The eye needs light to work. Sometimes people can have problems with bright light and this can happen for a number of reasons.
How the eye works
We need light to see what is around us and to see colour. Light bounces off the objects we look at. These reflect different amounts of light which we see as different colours.
Front of the eye
Light rays enter the front of our eye through the clear cornea and lens. It is very important that both the cornea and lens are clear as this allows the light to pass directly through the front of the eye to the retina.
The cornea and lens bend light so that it can focus on the retina at the back of our eye. This gives us a clear, precise image. The cornea focuses the light towards our retina. The lens fine tunes the focussing of this light.
Our tears form a protective layer at the front of the eye and also help to direct the light coming into our eye. The iris, the coloured circle at the front of our eye, changes the size of the pupil which allows different amounts of light into our eye.
The pupil is the dark hole in the middle of the coloured part of our eye. The pupil gets smaller in bright conditions to let less light in and bigger in dark conditions to let more light in.
Middle of the eye
The middle of our eye is filled with a jelly-like substance called the vitreous. The vitreous is clear and allows light to pass directly from the front to the back of our eye.
Back of the eye
The retina at the back of the eye is a light-sensitive layer which consists of rod and cone cells. These cells collect the light signals directed onto them and send them as electrical signals to the optic nerve at the back of our eye.
Rod cells are concentrated around the edge of the retina. They help us to see things that aren't directly in front of us, giving us a rough idea of what is around us. They help us with our mobility and getting around by stopping us from bumping into things. They also enable us to see things in dim light and to see movement.
Cone cells are concentrated in the centre of our retina where the light is focused by the cornea and lens. This area is called the macula. Cone cells give us our detailed vision which we use when reading, watching TV, sewing and looking at people's faces. They are also responsible for most of our colour vision.
The optic nerve is made up of thousands of nerve fibres. These fibres pass the electrical signals along to our brain where they are processed into the image we are looking at.
How we see
Seeing can be likened to the process of taking pictures on a film with a camera which you then get developed. The retina is like a camera film which stores an image of what we are looking at. The image directed onto the retina is then sent along to the brain where it is processed, like developing a camera film. Therefore we actually 'see' in our brain with the light information sent to it from our eyes. This whole process happens very quickly so that everything we see is in focus.
Effects of light on vision
Our eyes need light to work. Light entering the eye is collected by the retina and processed by the brain to obtain the pictures that we need to see. Light is an essential part of this process, for example it is difficult to read when light levels are low. Sometimes light can cause problems for our vision.
Sometimes light can make it harder to see
There are times when the amount of light or the quality of that light can affect our ability to see. Many people with low vision need more light than usual to read. However, too much light can cause problems from glare.
Glare is when a light source affects our ability to see clearly. There are two types of glare, discomfort glare and disability glare.
Discomfort glare occurs when a light source is just too strong for our eyes. It may cause us to screw up or shade our eyes, it can even cause us to close our eyes.
A good example of discomfort glare is leaving a dark room and moving into bright sunlight. As our eyes adjust, the light feels uncomfortable and sometimes painful. Usually our eyes adjust to the new source of light and we are able to see clearly again after a few seconds. The eye adjusts to the new level of light by making our pupil smaller (constricting). As the name suggests, discomfort glare can make our eyes feel uncomfortable.
Disability glare Disability glare reduces our visual performance. Disability glare can be caused by eye diseases and can occur with ordinary light sources and levels of light.
The structures of the eye are normally clear, which means the light passes directly through our eye. As we age certain eye problems can make the eye's structures less clear. The most common example of this is a cataract.
When someone has a cataract, the lens in their eye is not as clear as it should be. This causes the light entering the eye to scatter. The light does not pass smoothly through the lens and scatters throughout the eye. This effect, as well as making things unclear and blurred, also makes coping with bright light difficult. The amount of scattering of light depends on the amount of light entering the eye and how thick the cataract is. This means that a brighter light will scatter more, causing things to become more blurred and causing more glare.
People with cataracts often have a lot of trouble with glare.
Causes of glare
Some people are born with a lack of pigment in the eye, ocular albinism, and this causes glare. Eye diseases such as cataracts, macular degeneration and uveitis can cause glare. Other eye problems such as conjunctivitis and corneal problems can also cause difficulties with glare. There are some conditions such as meningitis that cause light to be painful.
If you start to experience light sensitivity then it is always best to have it checked out by an optometrist as it may be the first sign of an eye disease which may need treatment
If the onset of the light sensitivity is very sudden, then it should be checked as soon as possible in case it is the first sign of a more dangerous condition such as meningitis.
