The Eye and Vision — Page 84, Lesson 124

I want to walk you through the next part of our lesson on the eye and vision. We've covered the structure of the eye and how we see, and now we need to talk about some very practical limitations that affect a pilot's ability to see in the cockpit environment. Let's start with the blind spot. You already know that each eye has a blind spot where the optic nerve leaves the retina, and there are no light-sensitive cells there. Now, if both your eyes are open and unobscured, the blind spot is not a problem. Why? Because each eye is able to see the detail that falls in the other eye's blind spot. Your brain fills in the gap using the information from the other eye. However, here is the real danger for a pilot: there is a very real possibility that an approaching aircraft on a constant bearing will not be seen. Think about that. If that aircraft remains in the blind spot of one eye, and an object or a person inside the cockpit is obscuring that aircraft from the pilot's other eye, then the aircraft is completely invisible to you. That is a critical threat to see-and-avoid. Now let's move to stereopsis, or stereoscopic vision. Some of the optic nerve fibres cross over in the brain. Because one eye is a little more than 2 inches — that's about 5 centimetres — to one side of the other eye, the images on the two retinas are different from one another. This difference is called binocular disparity. The brain compares these slight differences seen by each eye and interprets this as depth and distance perception. So a person with two eyes has a far greater ability to judge relative distances when objects are nearby than a person who only has one eye. But here is the key limitation for aviation: stereopsis is virtually useless for depth perception at distances beyond 200 feet, or 60 metres. Beyond that range, your two eyes are too close together to detect any meaningful difference between the two images. So for judging the distance of an approaching aircraft that is still far away, stereopsis gives you no useful information. Next, I want to introduce a phenomenon called empty visual field myopia. In the absence of anything to focus on — an empty field — the natural focus point of the eye is not at infinity, as was long assumed. Instead, it is on average at a distance of between just under 1 metre and 1.5 metres. There are wide variations between individuals, but the key point is that your eyes will naturally focus at about arm's length, not at infinity. This is very significant when you are searching for distant targets and visual cues are weak. Your eye will not be adjusted to detect them. The condition is aggravated when there are other objects close to that empty field range — rain spots on the windscreen, for example — because the eye will naturally be drawn to them. So you end up focusing on the rain spot instead of scanning for traffic. This phenomenon can occur in cloudless skies at high altitudes, in total darkness, under a uniformly overcast sky, or when you are simply resting your eyes. Aircrew should minimize the risks associated with empty visual field by periodically and deliberately focusing on objects, thus exercising the eyes and keeping the focusing muscles active. Now let's talk about high light levels. It is possible for too much light to fall on the eye. Pilots are exposed to much higher light levels than most people. Very high light levels occur at altitude, where light may be reflected from cloud and, more importantly, where there is less scattering of the light rays by atmospheric pollution. Normal sunlight contains all the colours of the spectrum, but at high altitudes pilots are exposed to light that contains more of the high-energy blue and ultraviolet wavelengths than is experienced at sea level. The higher-energy blue light can cause cumulative damage to the retina over a long period. Ultraviolet wavelengths can also cause damage, mainly to the lens of the eye, but most UV is filtered out by the cockpit windows. Finally, sunglasses. Wearing appropriate sunglasses can provide complete protection against the problems I just described. When purchasing sunglasses, always consult a knowledgeable supplier. Sunglasses should have specific characteristics, and we'll look at those next.