Optimising Vision Behind the Wheel

Field of view is the area through which vision is possible. This can be restricted by facial features, spectacle frames, and lenses.

The absolute field of view is the field of view possible with the naked eye. When we introduce a frame that sits on the face, it affects this absolute view, leaving us with an apparent field of view, within the extremities of the frame.

Calculating Apparent Field of View (AFOV)

The two main factors that go into this formula are the size of the frame and how close this frame is positioned to the eyes. Let’s compare two frame sizes and back vertex distances.

Example 1. You select a 45 mm eye size frame fitted at a back vertex distance of 12 mm. Punching the numbers into the calculator you get an apparent field of view of 112.62°.

Example 2. You select a 55 mm eye size frame fitted at a back vertex distance of 8 mm. Using the same formula, the apparent field of view is 136.40°.

Using the formula, the ideal driving spectacles that you can recommend to drivers would have a large frame that sits as close to the eyes as possible. But alas! What about prescription? When we introduce prescription lenses, the field of view is impacted again.

Now we need to consider another form of field of view: real field of view (RFOV). The RFOV formula accounts for the frame size and position of wear, along with the power of the prescription, and the power of light exiting the lens.

The RFOV is dependent on the size of the frame, its position of wear, and the prescription required. The degree to which the lens diverges or converges light will also affect the size of the RFOV.

Nighttime driving presents another set of visual challenges. Because there is less available light, we don’t want to introduce a lens coating that absorbs even more light.

Tinting and Coating

A study from the Department of Ophthalmology at Harvard Medical School^, investigated whether yellow-tinted night driving lenses help drivers see pedestrians more quickly at night or reduce the effects of glare from oncoming headlights.¹ Twenty-two adults of varying ages completed simulated night-driving tests while wearing three different pairs of yellow-tinted glasses as well as clear lenses. They drove in scenarios with and without headlight glare. The results showed that yellow-tinted lenses did not improve how quickly drivers detected pedestrians compared with clear lenses. They also did not reduce the negative effects of glare from oncoming headlights. As expected, headlight glare made it harder to detect pedestrians, particularly for older drivers.

Anti-reflective coatings can be beneficial for night driving. They reduce reflections from the front and back surfaces of the lens, allowing more light to pass through. This reduces distracting reflections from sources such as headlights and streetlights. An anti-reflecting coating should also be applied to the back surface of sunglasses for daytime driving to help prevent reflections from becoming a distraction.

Another challenge drivers face is the need to assess information at different focal lengths. There are different types of dashboards in modern vehicles, including analogue dashboards, digital dashboards, and heads-up displays. Understanding the type and position of the dashboard a person is going to be using while driving can influence the lens that you recommend. For somebody wearing a progressive lens design, you may wish to select a design that enhances the distance and intermediate portions of the lens, with minimal unwanted astigmatism encroaching on these areas of the lens.

Fitting Imperatives

The bee’s knees of lens designs are only as effective as the accuracy of the dispenser’s measurements. Any frame selected for driving should be fitted to the person’s face before taking measurements. For your selected lens to perform to the expected standard, ensure that monocular pupillary distance measurements are taken, along with the manufacturer’s recommendations for monocular fitting heights for single vision, progressive, or bifocal lenses.

The difference between a good lens and a great lens can lie in taking the following measurements into consideration alongside pupillary distances and heights: back vertex distance, frame wrap, and pantoscopic tilt.

Frames and lenses work together. An ill-fitting frame can cause a person to lose confidence in their ability to safely drive while wearing spectacles, and it is the dispenser’s responsibility to consider the spectacles holistically as a completed pair. We need to take into consideration frame selection, adjustments to match the wearer’s anatomy, lens designs, treatment options, prescription requirements, position of wear, and the customer’s preferences. This enables us to tailor our selections to each individual to ensure the frame and lenses are well-suited and appropriate.

Nicole Grasso is a qualified optical dispenser and an optical trainer at the Australasian College of Optical Dispensing.

Reference

1. Hwang AD, Tuccar-Burak M, Peli E. Comparison of pedestrian detection with and without yellow-lens glasses during simulated night driving with and without headlight glare. JAMA Ophthalmol. 2019;137(10):1147–1153. doi: 10.1001/jamaophthalmol.2019.2893.

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