Suppression occurs during binocular vision when the brain does not process the information received from either of the eyes.

At both testing distances the patient is required to wear red-green goggles (with one red lens over one eye, usually the right, and one green lens over the left) When performing the test at far (distance) the W4LT instrument is composed of a silver box (mounted on the wall in front of the patient), which has four lights inside it.

With both eyes open, a patient with normal binocular vision will appreciate four lights.

The Worth Four Light Test is indicated for use when assessing the binocular functions, the ability of eyes to work in coordination, of an individual.

It can be used when wanting to assess whether the individual has a normal or abnormal binocular single vision response (BSV).

In cases of manifest strabismus the test can help in determining the nature and type of the diplopia or which eye is suppressing.

Therefore, is indicated in cases of a suspected central suppression scotoma as it can be used to detect where the lights may not be appreciated from the eye with the scotoma though in some cases of minimal deviation in the eye as demonstrated in a microtropic deviation a normal response of 4 lights may be reported.

[citation needed] Other indications for the test include establishing an individual's dominant eye dominant eye compared to the other and when evaluating reduced monocular visual acuity which shows no improve on pinhole testing.

Whilst there are no contraindications of the W4LT there needs to be caution in interpreting the results of individuals with BSV in natural conditions as they may show a diplopic response under the dissociation of the test.

For a near measurement, the test should be performed at approximately one third of a metre, or thirty three centimetres, with a handheld Worth's Four Lights torch.

It is also important to note the distance at which the test was conducted and whether or not the patient wore their own refractive correction.

When questioned they will report that: This is recorded as : W4LT (D): 4 lights (BSV) It will be demonstrated on cover test that the patient has a manifest deviation.

Despite their apparent deviation, when tested with the W4LT they will produce a normal BSV result, indicating the presence of Abnormal Retinal Correspondence.