VISUAL ACUITY--SNELLEN WHO?

SNELLEN CHART

We have all seen this chart before.  It is called the Snellen Chart.  It is the most commonly used chart to determine visual acuity.  This chart was developed by Herman Snellen in 1862.

When the eye is 20 ft from the chart and can read the 7th or 8th line clearly (depending on the chart used) then the eye is regarded as having 20/20 Visual Acuity.

20/20 = 6/6 metric conversion (meters)

Perhaps I should backtrack and explain what Visual Acuity means.

Visual Acuity:  Measurement –Assessment of the eye’s ability to distinguish object details and shape, using the smallest identifiable object that can be seen at a specified distance (usually 20 feet or 6 m)

The Numerator, or the number on top (first number) represents the test distance.  This is commonly 20 feet or 6 meters.  The Denominator, or the bottom number (second number) is the distance from the chart at which a person with normal vision can see the chart letters.

 

 

20/20—Means that a person can distinguish at 20 ft what a normally sighted person can see at 20 ft.   (normal vision)

20/50—Means that a person can distinguish only at 20 ft what a normally sighted person can see at 50 ft.  (worse )

What does 20/200 represent?