The following is the minimum contrast ratio when conforming to (Level AA) for screen applications:

4.5:1

except for the following: (Level AA)

• Large Text: Large-scale text and images of large-scale text have a contrast ratio of at least 3:1;
• Incidental: Text or images of text that are part of an inactive user interface component, that are pure decoration, that are not visible to anyone, or that are part of a picture that contains significant other visual content, have no contrast requirement.
• Logotypes: Text that is part of a logo or brand name has no minimum contrast requirement.

Note. The higher the contrast does not always equal better. Users suffering from dyslexia prefer off-black text on a off-white background.