Snellen hook
The Snellen-E (named after the Dutch ophthalmologist Herman Snellen ; synonym: E-hook ) is an optotype in the form of a capital Latin E, with all three bars having a length of 5 d. The bar and space widths are the same and are each 1 d. So the Snellen hook is just as long as it is high. Because, in contrast to the standard visual mark, the Landolt ring , it is not round, only four positions come into consideration, namely up, right, down or left. Therefore, the probability of a guessed result with 25 percent is also twice as high.
use
During an eye test , the test object should recognize where the opening of the Snellen-E is pointing. It is mainly used in ophthalmology to check the visual acuity of illiterate people or small children. In particularly difficult cases, the examinee is given a model of the optotype, the so-called optotype fork, which is then to be held as the sign is seen. The handling is not ideal for children, but more informative than the test with eye chart pictures, which are more likely to test the sense of form.
Even if the test specimen is not yet able to resolve the gaps, he can perceive one side to be blacker than the other. Therefore, the optotype is only used for non-expert visual acuity testing . In addition, there may be differences in the test result if the optotypes are presented “individually” or in a “row”.
A modification of the Snellen-E is the Pflüger-E-hook . In this case, the middle bar is shorter than the upper and lower bars by the extent of its thickness.
Snellen-E and Snellen hook
In the German-speaking area, the Snellen-E is often shortened and misunderstood as the "Snellen hook". This is misleading insofar as Snellen introduced two different optotypes at the time, the Snellen-E ("Snellen's E") and the Snellen hook ("Snellen hook"). The latter is also square, but has only two "prongs", i.e. H. takes the form of a C or U (or rotated versions). Edmund Landolt developed the Landolt ring from him.
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For example, Fick writes (1898, p. 339): “ Of course we took the measurements according to Snellen's method, but not with letters, nor with the three-pronged hooks ⋿ , but with the simple hooks ⊏ , which, like Steiger 1), are convincingly presented deserves preference for scientific exams. "
Or in Le Grand's textbook (1967, p. 84) it is explained: "A test object similar to the Landolt ring is the Snellen hook, in the form of a U, the opening of which can be placed in four different positions (Fig . 26). "
The confusion arises from the fact that in the older literature the knowledge of the shape of the optotypes is assumed to be known. Sloan's (1951) Fig. 7 shows "visual acuity for dark Snellen hooks on a light background", and it only follows from the context that this does not mean the Snellen E. On the other hand, the alternative designations "Snellen-E-hook" or "E-hook", the terminological use of which has been established in daily practice for decades, are clear. As long as the "E" is mentioned, there can be no confusion.
literature
- Herbert Kaufmann (Ed.): Strabismus. With the collaboration of Wilfried de Decker et al. Enke, Stuttgart 1986, ISBN 3-432-95391-7 .