well as their peers and may have failed multiple school screenings in previous years. Yet even the vanity of youth is trumped by the desire to operate a motor vehicle.

At the other end of the age spectrum, many seniors feel devastated when vision, mental capacity or response time is impaired to the point that driving would endanger the patient as well as others. With seniors the issue is usually not vanity. Most commonly for seniors, it is related to feelings of loss of independence.

As ECPs we must consider the importance of drivers’ vision for the safety and welfare of those who operate motor vehicles and also for passengers, pedestrians and the population at large. Most ECPs are careful and conscientious in prescribing and fitting eyewear for patients who drive. They caution against using sunglasses for night driving and alert those who cannot meet the legal vision standards for operating a motor vehicle. However, most drivers would also appreciate knowing ways to see more safely, avoid fatigue and reduce their potential for auto accidents.

WATCH OUT FOR THE GLARE BEARS

Glare is a general term for useless, sometimes uncomfortable light that interferes with the visual task at hand. There are many types of glare including blinding glare, central glare, eccentric glare and veiling glare. There are several kinds of glare that commonly interfere with safe driving. You might think of them as the “Three Glare Bears.”

BLINDING GLARE
“Papa Glare Bear” (aka dazzle or blinding glare) is an overabundance of otherwise useful light. Using good quality sun lenses addresses the “too much light” issue for daylight by cutting out about 82 percent of light that would otherwise enter the eye. For night driving, the most common causes of blinding glare relate to oncoming headlights. This is especially troubling for senior drivers because of their increased glare recovery time.

VEILING GLARE
“Mama Glare Bear” is sometimes called veiling glare, and may come and go because you are not always using the same kind of light. For example, when driving away from the sun the film that often builds up on the inside of your windshield is nearly invisible, but when you turn so bright sunlight falls on your windshield, the film can glow so brightly that your view is dangerously “veiled.” By keeping your windshield clean inside and out, this Mama Glare Bear trick is greatly reduced or eliminated.

Veiling glare also arises from light falling on your auto’s dashboard and then reflecting into your eyes. This useless light can seriously obscure your view of traffic conditions ahead. Serious and sometimes fatal accidents have been caused by light reflected in this way. The way to tame this problem is to use polarizing lenses when driving under daylight conditions (see Fig. 1).

GHOST IMAGE GLARE
“Baby Glare Bear” creates ghost images that interfere with night driving. Oncoming headlights, illuminated signs, traffic signals and overhead street lamps are reproduced as ghostly images by light reflected within the lenses. These reflections are annoying, distracting, fatiguing and at times, downright dangerous. They interfere with safe, comfortable driving. Using anti-reflective (AR) lenses is the best way to address ghost images. AR lenses not only reduce ghost images, they also increase the brightness of the light that makes up the major ray bundle, giving drivers a significant additional safety margin by extending the effective reach of the headlights by eight percent or more. AR lenses don’t reduce blinding glare or veiling glare other than that resulting from lens reflections.

UNDERSTANDING POLARIZING LENSES
Much of the glare that troubles motorists is reflected. Light vibrates vertically, horizontally and at all angles in between. It even vibrates clockwise and counterclockwise, which allows us to use circularly polarizing filters to view today’s 3-D movies. Light reflected at Brewster’s angle is linearly polarized and can therefore be blocked by a polarizing filter. (Brewster’s angle is the angle of incidence that causes maximal linear polarization of reflected light.)

The tangent of Brewster’s angle is the index of refraction of the reflecting material. This angle is about 56.88 degrees for a reflecting surface of index 1.53, so theoretically 100 percent of light reflected at that angle can then be blocked by a 100-percent efficient, linearly polarizing film embedded in a lens. This 100-percent polarization is decreased by the chromatic aberration of the reflecting material. It is also reduced if the exact angle of incidence is not Brewster’s angle. Polarization for other angles, especially for steeper angles from about 75 degrees to 90 degrees is reduced. Polarized light reflecting from horizontal surfaces outside the auto is effectively and selectively blocked by polarizing sun lenses, as is the light reflected from the top of the dashboard (Fig. 1).

Many auto dash displays and instruments are now polarized, as are the speedometers that are projected ahead of the windshield. Some of the early electronic dash displays used a horizontal angle of polarization; as a result they could not be easily viewed with polarizing sunglasses. Today, except for the projected speedometers, auto engineers have polarized the instruments and displays at an oblique angle or vertically, so they work well with polarized sun lenses. The windshield reflects the projection speedometers as shown by the white paper reflection in Fig. 2, so they will not be very visible with polarized sun lenses.

