A Snellen Test (Chart) is one of the most
common and widely recognised tools for assessing patient eyesight. It
is used by medical doctors, including ophthalmologists, as well as High
Street opticians.
A Snellen Chart is used for testing Visual Acuity, which
is a measure of sharpness of vision, i.e. a person's ability to distinguish
and resolve visual detail. This is generally tested in a controlled environment
with appropriate light level, each eye being tested separately.
The Snellen Chart itself consists of rows of capital
letters in a standard simple font.
The largest character forms the first "line", the remaining
characters (which are also capital or "upper-case" letters)
being on the lines beneath it and progressively smaller in size.
The "patient" (in medical/clinical situations) or "subject"
(if/when the test is carried out for the purposes of non patient-specific
reseach), is positioned a fixed distance from the Chart, generally
6 metres (6m) in Europe or 20 feet in the United States, those distances
being similar. He/she then reads the series of letters beginning at the
top of the Chart and continuing to read along each line then down to the
left-hand-side of the next line below until he/she is unable to read any
further because the characters on the chart are so small that they appear
blurred, hence indistinguishable, to the viewer. People who are able to
distinguish every letter down to the bottom line of the Chart are generally
deemed to have "perfect" Visual Acuity. In all other cases the
person's Visual Acuity is defined in terms of the last line on the Chart
whose characters they are able to correctly identify.
See also the definition of the Sheridan-Gardiner
Test.
This section includes short definitions of clinical and surgical procedures
concerning the eyes and human visual system. For definitions of other
terms in this category, choose from the list to the left. Other related
pages include the diagram
of the eye, and definitions
of parts of the eye, a description
of the human retina, and the section about disorders
of the eye and visual system. |
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