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18 May 2010Non-contact measurement of ocular microtremor using laser speckle
The human eye moves continuously even while it appears to be at rest. The involuntary eye movements causing this
motion are called fixational eye movements. Ocular Microtremor (OMT) is the smallest (150 - 2500nm amplitude) and
fastest (~ 80Hz) of these eye movements. OMT has been proven to provide useful clinical information regarding depth
of consciousness and neurological disorders.
Most quantitative clinical investigations of OMT have been carried out using an eye-contacting piezoelectric probe.
However, this measurement procedure suffers from a number of disadvantages which limit the potential of the technique
in the clinical environment. The need for eye contact requires the eye to be anaesthetised and not all subjects can tolerate
the procedure.
A promising alternative to the piezoelectric technique is speckle metrology. A speckle correlation instrument for
measuring OMT was first described by Al-Kalbani et al. The approach presented in this paper is a non contact
measurement technique implementing laser speckle correlation and using a highly light sensitive video camera operating
at 500Hz.
The OMT measurement technique in this paper was investigated using a human subject and an eye movement simulator.
Using this system, measurement of speckle on the eye takes only a few minutes, no eye drops are necessary and no
discomfort is caused to the subject. The paper describes the preliminary results of capturing speckle from the simulator
and from the human eye in-vivo at eye safe laser powers. The effects of tear flow, biospeckle and speckle shifting by
larger eye movements on the displacement information carried by the speckle are also discussed.
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E. Kenny, D. Coakley, G. Boyle, "Non-contact measurement of ocular microtremor using laser speckle," Proc. SPIE 7715, Biophotonics: Photonic Solutions for Better Health Care II, 771528 (18 May 2010); https://doi.org/10.1117/12.854557