It’s a good idea to have a loaner set of plano polarizing lenses in a frame so your patients can “check out” their dashboard in advance. Some gas pump screens are also inappropriately polarized. It’s easier to see those types of screens by tilting the head to one side. That’s a minor accommodation for the comfort and safety benefits of polarizing.

Although not all reflected glare is polarized or partially polarized, polarizing lenses have no equal in selectively reducing problems with reflected glare. Manufacturers also use a tint to bring the total transmission down to a level suitable for use in bright sunlight. 

PRESCRIPTION CONSIDERATIONS
Driving during daylight hours and driving after dark are both mostly photopic (high illumination) rather than scotopic (low level illumination) tasks. Even so, optometrists and ophthalmologists often check their patients’ refractive errors under low room as well as high illumination when prescribing glasses for night driving and sunglasses. This is especially important for patients with large and active pupils because there is often a shift toward more myopia or less hyperopia when the pupils dilate. By taking this shift into account, distance vision may become clearer and more comfortable for driving after dark or with darkly tinted lenses. For patients with small pupils and/or with two diopters or less of remaining accommodation, this myopic shift is reduced, but for younger patients you should be cautious about dispensing sunglasses fabricated to the Rx written for clear lenses unless the prescribing doctor has noted “ok for sunglasses.”

FRAME CONSIDERATIONS
The California Driver Handbook under Additional Driving Laws/Rules states: “Do not wear eyeglasses with temples wide enough to keep you from seeing clearly to the sides.” This is common sense advice and should be respected in all states. Wide temple frames are attractive and popular. Since the pantoscopic angle cannot be varied on these frames, individualized, digital lenses should be used for dress eyewear, and driving glasses with narrow temples should be a part of the patient’s eyewear wardrobe.

Frames with wide eyewires are commonly used for strong myopic corrections to reduce weight. Strong hyperopic prescriptions have an optically created blind spot (Fig. 3). In either case, using a frame with significant wrap can improve peripheral vision. If a frame with significant wrap is used, be sure your lab makes the correct power, PD and prism adjustments to the prescription when the eyewear is fabricated.

LINES ON LENSES
It’s axiomatic that any impediment to clear vision is not a good idea for those who operate motor vehicles. When your patient’s lines-of-sight pass through the bifocal line, trifocal line or slab-off line, a bi-prism effect results that gives double vision in each eye separately. Although most glasses wearers find that lowering their chin a bit resolves the problem with bifocals and trifocals, the all-the-way across slab-off line can be disconcerting, particularly when using the external rearview mirrors.

CASE HISTORY
A patient recently dispensed with her first PAL correction (including a slab-off in the left eye) returned to her optometrist complaining of problems related to the slab-off line. The optometrist made up her Rx in a trial frame, using a D-seg, trial lens add. She found that although she could see clearly and comfortably at distance, she could only see clearly and comfortably at near when the doctor used an appropriate BU prism to demo the effect of the slab-off at near. The result: The patient understood the slab-off was required and a needless, incorrect remake was avoided.

The slab-off line had been placed at the prism reference point. By specifying it several millimeters, the patient can achieve the needed prism balance at near and easily look over the line to use her left rearview mirror.

ANTI-REFLECTIVE LENSES

Anti-reflective lenses improve the optical performance of every lens prescription, every lens design and every lens material. Prescribing AR lenses for night driving reduces troubling ghost images and extends the reach of the headlights. For both twilight (mesopic light level) conditions as well as for night driving, they give clarity, comfort and a possibly critical margin of added safety.

TINTED LENSES
For daylight driving, tinted lenses increase comfort and reduce fatigue. For night driving, the use of any tint will have the effect of dimming the headlights by the same percent as the absorption value of the tint itself. Worse, it also dims the taillights of cars ahead, traffic signals, reflective and illuminated road signs by the same percent. A tinted lens may also reduce visual acuity and needed detail (a pedestrian) at the fringe of the headlights’ illumination pattern or along the side of the road.

For the best margin of safety while night driving with prescription lenses, AR with no tint is the appropriate recommendation. From an anecdotal standpoint, some doctors feel that lenses with a transmission as low as 82 percent are well-tolerated for night driving. If you are an optician and add a tint that was not prescribed, be sure that the laws in your state permit this. You should also carry adequate malpractice insurance to protect yourself in case of an incident that could be related to the tint.

Because light lost by reflection increases as index increases, the overall transmission of non-AR, higher index materials, especially when tinted, should be given careful consideration before recommending them for night driving (see Fig. 2).

