  The distinctiveness of the human iris has been measured by first extracting a
set of features from the iris, an encoding, and then comparing these encoded
feature sets to determine how distinct they are from one another. For example,
John Daugman measures the distinctiveness of the human iris at 244 degrees of
freedom, that is, Daugman's encoding maps irises into the equivalent of 2 ^ 244
distinct possibilities [2]. This paper shows by direct pixel-by-pixel
comparison of high-quality iris images that the inherent number of degrees of
freedom embodied in the human iris, independent of any encoding, is at least
536. When the resolution of these images is gradually reduced, the number of
degrees of freedom decreases smoothly to 123 for the lowest resolution images
tested.
