The Eye and Vision — Page 84, Lesson 123

I want to walk you through two important topics in night vision and visual scanning: the indifferent zone and the blind spot. Let's start with altitude effects on vision. From sea level up to 3000 metres, we have what's called the "indifferent zone." The name comes from the fact that ordinary daytime vision is unaffected up to this altitude. However, there is a slight impairment of night vision even within this zone. Without supplemental oxygen, the average percent decrease in night vision capability follows a specific progression: 5% at 1100 metres, 18% at 2800 metres, 35% at 4000 metres, and 50% at 5000 metres. So you can see the degradation becomes quite significant as altitude increases. Now, the most common factors affecting night vision are listed for you. Age is one — we'll cover presbyopia later in the chapter. Mild hypoxia is another key factor. Cabin altitudes above 5000 feet can affect night vision, but it's important to note that it's not detrimental below approximately 12,000 feet. Smoking is a major one: a consumption of 20 cigarettes a day results in a night vision degradation of approximately 20%. Alcohol, minor illnesses, and a deficiency of vitamin A are also listed. Let's move to the blind spot. This is a critical concept for visual scanning. The point on the retina where the optic nerve enters the eyeball has no covering of light-detecting cells. That means any image falling on this point will not be detected. This has great significance when considering the detection of objects which are on a constant bearing from the observer. If the eye remains looking straight ahead, it is possible for a closing aircraft to remain in the blind spot until a very short time before impact. That's why safe visual scanning demands frequent eye movement with minimal time spent looking in any direction. You can use Figure 5.3 to demonstrate the blind spot to yourself. Hold the drawing at arm's length, close your left eye and keep your right eye open. Now move the picture towards your face keeping your right eye focused on the cross. The aircraft will disappear, then reappear as it gets close. That's your blind spot in action. So the key takeaway here: at altitude, your night vision degrades significantly without supplemental oxygen, and the blind spot means you must keep your eyes moving during visual scanning — never fixate on one point for too long, or you could miss a closing aircraft entirely.