The Eye and Vision — Page 84, Lesson 127

I want to walk you through the practical side of vision for pilots — starting with what makes a good pair of sunglasses for flight, then how the eye moves, and finally the two most common visual defects and how they affect your licence. Let's begin with sunglasses. If you're going to wear sunglasses in the cockpit, they need to meet several specific requirements. First, they must be impact resistant — that's for safety in case of an accident or debris. The frames should be thin metal, because thick frames obstruct your visual field, and you need the widest possible view when scanning for traffic. The lenses themselves should be coated with polycarbonate for strength. They must be of good optical quality — cheap lenses distort your vision. The luminance transmittance — that's the percentage of visible light that gets through the lens — should be between 10 and 15 percent. And they need appropriate filtration characteristics, meaning they filter out the right wavelengths of light, particularly UV. Now, there are three types of sunglasses you should avoid. First, cheap sunglasses — they can allow light to be over-diffused across the eye, which causes perceptual problems in flight. Second, Polaroid sunglasses — these use polarised lenses that reduce light by selectively filtering certain electromagnetic spectral planes. The problem is that polarised lenses can cause distortion patterns when used with laminated aircraft windscreens, which many cockpits have. They can also alter the appearance of clouds and reduce ground reflections that VFR pilots rely on for judging height and texture. So their use is discouraged. Third, light-sensitive or photochromic lenses — these darken in bright light and clear when the light dims. They are generally forbidden for use in flight because of the time delay: when you move from a bright situation, like sunlight, into a low-light situation, like a dark cockpit or cloud, the lens takes time to clear. That delay can significantly reduce your visual acuity at a critical moment, such as during an approach. Let's move to eye movement. Your eye is moved by six extraocular muscles, and these allow movement in two planes: up and down, and left and right. Normally both eyes move together to view an object — that's called conjugate eye movement. The surface of the eye is lubricated by a liquid secreted by the lacrimal glands — those are your tear glands. Tears are drained by the tear ducts into the nose, which is why your nose runs when you cry. At low humidity levels, which is common in aircraft cabins, the eyes can become dry and may feel painful. Tears also have a slight antiseptic property, which helps protect the eye from infection. Now, visual defects. The two main ones are hypermetropia, which is long-sightedness, and myopia, which is short-sightedness. Both are caused by a distorted shape of the eyeball. Let's take hypermetropia first. In long-sightedness, the eyeball is shorter than normal along the visual axis. This means the image is formed behind the retina. Normally, the cornea and lens combine to bend light and focus it on the retina — that's the combined refractive index. But if the eyeball is too short, the image falls behind the retina, and unless the eye's accommodation — its ability to change the lens shape — can compensate, you get blurring, especially when looking at close objects. The correction is a convex lens, which bends the light inwards before it hits the cornea, effectively adding more focusing power. In myopia, short-sightedness, the problem is the opposite: the eyeball is longer than normal, so the image forms in front of the retina. If accommodation cannot overcome this, then distant objects are out of focus, while close-up vision may be satisfactory. The correction is a concave lens, which bends the light outwards before it hits the cornea, spreading the light so it focuses further back. Now, the important part for your licence. Pilots with either hypermetropia or myopia may usually retain their licences, provided that their corrected vision — that means with glasses or contact lenses — meets two standards. First, they must be able to read normal small print in good lighting at a distance of 30 centimetres. Second, they must have at least 6/9 vision in each eye individually, but with 6/6 vision when using both eyes together. That 6/6 vision with both eyes is the standard. To give you a practical comparison, 6/9 vision is equivalent to reading a car number plate at about 40 metres, whereas the UK driving test requirement is only 23 metres. So the aviation standard is significantly stricter. Bifocal spectacles may be used when flying.