Some drugs taken for other conditions can also cause light sensitivity for example tetracycline, an antibiotic, and digitalis, a drug which is used for heart problems.
The treatment for light sensitivity depends on the cause. Usually, if the light sensitivity is a symptom of an underlying eye problem such as cataract, then treatment for the cataract can solve the glare problem. Treating eye diseases, like uveitis, often means that the eye becomes less light sensitive.
Unfortunately not all light sensitivity caused by eye problems can be treated. For example the light sensitivity caused by macular degeneration. If this is the case then other methods may be needed to help people cope with bright light.
Other methods are there for coping with light sensitivity
The most obvious way to cope with glare is to limit the amount of light that is entering the eye. Some people find shading their eyes with their hand or wearing a hat with a wide brim can help cut down on glare with little expense.
Tinted lenses help to minimise the light entering the eye and so cut down on the amount of glare someone experiences.
Sunglasses should have an UV filter so they also protect your eyes from the harmful UV rays of the sun. The best lens will have protection against UVA and UVB light.
Some people prefer to have light activated sunglasses, which get darker in brighter conditions, though this is a personal choice.
Polarized lenses can cut down on reflected glare from flat surfaces, for example, light reflected off water or off the bonnet of a car.
There are sunglasses, sometimes called solar or UV shields or wrap-around shades, which are larger than normal. They have built in sides which stop the light entering that way and they are also made to stop the light entering from above the eyes. They can also be worn over regular spectacles and come in a variety of tints. These can be very helpful but the fit of them is important. Everyone has a different shaped head and several pairs of wrap-around sunglasses should be tried to find the ones which fit closest and stop the most light from coming in around the edge of the frame.
If you visit a low vision specialist they may be able to give you advice on coping with glare. They can talk to you about the best way to use lighting to avoid glare. This is especially important since it can be difficult to balance the amount of light someone with a sight problem needs for tasks such as reading and the fact that they may have a problem with glare.
Some eye conditions can cause people to have great difficulty when moving between areas with different lighting levels. For example, when moving from sunlight into a dim room, or vice versa. It may be necessary to pause to put sunglasses on or to take them off and give the eyes time to adjust, and it is very important that the person does not feel rushed. Often, these changes in light levels occur at the entrance of buildings, this is particularly dangerous as there are often steps in these places.
Many people with a sight problem need to use a task light for activities such as reading. The best lights for this type of task are adjustable lamps. The best position for an adjustable lamp, when you need more light for a task such as reading, is between you and what you want to look at, directed at the task, and below your eye level.
More information on lighting is given in making things easier to see.
Light sensitivity in people without eye disease
Some people experience light sensitivity without any eye disease. It is still important, if sensitivity to light starts, to have your eyes checked by a professional since there may be some eye disease that hasn't yet been diagnosed. Unfortunately some people are sensitive to light without a physical cause.
People who suffer from migraine tend to be more sensitive to light, to a marked degree during a migraine headache and also to a lesser degree at other times. Sometimes, certain types of light or patterns can trigger migraine. People with migraine may therefore benefit from special tinted lenses, called precision tinted lenses. These can also help people who experience glare from pages of text, including some people who have dyslexia. Precision tinted lenses are discussed below.
It is important to remember that there is a scale of light sensitivity. Some people are just more sensitive to light than others. Also as we grow older we can also become more sensitive to light, this is because the eye changes even though there is no disease.
Even though there may be no physical cause for the light sensitivity it can sometimes be very intense. The advice about sunglasses and hats can often help someone with light sensitivity. Wearing correct sunglasses will not harm the eyes or make them lazy.
Coloured lenses with particular tints which restrict different wavelengths of light, are often used to help people with light sensitivity.
The colours of these tints range from yellow, to reds and to blues. At the moment there is no strong evidence that a particular shade of tint suits one eye condition over another, and it would appear that choosing the tint is a matter of personal choice. For example two people with macular degeneration may prefer two completely different coloured tints. However, people with conditions where light is scattered in the eye (eg cataract) often benefit from filters that block out short wavelengths, such as yellow filters.
Most optometrists and low vision providers will be happy to discuss with someone the tints available; they can also usually lend some form of tint to test over a period of time. This may help you find the best tint for helping with your light sensitivity.
A special instrument, the MRC Intuitive Colorimeter, is sometimes used to determine the colour of filter that will be optimal for a given person. This instrument is not often used for people with low vision, but is commonly used to prescribe coloured lenses for people with migraine, reading problems (eg dyslexia), or epilepsy. This is because people with these conditions often find a precise colour, which is different for each person, to be much better than general colours.
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