Colour blindness test. Caps of different colours, representing the hue circle of the visual spectrum, being used to check for colour vision abnormalities. This is the Farnsworth-Munsell 100 hue test (F-M 100 hue), which is designed to detect all colour vision defects from the mildest red-green colour blindness to total achromatopsia (absence of normal cone vision). The test is also used in industry to grade employee's colour discrimination. The person being tested must place the caps in hue order in the four boxes. The boxes keep reds, greens, blues and yellows separate. Colour vision defects may be inherited or can be caused by optic injury or nerve diseases.