Introduction to Low Vision

As a fully sighted individual, it can be difficult to understand why people with low vision see if very different ways to one another and to fully sighted people.

This unit will provide you with an overview of how the eye works, what can happen if parts of the eye aren't working properly and its impact on how people see in their day to day lives.

Overview of How the Eye Works

Click on the tabs to find different information

We need light in order to see. The passage of light through the eye is as follows:

Light bends as it travels through the eye.
It travels through a clear cornea where the light rays are bent
It then travel travels through the pupil which constricts (in bright light) or dilates (in low light) to control the amount of light entering the eye
Next it meets the lens which changes shape enabling objects to be seen clearly at different distances from the eye
The light continues through the vitreous humor, a jelly-like substance, that helps to keep the shape of the eye
After this, it meets the retina (which is nourished by the choroid) where the light rays are converted to electrical neural impulses by photoreceptor cells called rods and cones. These electrical neural impulses are transmitted via the optic nerve to the brain, where messages are received and processed.

Diagram of the human eye highlighting the cornea, pupil, lens, vitreous humor, retina, choroid and optic nerve

Diagram of the from above of the eyes and brain, identifying optic nerve, optic chiasm, optic tract, lateral geniculate nucleus and optic radiation

The diagram above illustrates how the eye and the brain work together to allow us to see and understand an object in our environment.

The image that we see in the environment is received in both eyes and then transmitted via the optic nerve and visual pathway to the brain, where the message is received and understood.

Elements along the optic path are the:

Optic nerve
Optic chiasm
Optic tract
Lateral geniculate nucleus
Optic radiation (visual centre of the brain)

We use our eyes to see in different ways. The allow us to see:

Detail, which allows us to read (Visual Acuity)
Clearly at difference distances (Focus and Acuity)
A complete picture of the world (Visual Field)
Colour, Contrast and Movement

Photograph of a person holding a globe at their waist level, standing outside on the grass with a sunny sky in front of them.

Possible Types of Low Vision

Click on the different headings to learn about possible types of low vision

Visual acuity is our ability to see clearly and in detail. It is tested using a Snellen Chart.

The Snellen Chart measurement is used to determine a person's level of low vision.

The chart shows letters in decreasing size, with a number underneath each row of letters. (60, 36, 24, 18, 12, 9 and 6)

The testing distance from the chart is 6 meters. Results are displays as a fraction. The top number is the testing distance. The bottom number is how far down the chart the person is able to read.

Normal vision: 6/6 - This means that someone at a testing distance of 6 meters can read the bottom line on the chart (the line with 6 underneath).

Someone with a mild visual impairment would be able to read the fifth line down (line with a 12 underneath) so would have a reading of 6/12

Someone with a severe visual impairment would only be able to read the top line with the 60 underneath and so would have a reading of 6/60

If the top number could not been seen at 6 meter distance but at 3 meter distance, the reading would be 3/60, which is a profound visual impairement

Snellen Chart showing letters of decreasing size.

What is Visual Field?

"Imagine that you are sitting in a room, looking at a large wall, the visual field is the area of the wall that can be seen without moving your eyes or head" David B Henson

Full Visual Field = Central Vision + Peripheral Vision

Peripheral Vision Loss means that only central vision is retained.

Example illustrating a peripheral field loss:

Picture 1 is how a fully-sighted individual would see a picture of a dog.

Picture 2 is how someone with Peripheral Field Loss might see the same picture of the dog. This is known as tunnel vision.

Example of a limited field of vision showing a dog but only a circular portion of the fur in the centre of the dog's back is visible.

Remaining Vision: Tunnel Vision ("What" Vision)

Photoreceptor cones are working but there are problems with the rods
Cones need light to work and are responsible for colour and detail
With Tunnel Vision, someone may be able to read, do detailed tasks and see colour
However, someone will experience night blindness, difficulty adjusting to changes in lighting levels and in bright sunshine
Movement can be difficult as obstacles disappear or suddenly appear
Also, people can appear without warning if they approach from the person from the side
Standing to close to the person will mean than they may not be able to see your entire face which makes communication difficult (Too close; too big)

Central Field Loss means that only peripheral vision is retained.

Example illustrating a central field loss:Picture 1 is how a fully-sighted individual would see a picture of two children holding balls.

Two boys with balls and the entire image can be seen.

Picture 2 is how someone with Central Field Loss might see the same picture the two boys.

Two boys with balls and the centre of the picture is obscured but the outer portion is blurred but can be seen.

Remaining Vision: Peripheral Vision ("Where" Vision)

Photoreceptor rods are working but there are problems with the cones
Rods work in low light and are responsible for shape, shadow and movement
The central loss may appear like an image vacuum/absence or as a scotoma which appears like a dark blob
Moving might be OK as someone has enough vision to see and avoid obstacles and possibly detect steps but will experience difficulty with detailed tasks such as reading, identifying colours and recognising faces
The person may adapt unusual head positions and task position to make best use of their vision, this is called eccentric fixation
The person may not make eye contact in attempt to see faces

Central and Peripheral Field Loss is complex and results from damage to different parts of the eye impacting both visual fields.

Cause of loss may be situated at the front or back of the eye or along the visual pathway
May have any one of or combination of types of vision depending on cause of loss – Distorted vision, blurred vision, patchy vision, altered colour vision, half/quarter visual field, light perception, total loss of sight

Introduction to Low Vision