Ishihara Colour Plates

Pseudoisochromatic plates
Pseudoisochromatic plates are the most famous type of color blindness test. Most people know them under the name Ishihara plates test, because Dr. Shinobu Ishihara was one of the first persons who designed a very reliable plate test, introduced in 1917. He produced different test sets. Ishihara plates are still widely used all around the world.

The copunctual points build the source for this type of color vision test. The fact that colorblind people can’t distinguish colors along the confusion lines is used to build a pattern of differently colored dots. If you are color blind you won’t spot the dots which are shifted along the confusion lines and therefore numbers, letters, lines or anything else can be hidden from you.

There exist four different type of plates:

Vanishing design: Only people with good color vision can see the sign. If you are colorblind you won’t see anything.
Transformation design: Color blind people will see a different sign than people with no color vision handicap.
Hidden digit design: Only colorblind people are able to spot the sign. If you have perfect color vision, you won’t be able to see it.
Classification design: This is used to differentiate between red- and green-blind persons. The vanishing design is used on either side of the plate, one side for deutan defects an the other for protans.
Why can colorblind people see something which is not visible for people with perfect color vision?
If you are colorblind you are not distracted by hue differences along the confusion lines. You will be more focused on lightness differences. This two different facts are used to design the hidden or invisible plates.