Diagnosis of Visual impairment

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The main diagnostic tests is a simple eye test involves reading letters from a chart. The letters become progressively smaller on each line of the chart. If you are taking the test for the first time, you should not use your glasses or contact lenses. If you have taken the test before and have previously been prescribed glasses or contact lenses, your optician may want you to take the test while wearing your glasses or contact lenses.

If you can only read the top line of the eye chart, you may be eligible to register as blind or partially sighted. You will need to be referred by your GP or optician to a hospital eye doctor who will carry out further tests and write to you local social services department confirming that you have been diagnosed as blind or partially sighted.You may also be registered as blind or partially sighted if you can read the first three lines of the chart but have a very restricted field of vision.

If your optician decides that you may need to wear glasses or contact lenses, you will need to take further tests to work out exactly what kind of glasses you need. You may be asked to wear special lenses to look at various charts containing different letters or colours.

Your optician will also look for signs of eye conditions such as glaucoma and diabetic retinopathy, by shining a light into your eyes to see how well they react to light.

At the end of the eye test, your optician will discuss the results of the test with you and advise you of your best course of action. They will also provide you with a certificate to say that you do not need to wear glasses or contact lenses, or give you a prescription for the type of glasses or contact lenses that you need to wear. You can use this prescription at any optician.

If you are at a higher risk of conditions such as glaucoma or diabetic retinopathy, or your optician has found signs of these conditions, you may need to have further tests:

A Visual Field Test (also sometimes called the Humphrey Visual Field Assessment or perimetry) tests for peripheral or side vision. During this test one of your eyes is covered while you focus your other eye on a central point. Random lights are flashed around this central point and you must hit a buzzer every time you see a flashing light. The lights appear at various intervals and in different places on the screen. They may also be of varying brightness. The test is then repeated with the other eye covered. In total, the test should take about 45 minutes to complete.
Tonometry is also used to test for conditions such as glaucoma. Tonometry is a method of measuring the pressure inside the eyeball. This can be done using a tonometer, a sensor that blows a small amount of air at the eyeball and then tests how much indentation the air has caused. Sometimes the tonometer is placed directly against the eyeball and pressure is applied in order for a reading to be taken. Tonometry may cause a small amount of discomfort.
Ophthalmoscopy is a method a doctor can use to look at the inside of the eye in detail including the optic nerve. A small torch is used very close to the eyeball to allow the doctor to spot any abnormalities.