The Eye and Vision — Page 84, Lesson 121

I want to walk you through the limitations of our vision and how the eye adapts to light and darkness — this is essential knowledge for night flying and for understanding why your visual performance changes in different conditions. Let’s start with the limitations of acuity. Visual acuity — your ability to see fine detail — is limited by several factors. I’ll list them so you know exactly what degrades your vision in the cockpit. First, the angular distance from the fovea — that’s the central spot on your retina where cones are packed tightest for sharp vision; the further an image falls from the fovea, the less detail you see. Next, physical imperfections within the visual system itself — things like astigmatism or minor lens irregularities. Age reduces acuity naturally. Hypoxia — lack of oxygen at altitude — impairs your vision. Smoking and alcohol both degrade visual performance. Visibility conditions such as dust, mist, or haze reduce contrast and sharpness. The amount of light available matters — too little and your acuity drops. The size and contours of an object affect how easily you see it. The distance of the object from the viewer changes the retinal image size. The contrast of an object with its surroundings — how much it stands out against the background — is critical. Relative motion of a moving object can blur it. And finally, drugs or medication can affect your vision. Now let’s move to light and dark adaptation — how your eye adjusts to changing brightness levels. First, light adaptation. When you experience a sudden high level of illumination — like walking out of a hangar into bright sunlight — your eye adjusts quickly, in approximately 10 seconds. But here’s the important part: if you have been in bright light for a long time, large proportions of the photochemicals in both the cones and the rods are reduced. Cones handle colour and detail in bright light; rods handle low-light, black-and-white vision. When those photochemicals are depleted, the sensitivity of your eye to light is reduced. So if you go quickly from outside on a sunny day into a darkened room, your vision is severely reduced until dark adaptation takes place — your eye needs time to rebuild those photochemicals. Now, dark adaptation — the reverse process. If you remain in darkness for a long time, the cones and rods gradually become supersensitive to light. They build up photochemicals so that even the minutest amount of light causes excitation of the receptors. That’s why after enough time in the dark, you can see things that were invisible moments earlier. This brings us directly to night vision. You may have noticed that in dim light it is easier to focus on an object if you look slightly away from it. Here’s why: the fovea — the centre of your retina — contains no rods. Rods are the receptors needed for vision in very low brightness levels. So the centre part of your eye becomes blind to dim light. To see a faint object at night, you must look away from the visual target so that the peripherally located rods can perform their sensing task. This is most noticeable when night flying. You can demonstrate this yourself by looking at dim stars on a clear night. Some of them will be invisible with direct viewing — looking straight at them — but they will be discernible if you look 10° to 15° off to one side. That’s your rods picking up the light that your fovea missed. Now, adaptation takes time. It takes about 7 minutes for the cones to adapt to darkness, and about 30 minutes for the rods to reach full dark adaptation. But here’s the critical point: even a brief exposure to bright light will require a further period of adaptation to recover effective night vision. So if you glance at a bright instrument light or a flash of lightning, you reset that clock. This leads to good airmanship. It is good practice to avoid bright lights for about 30 minutes prior to a night flight — that gives your rods time to adapt. It is also advisable to turn up cockpit lights when approaching a weather pattern that might produce the possibility of lightning. Why? Because if lightning flashes, the sudden bright light will ruin your dark adaptation; having the cockpit lights already up reduces the contrast shock and helps preserve some usable vision. Finally, fatigue may also necessitate increasing instrument lighting — when you’re tired, your visual sensitivity drops, and you may need more light to see the instruments clearly.