The human eye is a marvellous instrument. It is however distinctly different to a camera which can observe a 15 degree arc of horizon and bring everything at one distance into sharp focus.
Our eyes can observe about a 200 degree arc of horizon at one glance, but not everything we see will be sharp. This is because only a small area at the back of the eye called the fovea is capable of sending sharp images to the brain. Any image not processed directly through the fovea will be blurred. For example an aircraft we can see distinctly with the foveal centre at 5 kilometres would have to be as close as 500 meters in order to be recognised if the angle of sight caused the image to reach the eye just outside the fovea.
Each eye also has a blind spot. The image falling on this region does not register at all. Fortunately for us the images seen by each eye overlap with the left eye usually covering the blind spot of the right and vice versa. Sunglass frames and your nose can obstruct this visual overlap and leave you with real and potentially hazardous blind spots.
The lens of the eye is elastic and changes its curvature to focus at different distances under the control of the ciliary muscle. It was once believed that relaxation of this muscle caused the eye to fucus on optical infinity. We now know that it actually focuses at a relatively short distance when at rest. When your vision contains no distinctive objects such as at high altitude, or in haze, your eyes tend to relax and come to rest at a comfortable focal distance of about 6 meters. This sort of nearsightedness, or “empty field myopia” as it is formally called represents a real danger when flying because even though we may not be aware of it objects further than about 6-10 meters away will be out of focus. It explains the frequent statement of pilots following a near midair collision “…..the other aircraft suddenly materialised out of a clear blue sky.” Chances are that the other aircraft was visible in the distance long before the pilot’s eyes focused upon it. Perhaps somewhat surprisingly clouds do little to draw our focus into the distance.
These three physiological facts about our eyes – the fovea, blind spot, and empty field myopia – mean it is possible to look but not actually see. To really see all that is contained within apparently empty airspace you have to direct your eyes to move in a slow deliberate pattern. This is called scanning. If we try to take in the whole scene with one glance we will miss 95% or more of the detail. The basic rule of scanning is to examine relatively small blocks of airspace successively.
There is no special scanning technique which works magic. You simply need to divide up the sky into small blocks and examine them sequentially. Consider how you might practically do this. Because we are in the habit of absorbing so much information from reading this kind of motion often seems to work best. Imagine they sky is completely covered by adjacent pages of text. These imaginary pages are your blocks. Pick a “page” and “read” it by scanning each imaginary line from left to right then moving down to the next line until the page is finished. Pick the next “page” and repeat.
Regardless of how you chose to examine a block of airspace after each is finished you move on to the next. In time as you learn to control your eye movements, you will see more and more objects that you missed earlier. Most of us are probably familiar with this technique. It is rather like looking for that safety pin you just dropped on launch: if you examine the ground, imaginary square by square, you stand a much better chance of finding it than if you just stare at the ground. Using the scanning method you can logically examine any area of interest with your sensitive fovea to give you maximum probability of picking out details.
So far we have seen how to get sharp images of the entire sky into our brain by using the technique of scanning to present those images to our fovea. What use you may ask is the rest of the eye? Despite the fact that images which fall outside the fovea are not seen as sharply this region of peripheral vision is immensely useful. Our peripheral vision keeps us in touch with the big picture and also acts as an exquisitely sensitive motion detector. This is why you turn to look at the movement you caught from the corner of your eye.
While peripheral vision is useful throughout our entire flight it comes in most useful on landing during the final approach, round out, hold off and flare phases. To execute these manoeuvres we must accurately judge both our height and vertical descent rate. Pilots who have trouble doing this successfully are almost invariably staring down at the ground. It is widely known in all aviation that staring at the ground inevitably leads to poor landings and the key to good landings is to look up and along the “runway”.
Why? The reason for this phenomenon is that by staring down we lose the immensely valuable motion sensing information available from our peripheral vision. Couple this with the fact that we have a lifetime of experience as to what 5-6 foot altitude looks like when looking ahead. This altitude is what we see in our peripheral vision every day of our lives when we are walking around. So looking down robs us of valuable cues about height and descent rate from our peripheral vision. Little wonder flying instructors the world over repeat the same monotonous “look ahead on landing” to their students
A pilot who can’t see is an accident waiting to happen. Without good eye protection flying on bright sunny days can be tiring and hazardous. There are three problems caused by bright sunlight: glare, infrared (IR) and ultraviolet (UV) radiation.
Glare, though the most obvious nuisance in causing squinting, discomfort, and fatigue, is responsible for less serious problems than IR or UV radiation. IR damage, caused by looking at the sun or its reflections can quickly injure the sensitive retina at the back of the eye and should be avoided. On the ground UV is partially filtered by the earth’s atmosphere, but the higher you go the less the protection. UV light damages the eye and causes lens opacities (cataracts) which steadily reduce your visual acuity over a period of years.
Pilots should be aware that cutting down on glare by using very dark sunglasses can cause problems because reducing transmitted light too much can reduce visual acuity. Dark sunglasses can, on a bright day, cut your visual acuity down by over 50%! Cheap sunglasses often only cut down on glare but allow your pupils to dilate and actually increase the risk of damage from IR or UV radiation.
So what is best? Unfortunately there a numerous cheap sunglasses, but few good inexpensive ones. How can you best evaluate those shades you’ve been thinking of buying? Of course they must look cool and match your glider but what then? The National Society for the Prevention of Blindness suggests several criteria:
- Transmission factor: The experts recommend lenses that block 85-90% of the sunlight allowing 10-15% to reach the eyes.
- IR Transmission: 90% or more of IR radiation should be blocked
- UV Transmission: 95% or more of UV radiation should be blocked
- Colour: Neutral grey is the lens colour that retains colour fidelity best, but there are advocates of brown and green lenses. The transmission curve of green lenses resemble the colour sensitivity of the eye. Brown lenses block scattered blue light rays prevalent with dust or moisture in the air, thereby reducing haziness, improving contrast and sharpening details – at least that is how the argument goes.
- Optical quality: Both lenses should be evenly matched, equal in colour and absorptive qualities. The lenses should be free from waves, surface blemishes, scratches, or other distortions which can cause eye strain. To test the lenses, hold the glasses at arms length and focus on a distant vertical line. Move the glasses vertically and horizontally. If the line waivers the glasses contain distortions. NB This is not a valid test for prescription lenses.
- Impact resistance: For obvious reasons this is important in our sports.
Most manufacturers of quality sunglasses publish this information. A potential problem with cheap sunglasses is, although they may comply to the necessary regulations in respect to UV and IR filtering, the filter is generally a thin layer fixed onto the outside of the lens. This means that after you have kicked your glasses round the launch, or they have been sliding on the dash of your car, this protective coating is scratched. When we wear them, the pupil is dilated due to the reduction in light, and the UV, IR streams in through the cracks. Not Good. If you wear cheapies, replace them frequently. The more expensive sunnies have the filter integrated into the lens material so do not suffer from this problem.
A couple of other issues are that the frames block some of your peripheral vision leaving enlarged blind spots. This means you must move not only your eyes but also your head to adequately scan all the sky. Polaroid sunglasses will make it impossible to read LCD displays when held at certain orientations.
Thought for the day: We see largely what we expect to see. Seek and ye shall find.