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Revolving Magnetic Field Monitor

Eye tracking with search soil techniques

Since its first introduction by Robinson in 1963, search coil techniques have been one of the most powerful methods for high resolution eye tracking. The operation principle of this technique is simple: A search coil worn on the eye (seen on the right) within an alternating magnetic field will be induced current proportional to the cosine of it angular relative to the field. Most search coil techniques measures the change in the amplitude of the induced signal and convert that to the angle of the eye's line-of-sight.

The generation of magnetic field is commonly achieved by means of Helmholtz Coils that consist of two circular coils of fixed radius, placed in parallel along a common axis and separated by a distance. An important drawback of this technique is that the small size of the area within which the magnetic field is uniform (see the figure on the left showing the magnetic field lines generated by Helmholtz coils). Notice that if the strength of the magnetic field changes, so will the amplitude of the induced signal, even if the search coil's orientation stayed constant. This means that if the eye translated in space without rotating, the non-uniformity of the field would results in erroneous readings of its orientation. For accurate measurements in a cubic volume with 1 cm dimension, the size and the coils and the power need to generate the field reaches to impracticable values, which forces experimenters to restrained the head to prevent translation. 

In 1977 Han Collewijn proposed a different design, which circumvented this caveat and allowed high precision recordings of eye movements during head-free viewing. 
When the head is restrained the method can measured eye rotations as small as 1 minute of arc (1/60th of a degree).makes it currently the only method that can allow measurements of eye movements as small as 1 minute of arc (1/60th of a degree) while subjects can freely move their head.