2. shihara color plates
In 1917, this method with test images of different colored dots that form a complete picture was developed by the Japanese ophthalmologist Shinobu Ishihara. The test is based on the fact that “normal sighted people” can recognize different motifs by differentiating between red and green on the test images than people who perceive these color differences less well due to a red-green weakness.
3. Snellen hook/E-hook
The Snellen hook eye test (named after the Dutch ophthalmologist Herman Snellen) is an eye test with the shape of a capital E, where all three bars have a length of 5 d. Both bar width and space width are 1 d each. Thus the Snellen hook is exactly as long as it is high. Since it does not have a round shape in contrast to the Landolt ring, only four positions of the “opening” are possible, namely up, left, down or right.
For this reason, the rate rate is twice as high and therefore the quality of the visual test is significantly worse than that of the Landolt rings. Just as with Landolt rings, Snellen hooks can be used to test small children and illiterate people, although not ideal. Even if the opening direction of the E-hook is not recognized, the test person may feel one side to be blacker than the other and thus arrive at the correct result. For this reason, this eye test is not intended to be a visual acuity test by an expert.