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LETTER CHARTS

The Snellen 20/20 E-Chart :: The Snellen 20/20 letter chart was developed in 1862 by Hermann Snellen, a Dutch doctor. This letter chart tests visual acuity, or one's ability to identify high contrast small black letters on a white background. 20/20 means that a patient can read a certain letter target at 20 feet, 20/40 means that a person must stand at 20 feet to read the same letter that a person with 20/20 vision can read at 40 feet, and so on.

Pictured left is the traditional Snellen 20/20 E-chart. Many times people may be able to read the 20/20 letters, which indicates that they have a normal visual acuity, yet they feel that the quality of their vision is progressively getting worse. The Snellen test is not sensitive enough to detect such declines in the quality of our vision, which is our functional vision. Refer to Functional Vision and Contrast Sensitivity Defined for more detail on vision quality and blur vs. contrast.

(In references, see: Ginsburg AP. Spatial filtering and vision: implications for normal and abnormal vision.)

Below are two examples that illustrate how a typical Snellen-type letter chart and a typical street scene looks to a person with normal vision (Image 1) and to a person with low contrast sensitivity (Image 2):

Image 1 - Normal vision:

Image 2 - Low contrast vision:

As you can see in Image 2, a person with low contrast vision can still read the letters on the Snellen type 20/20 letter chart, but suffers a severe loss in the quality of their functional vision which is used to see the street scene. (In references, see: Ginsburg AP. Forensic aspects of visual perception.)

Additional Issues to Consider When Using Letter Charts :: Letters are complex targets. The letter E needs almost two times the number of spatial frequencies than does the letter L for minimum letter identification. This is why some letters are easier to read on the same letter chart line even though they are the same size and have the same letter stroke width. (In references, see: Ginsburg AP. Spatial filtering and vision: implications for normal and abnormal vision.)

Letters have complex contrast thresholds. Large letters will be detected and identified simultaneously as contrast is increased from not seeing to just being seen. However, the small letters need ten times the contrast after detection to be identified. This is illustrated in Figure 2 below. (In references, see: Ginsburg AP. Spatial filtering and vision: implications for normal and abnormal vision.)

Figure 2 - Letters have complex thresholds.

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