Describe the visual system resting state focus and its effects on object detection.

With normal vision the lens focuses an inverted image onto the rear of the eyeball – Retina.
The lens shape changes to focus the object according to it’s distance

Describe the effects of hypoxia on vision.

When one is hypoxic, you can suffer from symptoms such as:

• Dimming of vision
• Loss of sharpness
• Dulling of colour vision 

Describe the factors associated with the selection of suitable sunglasses for flying.

Should be impact resistant
Should transmit 10-15% of light
Should filter out UV rays

Should not be worn in decreased light
Preferably should not be polarised

Describe the following visual conditions:

Short sightedness

Distant objects are blurred and near objects are clear
When the eye is relaxed and the cornea and the lens focus the rays in front of the retina 

Presbyopia

Occurs naturally in people over 40 yrs
Long-sightedness caused by the lens material losing some of its flexibility
Muscles are less able to increase the curvature

Astigmatism 

The curvature of the cornea is not perfectly round
Causes uneven refraction of the light
Causes distorted images

Identify precautionary actions to protect night vision adaptation.

Ensure that if using usual visual references on takeoff at night – make sure the eyes are adequately night adapted (30 mins)

Use only red and UV cockpit lighting to preserve night adaptation

State the normal time for full night vision adaptation.

For full night adaptation, this will take 30 minutes

Half night adaptation takes 10 minutes 

Explain the process of dark adaptation.

Eyes can adapt to low light levels but it takes time
The cones shut down once they cease to receive sufficient light
This takes approx 7 minutes

While the cones are shutting down the rods are taking over
The rods take 30 minutes to reach their most sensitive

It takes 10 minutes to gain half of your night vision and 30 minutes for total dark adaptation

One second of bright light exposure will destroy night adaptation for a further 30 minutes
Red and UV cockpit lighting will preserve night vision

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

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

Distinguish between rod and cone cell functions and distribution in the retina.

Cones

• Sensitive to bright light energy
• Help sharp and detailed vision
• Register colour vision
• Concentrated in central section of the retina, especially the Fovea region directly opposite the lens
• Most effect in daylight and less effective in darkness

Rods

• Sensitive to movement
• See in black, white and shades of grey only
• Help night vision
• Responsible for peripheral vision
• Concentrated in a band on the Retina outside the central Foveal area 

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. 

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

Rod cells