Ocular microtremor (OMT) is a physiological high frequency (up to 150Hz) low amplitude (150-2500nm) involuntary
tremor of the human eye. It is one of the three fixational ocular motions described by Adler and Fliegelman in 1934 as
well as microsaccades and drift. Clinical OMT investigations to date have used eye-contacting piezoelectric probes or
piezoelectric strain gauges. Before contact can be made, the eye must first be anaesthetised. In some cases, this induces
eyelid spasms (blepharospasm) making it impossible to measure OMT. Using the contact probe method, the eye motion
is mechanically damped. In addition to this, it is not possible to obtain exact information about the displacement. Results
from clinical studies to date have given electrical signal amplitudes from the probe. Recent studies suggest a number of
clinical applications for OMT, these include monitoring the depth of anaesthesia of a patient in surgery, prediction of
outcome in coma, diagnosis of brainstem death. In addition to this, abnormal OMT frequency content is present in
patients with neurological disorders such as Multiple sclerosis and Parkinson's disease. However for ongoing clinical
investigations the contact probe method falls short of a non-contact accurate measurement solution. In this paper, we
design a compact non contact phase modulating optical fiber speckle interferometer to measure eye motions. We present
our calibration results using a calibrated piezoelectric vibration simulator. Digital signal processing is then performed to
extract the low amplitude high frequency displacement information.
Ocular Microtremor (OMT) is a continual, high frequency physiological tremor of the eye present in all subjects even
when the eye is apparently at rest. OMT causes a peak to peak displacement of around 150nm-2500nm with a broadband
frequency spectrum between 30Hz to 120Hz; with a peak at about 83Hz. OMT carries useful clinical information on
depth of consciousness and on some neurological disorders. Nearly all quantitative clinical investigations have been
based on OMT measurements using an eye contacting piezoelectric probe which has low clinical acceptability. Laser
speckle metrology is a candidate for a high resolution,
non-contacting, compact, portable OMT measurement technique.
However, tear flow and biospeckle might be expected to interfere with the displacement information carried by the
speckle. The paper investigates the properties of the scattered speckle of laser light (λ = 632.8nm) from the eye sclera to
assess the feasibility of using speckle techniques to measure OMT such as the speckle correlation. The investigation is
carried using a high speed CMOS video camera adequate to capture the high frequency of the tremor. The investigation
is supported by studies using an eye movement simulator (a bovine sclera driven by piezoelectric bimorphs). The speckle
contrast and the frame to frame spatiotemporal variations are analyzed to determine if the OMT characteristics are
detectable within speckle changes induced by the biospeckle or other movements.
Ocular microtremor (OMT) is a physiological high-frequency (up to 150 Hz) low-amplitude (25-2500 nm peak-to-peak) involuntary motion of the human eye. Recent studies suggest a number of clinical applications for OMT that include monitoring the depth of anesthesia of a patient in surgery, prediction of outcome in coma, and diagnosis of brain stem death. Clinical OMT investigations to date have used mechanical piezoelectric probes or piezoelectric strain gauges that have many drawbacks which arise from the fact that the probe is in contact with the eye. We describe the design of a compact noncontact sensing device to measure OMT that addresses some of the above drawbacks. We evaluate the system performance using a calibrated piezoelectric vibrator that simulates OMT signals under conditions that can occur in practice, i.e., wet eye conditions. We also test the device at low light levels well within the eye safety range.
Ocular microtremor (OMT) is a biological high frequency (up to 150Hz) low amplitude (25-2500nm peak to peak)
involuntary motion of the human eye. Clinical OMT investigations to date have used eye-contacting mechanical
piezoelectric probes or piezoelectric strain gauges. Before contact can be made, the eye must first be anaesthetized. In
some cases, this eyelid spasms occur making it impossible to measure OMT. Using the contact probe method, the eye
motion is mechanically loaded. Results from clinical studies with this method to date have given electrical signal
amplitudes from the probe proportional to the displacement, but not the exact displacement information. Recent studies
suggest a number of clinical applications for OMT, these include monitoring the depth of anesthesia of a patient in
surgery, prediction of outcome in coma, diagnosis of brain stem death. In addition to this, in patients with neurological
disorders such as Multiple Sclerosis and Parkinson's disease, abnormal OMT frequency content is present. In this paper,
we design a compact non-contact phase modulating optical fiber speckle interferometer to measure eye motions. We
simulate OMT motion using a calibrated piezoelectric vibration simulator and compare results produced using a contact
method with those using our optical non-contact method.
KEYWORDS: Speckle, Interferometers, Digital signal processing, Signal to noise ratio, Sensors, Modulation, Signal processing, Motion measurement, Calibration, Demodulation
Contact techniques exist to measure low amplitude low frequency mechanical vibration, however, by mechanically
loading the system of interest, they affect the measured results. In this paper, we design a compact non-contact optical
fiber speckle interferometer to measure inplane displacements. We implement this under laboratory conditions, and
present our calibration results, measuring low-amplitude microvibrations from 0.34 nm to 1.5 μm over a frequency range
from 10 Hz to 150 Hz.
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