gothe,
The Ishihara test has a few flaws, there are a few different books but they are mostly the same 14,24,38 plate edition, as the manufacturer states 2 errors or misreading are accepted as ok, misreadings happen when the light used is not correct, its supposed to be used with a blue filter that simulates true norther daylight, but the other issue is that the books need to be replaced very few years dependent on use..
also practitioners will arrange the plates in a random format to prevent learning or any visual hint,
If you have done the test before and passed all the plates, and then did the test again on a older book, one plate might be slightly faded and will cause a misread
we had all had to take a few seconds more to look at a plate to decide if its a number 8 or 3, as one part of the 8 is darker and the other side is lighter
hope it helps if you need anything else just PM me
