Vision And Visual Perception Flashcards
Cones
- Used for direct vision in good lighting
- Good acuity and fine detail
- Colour sensitive
- Single connection to nerve fibre (1:1)
- Approx. 6-7 million cells per eye
Rods
- Light sensitive, good for poor lighting
- Good at detecting movement (peripheral vision)
- Insensitive to colour
- Bundled nerve connection to nerve fibre (17:1)
- Approximately 120 million cells per eye
Limitations of the eye - the ability to discern objects at night
- Night vision AKA Scotopic vision
- Light intensities are below which cones are no longer sensitive
- Identification range of dim objects is limited and the detail is poor
- Poor acuity and lack of colour discrimination
- Scanning procedure to permit off centre viewing is more effective
Limitations of the eye - the ability to discern objects in daylight including wires and other aircraft.
- Day vision AKA Photopic vision
- Central (foveal) vision achieves maximum visual acuity 2-3 degrees entire visual field)
- Peripheral vision (covers (180 degree field) helps to detect our surroundings (movement or flashing objects)
- Binocular vision = depth perception
Where is the concentration of cones maximal?
At the centre point of image focus and decreases rapidly towards the peripheral areas
Where does the concentration of rods peak?
20 degrees
Eye limitations - poor lighting
- Mesopic vision occurs during dawn, dusk or full moonlight
- Decreasing visual acuity and colour vision
- An effect can be a colour-shift to blue
Eye limitations - glare
- Glare = excessive illumination
- Sunglasses or visors can provide protection against external glare and facilitate the view of flight instruments
Eye limitations - lack of contrast
- Occurs in periods of low contrast e.g. fog, haze or situations lacking prominent reference features (flying over water.
Eye limitations - the blind spot
- Where the optic nerve leaves the retina = ‘day’ blind spot - brain fills what it expects to see.
Potentially dangerous when looking out for other aircraft. - Fovea centralis, location of cones = ‘night’ blind spot - right in the centre of your vision
Eye limitations - colour perception
- Rods = 1 type of pigment, perceive great in dim light conditions
- Cones = 3 types of pigment, each type sensitive to short, medium or long wavelength.
What is the process of dark adaptation?
- Pupil opens to allow more light to fall on the retina - less than a second
- When cones are not stimulated, they begin to shut down and stop sending information to the brain - 7 minutes
- Rods take over - 30 minutes to be the most sensitive
What is the normal time for full dark adaptation?
30 minutes
Identify precautionary actions to protect night vision adaptation
- If dark adapted eyes are exposed to a bright light source for any more than 1 second, night vision becomes impaired.
- To help maintain dark adaptation you should:
Adjust instrument and cockpit lighting to the lowest level possible
Avoid inhaling CO from smoking or exhaust fumes
Get enough Vitamin A in your diet
Avoid exposure to bright lights
What are the methods of cockpit/flight deck lighting and the problems associated with each?
- Outside ambient lighting: natural or artificial
- Interior compartment lighting: instrument panel overhead lights
- Supplemental utility: flashlights, chemical light sticks
Avoid/cope: Use red light and low-intensity white cockpit.
What are the requirements for corrective lenses?
- Near vision use half moon or lower segment glasses
- Near and distant vision use bifocals
- Contact lenses permitted as long as spare pair of glasses are carried
What makes sunglasses suitable?
- 100% UV protection
- Avoid tints - minimise colour distortion
- Avoid polarised - inference with display
- Avoid photo chromatic - takes long time to clear in low light situations
- Avoid thick frames - obstruct field of vision
What is resting state focus and its effect on object detection?
- When unstimulated, eye will take up relaxed resting state focus at 3m
- Eye fails to detect any object past this distance
Avoid/cope: actively refocus eye by focusing on things at different distances, and an active scan rate
What is an effective visual search technique?
- Making eye movements as small as possible
- 20 degree sector every 2 seconds
What are the see and avoid method for mid-air collisions?
Looking = 0.1 to 0.2s
Seeing = 0.2 to 0.3s
Recognising = 0.7 to 1.0s
Deciding Action = 2.0 to 3.0s
Physical Action == 2.0 to 3.0s
Total time = 5.0 to 7.5s
Autokinesis
- At night a single light source can appear to move around even though it is stationary
Avoid/cope: frequent eye movements, focus on a stationary landmark, or monitor instruments to resolve conflict
Flicker vertigo
- AKA ‘stroboscopic illumination illusion’ - when a bright light source is flashed into the eyes at a rate of 10-30 flashes per second, may occur when sun is shining through propeller blades.
Can occur within seconds and cause vertigo convulsions and unconsciousness
Avoid/cope: turn away from light source, DO NOT close eyes
The break-pff phenomenon
- Feeling of physical separation from the earth, a feeling of remoteness, loneliness, insecurity and anxiety or exhilaration - thought to be related to pilot personality.
Avoid/cope: increasing workload or descending to a lower altitude.
Sector whiteout
- Occurs over unbroken snow cover and overcast sky.
Pilot is often not aware of phenomenon.
Avoid/cope: checking conditions and being aware and prepared, refer to instruments
The black hole phenomenon
- At night where there are few lights on the ground to aid in depth and height perception.
- A night approach with no lights before the runway and city lights or rising terrain in the background can produce an illusion on being high on final appraoch resulting in a dangerous undershoot
Avoid: Long in approaches
What are some conditions which can lead to the creation of a false horizon and its effect on visual perception.
- Flying over a sloping cloud
- Night flying over featureless terrain with ground lights that are indistinguishable from a dark sky with stars
- Night flying over terrain with a clearly defined pattern of lights and a dark starless sky
- Can cause pilots to fly aircraft in a banked attitude
What is relative motion?
- An aircraft on exact opposite headings tracking towards one and other will not appear to be moving towards each other until the very last second when it may be too late
What is the effect of fog, haze, or dust on visual perception
- Interfere with light transmission
- Obscure horizon and the surface beneath the aircraft, they refract light rays differently than clear air, causing eyes to perceive that objects are out of focus
What are the optimal characteristics of the windshield?
- Clean
- Scratch free
- Flat
What is the effect of rain on the windshield?
- Visibility is reduced and distorted
- Built up water on windscreen refracts light rays differently causing an illusion of displacement
- Heavy rain can defuse lights - lighted objects in the distance will appear less bright, seeming further away, lighted objects at close range (runway lights) will appear larger, which in turn looks closer
What is the effect of up-sloping terrain on visual perspective
- Terrain sloping up the appraoch threshold can cause the illusion of being high which may lead to a pilot undershooting.
What is the effect of down-sloping terrain on visual perspective?
- Terrain sloping down to the approach threshold can cause the illusion of being low on the approach which may lead to a pilot overshooting
What is the effect of a steep approach angle on visual perception?
Pilot will appear high
What is the effect of a shallow approach angle on visual perception?
Pilot will appear low
What illusion will a wide or short runway give?
Of a low altitude approach, causing pilot to overshoot.
What illusion will a narrow or long runway give?
A high-altitude final approach, causing pilot to undershoot
What effect does a wet runway have on visual perception?
- Does not reflect light thus appearing to be further away
What is the effect of the intensity of the appraoch lights on visual perception?
- Very bright lights or lights viewed through extremely clear air, appear closer than they really are
Can lead to an approach above the glide slope and an early flare - In low-visibility there is a temptation to make the approach lights the aim point when they first come into view causing the pilot to undershoot
