Can you see the 74? This is an Ishihara Color Test. Its used to help diagnose colour blindness, people with certain forms of colour blindness would not be able to read the text contained in the image. The full set of 38 plates would allow a doctor to accurately diagnose the colour-perception deficiencies affecting a patient. The test is ingenious in its concept, yet remarkably simple in its execution. No complicated lenses, lighting, tools or measuring devices are required. The doctor or nurse can quickly administer the test with a simple and portable pack of cards. The Ishihara test is an end to end test. Anything, from lighting in the room, to the brain of the patient can influence the result. The examiner will endeavour minimise many of the controllable factors, such as switching off the disco lights, asking the patient to remove their blue tinted sun-glasses and maybe checking they can read normal cards (e.g. your patient might be a child.). End to end tests like this are messy...
My thoughts on developing & testing.