What is normal vision?

Normal vision occurs when there is no abnormality of structure or function of the eye, eye muscles or orbit, the nerve pathways and blood supply to the eyes and structures in the orbit, the optic nerves, and subsequent pathways in the brain to the visual cortex in the back part of the brain.

How do we see?

The Eye

Light enters the eye through the cornea, at the front of the eye. This is a transparent dome, which has both protective and focussing properties. Light is refracted at the junction of air and the tear film on the surface of the cornea.

It then travels through the pupil (the aperture in the centre of the iris, the coloured part of the eye), which varies in diameter, depending on the amount the light entering the eye. If the environment is bright, the pupil constricts and if it is dark, the pupil dilates. The lens sits behind the iris, and it again, refracts light, to focus it on the retina. The lens can change its shape to ensure the image of both near and distant objects are focussed on the retina. The lens will become thicker, to focus near objects, and thinner, for looking in the distance.

The retina is composed of millions of light-sensitive cells (photo receptors) called rods and cones. Cones are vital for the clarity and sharpness of the vision, as well as being sensitive to colour, and are concentrated around the macular area where vision is at its sharpest, the rods are responsible for peripheral vision and night vision, but are not colour sensitive. They are more numerous than cones and much more sensitive to light, and form the large majority of the photoreceptors in the remaining retina, being grouped mainly in the periphery. They do not contribute to visual clarity as cones do. The image appears inverted on the retina.

The nerve impulses travel through the optic nerve from the retina, half to the visual cortex in the back of the brain on the same side, and half (through the splitting of the pathway) crossing over to the opposite side.

The brain normally assembles the impulses into a three dimensional image, which is perceived the right way up.

• Key Medical Assessment criteria
• The Eye
• Examination of the eyes
• Deeming Provisions
• Visual Impairment
• Registration of blindness
• Disabling effects
• Visual Acuity

Amended April 2011