For a visual stimulus, the size of detail is always expressed in terms of the angle subtended by the detail at the eye. Where the detail in question can be characterized by one dimension, e.g., the stroke width of a letter, the size is usually given in minutes of arc. Where the detail in question is characterized by two dimensions, e.g., an irregular spot, the size is expressed as the solid angle in steradians.

Original Chapter Source: Health & Performance
Provided By: Peter Boyce