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Describe the limitations of the eye in terms of the following...
Night Vision
The eye can adapt to low levels of light but this takes time.
Day Vision
The best vision in daylight is central vision using the rods and cones of the central retina. Sharpness of vision decreases rapidly away from this central area, outside which the pilot is quite blind. Detection of movement is not so adversely affected.
Poor Lighting
At low levels of luminance (brightness) the rods come into use after a period of night adaptation. The eye can then detect very low levels of light – though colour vision is lost.
Glare
When at altitude, the pilot is exposed to light of very high intensity eg bright sunlight reflected from cloud tops. The contrast between the glare of a very bright outside environment and the darker cockpit interior makes it difficult to read instruments and charts inside the cockpit.
Glare causes both a decrease in vision and visual sharpness (acuity)
Lack of contrast
Where another object in the sky “stands out” against the background colour will determine how easily it is seen Where there is low contrast – scan and rescan the area
Blind spot
The blind spot is not usually seen continually as the brain fills in the space with what it thinks should be there automatically While scanning the pilot should always consider artificial blind spots such as spars and wings
Colour perception
Colours detected by the cones Mainly in the central fovea region of the retina Defective colour vision is difficulty in distinguishing between red and green.
Empty field myopia
When flying in conditions of low visual stimulation a significant form of “short sightedness” can occur. Eg – over an expanse of sea or above a cloud layer where everything appears much the same and is unbroken by any features Relaxed focal length of the eyes is 0.5 – 3metres So it takes an effort to focus on distances esp in the absence of visual clues
Identify the following eye structure components ...
Lens
The lens is like the lens of a camera – it is able to focus the incoming rays so they fall onto the back of the eye – the Retina.
Cornea
This is the tough protective lining of the eye.
Retina
The back part of the eye which receives the light energy rays and signals then, contains the rods and cones and the optic nerve disc.
Fovea
This is the area in the centre of the Retina made up entirely of Cones.
Optic nerve disc
Where the small nerves from behind the Retina carry the light generated by nerve signals to the visual centre of the brain from the rods and cones – this forms the Optic Nerve. There is no room here for rods and cones, any light falling here is not registered. This forms the basis of the Blind Spot.
Cone cells
Concentrated in the centre of the Retina, the Cone make up a structure called the Fovea. They receive bright light energy and are responsible for sharp vision, detail and register colour.
Rod cells
There are no rods in the Fovea, they are found around the rest of the Retina They receive weak light energy and are used for night vision, detecting movement and peripheral vision.
When light energy enters the eye it passes through the Cornea – the tough protective lining of the eye, and then through the Lens which together focus the incoming rays against the Retina – the back of the eye.
If the light energy is strong enough it will cause a light chemical reaction within the Retina generating a nerve signal that passes from the Retina to the Vision centre located at the back of the brain.
The eyes are able to function at very bright levels of light and can also detect very low levels of light – made possible by the Pupil – the variable hole at the front of the eye. There are 2 different types of light receptor within the Retina.
After diving the increased nitrogen solution may have been insufficient to cause decompression sickness, but if further decompression occurs associated with a flight at altitude then decompression sickness may be triggered
This blood vessel blockage causes pain and various other symptoms, for example, sometimes similar to those of a stroke (such as sudden weakness on one side of the body, difficulty speaking, or dizziness), or even flu-like symptoms. Nitrogen bubbles also cause inflammation, causing swelling and pain in muscles, joints, and tendons.
Any reduction in pressure / increase in altitude causes nitrogen to come out of solution and form nitrogen gas bubbles
These gas bubbles move about the body and can become lodged in the joints, brain, spinal cord and under the skin
As outside pressure decreases during a climb, the accumulated nitrogen that cannot be exhaled immediately forms bubbles in the blood and tissues. These bubbles may expand and injure tissue, or they may block blood vessels in many organs—either directly or by triggering small blood clots.
