A book containing several Ishihara test charts. Invented by Japanese physician Dr Shinobu Ishihara in 1917, these charts use arrangements of coloured dots that appear random at first but contain numbers. There are four types of plate: transformation plates (people with defective vision see a different number to those with normal colour vision), vanishing plates (number only visible with normal colour vision), hidden digit plates (number only visible with defective colour vision) and diagnostic plates that test the kind and severity of deficient colour vision. The full test comprises 38 plates, although a diagnosis often requires far fewer.