| Videoaberroscope & Videokeratoscope | |
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The Nidek ARK 10,000 instrument is described by the manufacturer as an OPD-scan (Optical Path Difference Scan). It combines four different instruments in one: Autorefractor - Autokeratometer - Videoaberroscope - Videokeratoscope The instrument is the first used in the pre-consultation and the basis of the determination of the prescription and detection of any optical anomaly of the eye. The functions of the instrument are described as if it was four different instruments.
Auto-Refractor
The auto-refractor gives an automated unbiased measurement of the patient's optical/spectacle correction. It does that by focusing an infra-red light towards the retina (back of the eye) while the patient looks at a distant scene with a road and inflatable balloon. The instrument then displays the correction made up of two parameters
Sphere: level of myopia or hyperopia.
Cylinder power and axis: level and direction of astigmatism
The sphere and the cylinder power and axis of the optical/spectacle correction are automatically fed to the refractor head to be used by the optometrist as the starting point for the subjective refraction to obtain the final spectacle correction.
Videoaberroscope
The videoaberroscope simultaneously to the auto-refractor, measures the optical quality of the eye. Once the sphere and cylinder are corrected, there are remaining optical defects of the eye called aberrations (at times they are called higher order aberrations). Such aberrations are particularly problematic when the pupil is large, in dim light conditions. One such condition is night time driving.
The videoaberroscope gives three sets of measurements:
The Total Aberration RMS
The RMS for each of 26 individual aberrations
The best optical correction for the eye under dim and low light conditions
Auto-keratometer
The corneal front surface is the most important optical surface of the eye; alone that surface accounts for about two-thirds of the total optical power of the eye. An accurate measurement is therefore very important. The auto-keratometer determines the curvature of the central part of the front surface of the cornea.
The instrument displays the curvature in mm and the axis of the measurement in degrees for two extreme positions
Flattest central corneal radius (mm).
Direction of flattest central corneal radius (o).
Steepest central corneal radius (mm).
Direction of steepest corneal radius (o).
From the measurements above the corneal cylinder (corneal astigmatism) is calculated.
Videokeratoscope
The Videokeratoscope measures the curvature of the front surface of the cornea over most of its surface. The measurements, which are made simultaneously to the measurements of the central corneal radii by the auto-keratometer, produce several parameters:
The curvature at any point of the cornea
The rate of change of curvature of the cornea ( Eccentricity)
The aberrations produced by the cornea |
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