Perimetry is the quantification of the extent of the visual fields with a perimeter apparatus.
Manual and computerised machines are used, but both work on the same following principles:
- the patient sits opposite the machine and places their head on a fixed rest so as to provide a constant position for visualisation of targets
- the patient closes, or has patched, one eye
- with the other eye they focus on the central hub of the instrument while a dot of light moves in on successive meridians
- they are asked to give a response - e.g. verbal or pressing a button - when the target is first seen
- the meridians are changed so that the whole field is mapped on perimetry paper, and successive maps can be drawn with lights of different colours and sizes
- the other eye is tested in the same way