We've already covered the basic structure of the eye and how it focuses light — Page 88, Lesson 131

I want to walk you through the next part of the eye and vision topic. We've already covered the basic structure of the eye and how it focuses light. Now, let's look at some practical considerations for aircrew, starting with contact lenses. The cornea, which is the transparent front part of the eye, normally gets its oxygen supply from the ambient air — the air around us. When you wear a contact lens, it sits on the cornea and can interfere with that oxygen supply. Two specific in-flight factors increase the risk of corneal damage when using contact lenses: mild hypoxia, which is a reduced oxygen level in the body, and dehydration caused by the low humidity typical on a flight deck. Both of these conditions make the cornea more vulnerable. There are also specific mechanical risks. If the aircraft experiences a cabin decompression, a sudden drop in cabin pressure can cause gas bubbles to form under the contact lens, which is obviously a problem for vision and comfort. Additionally, the lens can be physically dislodged. This can happen from careless rubbing of the eyes — which pilots might do when humidity is low and eyes feel dry — from an accidental knock, or from increased g-forces during manoeuvres. Now, the regulations. The use of contact lenses by aircrew is permitted, but it must be under authorised medical supervision. However, there is a specific prohibition: bifocal contact lenses are not allowed. There's also an important operational note: if a pilot is cleared by the authorities to use contact lenses for flying, it is always on the proviso — the condition — that a pair of ordinary spectacles is carried at all times while they are exercising the privileges of their licence. So, glasses must be in the flight bag as a backup. Let's move on to a completely different aspect of vision: colour vision. Good colour vision is considered essential for flight crew. Why? Because of the extensive use of colour in the aviation environment. The excerpt lists the key areas: the navigation lights of aircraft, runways and airfields, ground obstructions, cockpit displays and instruments, maps and charts, emergency flares, and light signals from air traffic control. Being able to distinguish these colours correctly is a safety-critical requirement. This leads us to colour blindness. Total colour blindness is a complete inability to see colour, and the book states it is a bar to the issue of a flying licence — meaning you cannot get a licence if you have it. It is caused by a defect in the structure of the colour-sensitive cones in the retina, normally when a single group of those cones is missing. Total colour blindness is extremely rare. However, many people suffer from this defect to a degree — they are described as "colour defective." The most common form is red/green blindness. In this condition, red and green are not seen as distinct colours; instead, they are seen in shades of yellow, brown, or grey. Importantly, this condition does not affect visual acuity — the sharpness of vision — and many people go through their entire lives with no knowledge that they have this imperfection. Colour blindness is rare in women, but women do act as carriers of this incurable and congenital flaw — meaning it is something you are born with and cannot be treated. Finally, let's look at the relationship between vision and speed, particularly in high-speed flight. The book defines high-speed flight as greater than approximately 450 knots, and it says the principle problem at very low altitudes — below 500 feet — is the extension of the total reaction time. Let me break down what that total reaction time includes. It is a sequence: visual input, brain reaction, perception, recognition, evaluation, decision, action, and response. In ideal circumstances, this entire chain takes about 5 to 7 seconds. Many factors can prolong this period, including workload and fatigue. But the excerpt highlights that the initial part of that chain — the "visual perception cascade" — is especially important. The visual perception cascade covers visual input, brain reaction, perception, and recognition. In perfect conditions, this cascade takes approximately 1 second. However, several factors can extend this time period: poor atmospheric conditions, darkness, and the size and contrast of the object you are trying to see. The excerpt cuts off there, but you can see that at high speed and low altitude, even a small delay in that initial visual perception can be critical.