The American Optician

Spring 2017

Issue link:

Contents of this Issue


Page 21 of 22

2 2 While most people in a profession strive to make some contribution to their field, a rare few have a profound and lasting impact on it. Think Bill Gates and computer technology, Jonas Salk and the polio vaccine, Albert Einstein and the theory of relativity. In the ophthalmic profession, Herman Snellen was such a luminary. Snellen was born in Zeist, The Netherlands, in 1834. His father, F.A. Snellen, was a well-known and well-liked physician. He received his medical training at the University of Utrecht from some of the most knowledgeable and prodigious ophthalmologists of the time, most notably Franz Cornelius Donders , one of the founders of scientific ophthalmology. He graduated in 1857 and worked in the Netherlands Ophthalmic Hospital founded by Donders, and he became the hospital's director in 1884. In 1887, he was made a professor at Utrecht and was the first to specialize solely in ophthalmology. DONDERS' FORMULA The story of Snellen's eyechart development is interesting and perhaps a bit surprising. Like so many other inventions, Snellen relied on information discovered by others before him. In 1861, Snellen's teacher, Donders, was working on his studies of refraction and accommodation and needed a reliable and repeatable way of measuring visual acuity. For his work on vision testing, Donders created a formula that expressed visual acuity as a function of magnification. This formula compared the size of the test target object a person could see clearly at a specified distance to a standard test target object size. Instead of expressing the standard test object size in millimeters, SPOTLIGHT ON HERMAN SNELLEN THE MAN WHO STANDARDIZED ACUITY TESTING he expressed it in angular size, which was 5 minutes of arc (5/60th of 1°). Donders' formula is as follows: Visual Acuity = 1/Magnification Requirement: Magnificaiton = (Angular) size seen by the subject / (Angular) Size of the reference standard Example: Magnification Requirement: 1x Visual Acuity = 1 = 1.0 = 20/20 2x Visual Acuity = ½ = 0.5 = 20/40 4x Visual Acuity = ¼ = 0.25 = 20/80 10x Visual Acuity = 1/10 = 0.1 = 20/200 It is thought that Donders chose the 5-minute object size based on Robert Hooke's work 200 years before, who found that the human eye could distinguish two closely positioned stars separated by 1 minute of arc. This suggested that any object a person wishes to see clearly must have no less than 1 minute of arc separating its components. Donders' angular system is useful because it allows the testing to be done at any distance. For example, if a person can read X mm size letters at 20 feet, they will also be able to see 10X mm size letters at 200 feet because the angular size of both test letters at their respective distances are the same for the eye . CHARTING PROGRESS There were already a number of eye charts that had been developed at this point, none of which became accepted. The most popular one in the 1860s was Eduard von Jaegar's, in which he used increasing letter sizes he found in the Vienna State Printing House catalog. Each line on Jaegar's chart was associated with a number that related to a printing type font size (known as point size). While his "point system" was useful, his testing charts used no external standard, so it couldn't be accurately reproduced. Many others developed similar "point" vision testing charts using Jaegar's concept, but all of them used different letter sizes for each point size. BY ED D e GENNARO

Articles in this issue

Links on this page

Archives of this issue

view archives of The American Optician - Spring 2017