The Catford Drum is designed to be used as a method for the objective examination of how much babies and young children see, a subject which has been problematic for some time.
Optokinetic Nystagus was first observed in 1825 and evaluated in 1867. The eyes of a normal subject fix and follow an object to periphery of the field and then jerk back to follow the next. It has been found that targets of decreasing size may be presented until they become too small to be fixed and nystagmus is no longer elicited. Use of this phenomenon is made with the present technique.

The Catford Drum is made up of a hand held screen with a central aperture, a drum with targets consisting of a black line five minutes of arc thick offset by one minute placed around aperture. The drum is rotated by a motor which oscillates the appropriate target across the aperture with a slow phase and a quick return.