Light sense and driving
Photopic and mesopic vision
Photopic (day vision) driving occurs during daylight hours or at night when driving along brightly illuminated roads.
Mesopic (between day and night vision) driving occurs with natural light at dawn or dusk, or with artificial light when passing through illuminated tunnels or insufficiently lit roads using the light from the cars headlights.
Mesopic vision is extremely important in nighttime traffic and is characterized by identification of objects by their silhouettes or shapes according to their surface area and especially their contrast. During this type of driving, the eye always has a certain degree of mydriasis.
The visual acuity of a driver at night depends on the color of the object, its reflective capacity, contrast with the background, distance, movement, the position of the object in the light beam of the headlights, and the driver’s fatigue and ability to adapt to variations in light from different sources.
The ability to judge distances, movement and colors is poorer during night driving, and the speed of perception of objects is slower. Also, colors are distinguished by their relative luminance; and consequently a red car will appear to be at a greater distance than it really is.
The change from photopic to mesopic vision usually requires 5 minutes, and occurs gradually at dawn or dusk. However, because vision is insufficient on abruptly entering a tunnel, we should always use our vehicle’s headlights to aid us, even if they don’t seem necessary.
Mesopic visual acuity is reduced in disorders such as glaucoma or during treatment with myotic drugs.
Impairment of mesopic vision is usually associated with degenerative diseases of the retina and retinal dystrophies. Because these conditions are progressive and accompanied by other visual limitations, they will prevent driving in general in many cases.
Poor night or twilight vision and difficulty in adapting to darkness is typical in hemeralopia, night blindness in advanced myopia, cataracts and vitamin A deficiency, as well as with certain iatrogenically administered drugs.
Nighttime visual acuity can be reduced by visual fatigue, the action of certain toxins (CO, alcohol, tobacco smoke), noises and vibrations, hypoglycemia, and remaining for long periods during the day in very sunny environments. In the latter case, it is recommended to wear sunglasses if you are going to drive afterwards.
It should also be taken into account that during night driving other factors such as sleepiness, alcohol, drugs and general fatigue are often present in some drivers.
As we grow older, we require more light. The older driver tends to overuse high beams, which are more powerful but may dazzle the driver of an oncoming vehicle.
Dazzling occurs when a light source brighter than the general level of illumination appears in the visual field.
It causes maximum pupil contraction, and transient visual complaints such as lacrimation, blepharospasm, frequent blinking and a momentary loss of vision or “darkening”, especially if the eyes are adapted to darkness.
This is followed by slow recovery of visual functions, which is considered normal if it occurs within a maximum of 50 seconds.
Acute dazzling increases tiredness and perturbs color perception, visual field, contrast sensitivity and depth vision.
The effect of dazzling is aggravated by increasing age, alcohol, fatigue, various drugs, some diseases and certain medicines.
It is different from the dark area, which is the point in which the light from the oncoming vehicle completely fills the visual field of the driver, preventing him/her from seeing what is behind the barrier of light. Its duration varies depending on the speed at which the vehicles are moving.
There is no treatment for a driver who does not recover well after being dazzled.
This problem may arise during daytime driving when on entering a tunnel dazzling is added to existing dim lighting conditions.
Dark adaptation is considerably slower than light adaptation and is more important. It requires a period of approximately 30 minutes, after which objects can be distinguished in darkness, but not their colors.
- The physician should assess all symptoms and signs leading to suspect an abnormal sense of light and confirm the diagnosis. Our patient will be happy with the advice on schedules and rests recommended for driving in his case.
- The driver with an abnormality in the ophthalmoscopy or the campimetry can have a disturbance of light adaptation that forces to a more comprehensive study for suspected glaucoma, retinal degeneration, or other diseases.
- Pupillary areflexia that prevents from obtaining the visual capacity legally established is disabling for driving.
- People with poor recovery after blinding by headlamps cannot drive at night and should use smoked glasses to prevent daytime blinding.
- It is fundamental to explain well the problem to drivers with a poor recovery from blinding, to adopt precautionary measures for driving, and that they anticipate the event.
- At night, the pedestrian should be concerned with drivers to see them and follow some rules to alert them of their presence, walking front to the cars, using clear clothing and reflecting strips, and avoiding doing it under the effects of alcohol.
- People with mesopic vision disorders, suffering delays in the recovery from blinding, should be recommended by their physician, depending on the type of driving license, on the impossibility to drive between sunset and dawn, or the impossibility of driving in any case. The report given to the patient by the specialist will help for an adequate assessment of the case for obtaining or extending the driving license.
- Mesopic visual acuity is substantially reduced in conditions such as glaucoma, degenerative diseases of the retina and retinal dystrophies that will prevent in many cases driving in general, and we should inform or driver patient of it.
- The hemeralope (*) is particularly sensitive to headlamp glare, so he cannot drive at night.
- All drivers should rest adequately before start night driving, in order to avoid the delay in the recovery from possible blinding caused by fatigue.
- On long travels at night, the patient should eat from time to time in order to prevent the hypoglycemia leading to reduced night visual acuity.
- Drugs that affect the ability to recover from blinding should be avoided, and if it is not possible, they should not drive at night.
- After a long stay in very sunny places, the use of sunglasses is recommended for driving later.
- Elderly drivers should avoid driving in roads with low lights.
- It has been demonstrated that a good public lighting reduces by 30% the fatal accidents in roads, so we should collaborate in our environment for an appropriate lightning increase.
(*) Person with low visual acuity in crepuscular or low light.