Perimetry is the systematic measurement of differential light sensitivity in the visual field by the detection of the presence of test targets on a defined background. Visual field testing can be performed clinically with confrontational field testing keeping the subject's gaze fixed while presenting objects at various places in their visual field. This is generally used to explore the extreme boundaries of the visual field.

Perimetry more carefully maps and quantifies the visual field. Simple manual equipment can be used such as in the tangent screen test or the Amsler grid. When dedicated machinery is used it is called a perimeter. Kinetic perimetry uses a mobile stimulus moved by a perimetrist such as in Goldmann kinetic perimetry. Kinetic perimetry is useful for mapping visual field sensitivity boundaries. Threshold static perimetry is generally done using automated equipment. It is used for rapid screening and follow up of diseases involving deficits such as scotomas, loss of peripheral vision and more subtle vision loss. Perimetry testing is important in the screening, diagnosing, and monitoring of various eye, retinal, optic nerve and brain disorders.

The term is often used in optometry and ophthalmology, where a visual field test is used to determine whether the visual field is affected by diseases that cause local scotoma or a more extensive loss of vision or a reduction in sensitivity (threshold).

It is also important to note that it also matters whether you avert right or left. The most effective direction is that which places the object on the nasal side of the vision. So, for right-eyed observers it is best to shift to the right, and for left-eye observers it is best to shift to the left. Some people also claim that it is better to avert up instead of down. The best thing to do is practice and find the best location for one's own eyes.