Have u ever think who was the introducer or inventor for the eye chart? Here are some points to share. Hermann Snellen , a Dutch professor of ophthalmology, put the E on top of the eye chart in 1862. Although his very first chart was headed by an A, Snellen quickly composed another chart with E on the top.Snellen succeeded Dr Frans Cornelis Donders as Director of the Netherlands Hospital for Eye Patients. Donders was then the world's foremost authority on geometric optic.Snellen was trying to standardise a test to diagnose visual acuity , to measure how small an image an eye can accept while still detecting the detail of that image.Donders's complicated formulas were based on three parallel lines, of all the letters of tha alphabert , the capital E most closely resembled the lines that Fonders had studied so intensively . Because Donders had earlierst determined how the eyes preceivers the E, Snellen based mush of his methamatical work on the fifth letter.
The three horizontal limbs of the E are saperated by an equal amount of white space. In Snellen's original chart , there was a ont-to-one ratio between the height and width of the letters, and the gaps and bars were all the same lenth (in some moden eye charts, the middle bar is shorter).
Louanne Gould of Cambridge Instruments says that the E, unlike more open letters like L or U, forces the observers to distinguish between white and black, an important constituent of good vision. Without this ability, Es begin to look like Bs, Fs, Ps or many other letters.
Of course, Snellen couldn't make an eye chart full of only E's. or else all hos patients would have 20-10 vision. But Snellen realised that it was important to use the same letters many times on the eye charts, to ensure that the failure of an observer to identify a letter was based on a visual problem rather than the relative difficulty of a set of letters. Ian Bailey, Professor of Optometry and Director of the Low Vision Vloni at the University of California at Berkeley, says that it isnt so important whether an eye chart uses the easiest or most difficult letters. Most eye charts incorporate only ten different letters, ones that have the smallest range of difficulty.
Today, manu eye charts do not start with an E - and there is no technical reason why they have to - but most still so. Dr. Stephen C Miller of the American Optometric Association suggests that the desire of optical companies ti have a standardised approach to the production of eye charts probably accounts fr the preponderance of E charts.
And we're happy about it. It's a nice feeling to know that even if our vision is falling us miserably, we'll always get the top row right.