The far right column compares the relative brightness of ghost images of oncoming headlights for non-AR, higher index materials compared to non-AR standard plastic. This clearly shows the increased importance of using AR with higher index materials.

VISUAL FATIGUE

Prolonged freeway driving, especially for those who only occasionally do long distance driving, leads to both general physical fatigue and visual fatigue. Physical fatigue is related to extended periods of immobility, failure to occasionally open the windows for fresh air and boredom.

Visual fatigue often causes reduced eye movements (i.e., staring at the vanishing point of the road straight ahead while details to the side rush past faster and faster as they approach you). The onset of visual fatigue is also accompanied by a lower blink rate, a reduction in stereopsis and mild to moderate ocular discomfort. Patients prone to dry eye problems may fatigue more quickly (especially during winter months) because relatively dry, cold air from the outside is heated and blown about the interior of the car absorbing available moisture rapidly.

For a huge portion of our population, the operation of a motor vehicle for pleasure, profit or necessity is part of life from adolescence to the age at which we are no longer physically or mentally capable. Whether your patient drives a little or a lot, he or she deserves the best vision care and eyewear technology you can offer. From the vision exam through dispensing of the eyewear, ECPs have the technology and ability to help their patients enjoy the best in safety and comfort for driving. Patients appreciate such detailed care—their lives depend on it.


Palmer R. Cook, OD, is director of professional education at Diversified Ophthalmics, Cincinnati, Ohio.

Vision-Related Suggestions for
Safer Driving for your patients
  1. Keeping the windshield clean inside and out is a big help. Windshield filming degrades the retinal image and can act as an extended light source under some conditions almost completely blocking the driver’s vision.
  2. Plastic headlight lenses are prone to optical deterioration that can severely limit your time to react. Various systems are available to at least partially correct this deterioration. These links may be helpful: wikihow.com/instantly-remove-oxidation-from-headlight-without-sandpaper and wikihow.com/clear-foggy-headlight-covers. Alternatively a car dealer can replace the fogged and pitted covers.
  3. Spectacle lenses and contact lenses should be absolutely clean for driving.
  4. Car humidifiers are available for prolonged driving for contact lens wearers or for those prone to dry eye problems. Using the recirculate selection on your car’s ventilation system keeps the air in your car more moist. If your windshield and windows fog excessively, use an anti-fog lens cleaner on the inside surfaces of the glass or try a clean cloth with a few drops of glycerin to make the glass fog resistant. Wearing good quality sunglasses that curve or wrap so they partially block air currents and adjusting the vents so air is not blowing on your face is also helpful. Using clear lenses in frames that also curve to restrict airflow across your eyes can also be helpful for long distance night driving.
  5. Keep headlights clean to increase your own safety margin at night, and clean the taillights to help the driver behind you accurately underestimate how close he is to you.
  6. Avoid fatigue and “highway hypnosis” by keeping your eyes moving as you drive and taking stretch breaks regularly.
  7. If your lens prescription is designed to relieve strain, and you wear your glasses intermittently, you will be more fatigue resistant on long trips if your wear your lenses from the outset, rather than waiting until you begin to tire.
  8. More than eight percent of the population has some form of color vision abnormality. gas permeable contact lenses have been used to help those with deutan (red-green) color vision difficulties. For protan (reduced red sensitivity) color vision problems, lenses such as the Coppertone HEV Brown design that is both polarized and has high transmission in the red end of the spectrum can be helpful. This lens also has low transmission in the high-energy visible(HEV) range of the spectrum, that may be damaging the eye.
  9. A lens technology that is particularly designed for driving is the Drivewear lens by Younger Optics. It has the advantages of Transitions’ photochromic technology, and it is polarized to reduce both the veiling glare of windshield reflections and the glare reflected from external sources. It has an absorption range that is appropriate from overcast daylight to moderately bright sun conditions. For extremely bright sun conditions many drivers should have a darker polarizing lens. The ultraviolet level inside a car’s passenger compartment does not achieve the maximum darkening of Drivewear. On the other hand, most people appreciate the convenience of the somewhat higher transmission of Drivewear when they are alternately in the car and in well-lighted malls and stores.
  10. Many auto dash displays are now polarized as are the speedometers that are projected ahead of the windshield. Engineers have designed auto instruments and displays so they work well with polarized sun lenses. Projection speedometers that seem to float over your hood may be less visible with polarized sun lenses. When selecting your sun protection for driving, ask how to “check out” your car’s instruments with polarizing lenses. Polarizing sun lenses offer comfort and safety advantages for drivers by selectively filtering reflected glare.

—PC

 

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