Welch Allyn SureSight Vision Screener - User Manual - Manual (Page 24)

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• S – Sphere: Negative numbers are representative of myopia, or near-
sightedness. Myopia is when distant images are blurry resulting from too
much focusing power in the lens and cornea (i.e., the eye is too long). The
light rays focus the image in front of the retina rather than at the retina.
Positive numbers indicate hyperopia, or far-sightedness. Hyperopia is a
condition in which blurred vision occurs at a close distance due to the rays of
light converging on the retina short of the focal length at which they would be
in focus. Causes also may include a weakness in the ability of the lens to
focus, particularly at closer distances.
• The ‘C’ displayed represent the cylinder, which is a measure of
astigmatism, or asymmetry of focus. Astigmatism is a refractive error due
to an uneven curvature of the cornea, blurring some of what the person sees.
An astigmatic eye typically has areas of the cornea that are steeper or flatter
than normal, which produces distorted vision.
• The ‘D’ found under the left eye measurements indicate the difference in
power between the two eyes. This is only displayed when the unit is in child
calibration. This difference is mean sphere (D=S+½C) computed taking into
account both the sphere and cylinder reading. It will typically be different
than the algebraic difference between spheres.
1 Peter’s H.G.,“Peter’s Multiple Tables,” Borish Clinical Refraction, Vol., 1, 3 rd Edition, p. 371