Ch 6 - The Eye And Vision

Question 1

Basic Anatomy of the Eye

Answer 1

• The Eye itself
• Optic nerve (which connects the eye to the visual cortex at the back of the brain)
• Visual Cortex

Question 2

Tear Ducts

Answer 2

Produce an antibacterial solution which helps clean the eye

Question 3

Reactive Power (of the lens):

Answer 3

The reactive power of the lens is on average 16-30 dioptres

P=1/f(meters (distance))

Question 4

Aqueous Humour

Answer 4

A watery fluid at the front of the eye that helps refract light towards the lens

Question 5

Cornea

Answer 5

Transparent and at the front of the eye, responsible for ~ 70% of the eyes focussing

Question 6

Iris

Answer 6

Coloured part of the eye

Controls how much light enters the pupil

Question 7

Pupil

Answer 7

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

Question 8

Lens

Answer 8

Behind the iris, flat flexible disc, refracts light to focus onto the retina

~20-30% of the focussing

Question 9

Retina

Answer 9

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

Question 10

Vitreous Humour

Answer 10

Fluid that keeps the shape of the eye ball

Linked to glaucoma

Question 11

Optic Nerve

Answer 11

Carries impulses from the retina to the brain

Question 12

Fovea

Answer 12

Most sensitive part and centre of the retina

Contains only cones and is where central vision occurs

Question 13

Ciliary Muscles

Answer 13

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

Question 14

How many muscles are responsible for eye movement

Answer 14

6 Extraocular Muscles

Question 15

Saccades

Answer 15

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.

Question 16

Attention

Answer 16

Our eyes perform the sac cadets as we are always trying to find something interesting or threatening to look at in our peripheral vision.

Question 17

Pursuit

Answer 17

After we have found something interesting to look at, we then track it in a smooth movement called a pursuit

Question 18

Accommodation

Answer 18

Focussing:
~70% Cornea
~30% Lens

Question 19

Distance vision

Answer 19

Lens become flatter or more concave

Question 20

Near Vision

Answer 20

Lens becomes thicker and rounded or convex for near vision

Question 21

Factors that affect focussing ability

Answer 21

Age

Fatigue

Question 22

Cones

Answer 22

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

Question 23

Rods

Answer 23

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)

Question 24

Light and Dark Adaption

Answer 24

Light -> Dark: Cones take 7 minutes, rods take 30 minutes

Dark -> Light: Cones and rods will take ~10s to adapt

Question 25

Flash Blindness

Answer 25

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)

Question 26

Night Vision

Answer 26

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)

Question 27

Visual Acuity

Answer 27

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

Question 28

Visual Field

Answer 28

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

Question 29

Depth Perception

Answer 29

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)

Question 30

Empty Field Myopia

Answer 30

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

Question 31

Scanning technique

Answer 31

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

Question 32

Blind Spot

Answer 32

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

Question 33

Monocular Vision

Answer 33

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

Question 34

Myopia

Answer 34

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

Question 35

Corrective Glasses

Answer 35

If you need them as a pilot, you must always carry an extra set with you.

Question 36

Contact Lenses

Answer 36

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

Question 37

Hypermetropia

Answer 37

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

Question 38

Presbyopia

Answer 38

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

Question 39

Astigmatism

Answer 39

A misshapen cornea or a misshapen lens

Question 40

Cataracts

Answer 40

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

Question 41

Glaucoma

Answer 41

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

Question 42

Colour Blindness

Answer 42

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

Question 43

Sunglasses

Answer 43

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