The American Optician

Spring 2017

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2 3 CLICK HERE TO SIGN-UP TODAY! Fresh, vital content. Delivered straight to your inbox every month. This only created incapability among test charts. Donders asked Snellen to develop a testing chart he could use for his refractive studies, and Snellen adopted Donders' 5-minute and 1-minute angle separation concept into his chart . Instead of using printing fonts like Jaegar, Snellen created optotypes, which he constructed using a 25-box grid. He originally tried symbols and characters but settled on letters. Using the grid to create the letters (or any other character or symbol) ensures that every character conforms to the 5-minute and 1-minute rules. Click here for an illustration . In following Donders' visual acuity formula, Snellen's chart also increased the letter size for each line, and each line's letters were proportional in size to all the others on the chart . In doing this, it was easy to conduct sight testing at any distance and simply convert the results. And because Snellen used a mathematical formula for his chart as an external standard, it was easily reproducible. Both Donders and Snellen realized that vision often was better than the standard letter size they had chosen. This is likely why Snellen's chart included test targets for 20/15 and 20/10 vision. Snellen's original eye chart used a distance of 20 Parisian feet. At that time the metric system had not been universally adopted, and when it was in 1875, he altered his chart for a 5-meter testing distance and a 6-meter chart for those wanting to stay with Parisian feet. Today, 6-meters and 20- feet are the recognized distances. Many other charts were developed after Snellen's, but nearly all of them found little use. One that was worthwhile and still in use is Edmund Landot's "rings" or "C" chart. The beauty of this chart is that it uses a single optotype and each has the same letter opening size . The only difference between each one is the orientation of the opening. This takes the variability of having different letters. The chart is also useful for young children and others who cannot speak; they simply point in the direction of the letter opening. While Herman Snellen was most noted for his vision testing charts, he was also a noted surgeon and invented several medical procedures including ones for entropion, ectropion and trichiasis. While our understanding of vision assessment is changing, Snellen's contribution to vision testing will no doubt be highly utilized for a very long time. Ed De Gennaro is Editor Emeritus for First Vision Media Group.

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