Ch 6 - The Eye And Vision Flashcards
Basic Anatomy of the Eye
- The Eye itself
- Optic nerve (which connects the eye to the visual cortex at the back of the brain)
- Visual Cortex
Tear Ducts
Produce an antibacterial solution which helps clean the eye
Reactive Power (of the lens):
The reactive power of the lens is on average 16-30 dioptres
P=1/f(meters (distance))
Aqueous Humour
A watery fluid at the front of the eye that helps refract light towards the lens
Cornea
Transparent and at the front of the eye, responsible for ~ 70% of the eyes focussing
Iris
Coloured part of the eye
Controls how much light enters the pupil
Pupil
Hole in the middle of the eye which allows light to pass into the eye
Constricts to object close or bright
Dilates for far away or dark objects
Lens
Behind the iris, flat flexible disc, refracts light to focus onto the retina
~20-30% of the focussing
Retina
Light sensitive lining of the inside of the eye
Contains two types of receptor:
Cones (colour)
Rods (black and white and night vision)
Triggers electrical impulses which are sent to the visual cortex
Vitreous Humour
Fluid that keeps the shape of the eye ball
Linked to glaucoma
Optic Nerve
Carries impulses from the retina to the brain
Fovea
Most sensitive part and centre of the retina
Contains only cones and is where central vision occurs
Ciliary Muscles
Connected to the lens. Adjust the shape of the lens to make it more or less curved to increase or decrease the refraction of light
How many muscles are responsible for eye movement
6 Extraocular Muscles
Saccades
Eyes are constantly moving in the socket. Jerky quick movements which are then followed by a rest. Process lasts about 1/3 sec which we are never aware of.
Attention
Our eyes perform the sac cadets as we are always trying to find something interesting or threatening to look at in our peripheral vision.
Pursuit
After we have found something interesting to look at, we then track it in a smooth movement called a pursuit
Accommodation
Focussing:
~70% Cornea
~30% Lens
Distance vision
Lens become flatter or more concave
Near Vision
Lens becomes thicker and rounded or convex for near vision
Factors that affect focussing ability
Age
Fatigue
Cones
For our colour vision (photopic vision)and central vision
3 Types:
- Red
- Blue
- Green
Most are found in the fovea ~150000 cones per mm
- In a night flight, look next to the object rather than directly at it
Any alphanumeric information
Rods
Responsible for our night vision and peripheral vision (sceptic vision)
Not good at detail
Highest concentration of rods is 10-15’ away from the fovea (best peripheral vision)
Light and Dark Adaption
Light -> Dark: Cones take 7 minutes, rods take 30 minutes
Dark -> Light: Cones and rods will take ~10s to adapt
Flash Blindness
When something bright temporarily blinds you
Can be seen flying at night with :
- Lightening
- Lazarus
To minimise damage, turn instrument lighting up (especially near storms)
Night Vision
With hypoxia, night vision can be affected as low as 5000ft in the reaction threshold and the indifferent zone.
Factors affect your night vision:
- Vitamin A
- Alcohol will reduce it
- Wavlength of the light (red doesn’t affect the rods as badly compared to white or blue)
Visual Acuity
Average visual acuity is 20/20 which is what you should be able to read at 20 ft away.
Can only see alphanumeric information with the fovea
5’ away from the fovea = 20/40
25’ away from the fovea = 20/200
The clarity depends on:
- Angular distance from the fovea
- Regular distance from the fovea
- Light level
- Smoking
Visual Field
The central and peripheral vision combined
Each eye can see 120’ left to right which means there is a 60’ overlap called the binocular vision
150’ up and down
Depth Perception
The binocular vision (60’ overlap) or stereoscopic vision does our depth perception up to ~1m
Up to 60m or 200ft Combines each image received by the eye, brain processes these and puts them together and gives us our depth perception
After 200m, the brain falls on past experience/colours/contrasts/sizes to scale depth
- Colour - brighter = closer
- Size - Bigger = closer
- Contrast - more contrast = closer
- Relative Motion - faster = closer
- Overlapping contours (obscuration)
Empty Field Myopia
Only happens when the eyes are resting and can then only see things up to about 1-2m
Most commonly happens:
- Clear sky
- At night
- When there is a lack of distant focal objects
To prevent:
- Scan technique
- Look at wing tips (in small aircraft)
- Look at the clouds
Scanning technique
Through the horizon, break up into 10’ segments, scan up and down each section for no less than 2 seconds a time
Rinse, repeat
*Keep scan going even if you have climb/descent clearance
Blind Spot
We have no rods or cone where the optic nerve leaves the retina to go to the brain
This causes a blind spot which can be detrimental when flying, hence you need to physically move your head when performing your lookout / scan
Monocular Vision
When you can only see out of one eye, leading to a parallax error where close up objects appear to move relative to each other
- Hence a person with 1 eye will not get a class 1
Myopia
Short sightedness
When the eyeball is too long, the refracted light meets before where the retina is
To correct this, the person must wear concave / diverging lenses
Corrective Glasses
If you need them as a pilot, you must always carry an extra set with you.
Contact Lenses
You will need to carry a spare set of normal glasses with you
Problem with contacts:
- Humidity in the cockpit is low so they can dry out the moisture in your eye preventing O2 from getting to the cornea
*Bi-Focal contact lenses are prohibited
Hypermetropia
Long sightedness as the eyeball is too short, meaning the refracted light meets behind where the retina is
Corrected by wearing convex or converging lenses
Presbyopia
Another form of long sightedness that develops with age
- Ciliary muscles weaken
- Means the lens is unable to accommodate like it used to
- Most commonly happens from the age of 45-50
Astigmatism
A misshapen cornea or a misshapen lens
Cataracts
A lens becoming opaque or cloudy
Age related
Can be treated with surgery: either remove or replace with plastic lens
Visual acuity will normally not be completely lost, may need glasses though
Glaucoma
Increased pressure inside the eye ball.
The vitreous fluid inside the eye ball that keeps it shape normally, replaces itself regularly by draining and then new fluid replacing it. However if the drainage gets blocked, the new fluid keeps coming in which increases the pressure inside the eye - can be extremely painful
Can cause damage to the eye which leads to blindness
Concealed and insidious (slow and unnoticed onset)
- Hard to knows its developing
- Leads to progressive narrowing of the visual field and eventually complete blindness
Colour Blindness
Can be born with it or it can be developed over time.
Can be treated with medications
Caused by a cone defect.
7% of men become or are colour blind
0.1% of women
Does not affect visual acuity
Sunglasses
UV light damages eyes, levels of UV get more intense the higher you go so pilots should use sunglasses all the time above cloud level or on clear days. Should also carry a spare set
Should be:
Impact resistant
Thin Metal frames
Coated in polycarbonate for strength
Good optical quality
Luminance transmittance of 10-15% (very dark)
Have appropriate filtration characteristics
Light sensitive lenses (photochromatic) are generally forbidden
Polarised lenses are also discouraged (definitely banned)
- Distort the windscreens
- Alter cloud appearance
