Assessing Image Quality, Eyelid Occlusion, and Possibility of Artifice

The operators described above for finding an iris also provide a good assessment of ``eyeness," and of the autofocus performance of the videocamera. The normally sharp boundary at the limbus between the iris and the (white) sclera generates a large positive circular edge; if a derivative larger than a certain criterion is not detected by the searching operator using the contour integral defined in (3), then it suggests either that no eye is present, or that it is largely obscured by eyelids, or that it is in poor focus or beyond resolution. In practice the automatic identifying system that has been built continues to grab image frames in rapid succession until several frames in sequence confirm that an eye is apparently present, through large values being found by operator (3), and through large ratios of circular contour integrals being found on either side of the putative limbus boundary. Excessive eyelid occlusion is alleviated in cooperating Subjects by providing live video feedback through the lens of the videocamera into which the Subject's gaze is directed, by means of a miniature liquid-crystal TV monitor displaying the magnified image through a beamsplitter in the optical axis.

A further test for evidence that a living eye is present exploits the fact that pupillary diameter relative to iris diameter in a normal eye is constantly changing, even under steady illumination [1],[11]. Continuous involuntary oscillations in pupil size, termed hippus or pupillary unrest, arise from normal fluctuations in the activities of both the sympathetic and parasympathetic innervation of the iris sphincter muscle [1]. These changes in pupil diameter relative to iris diameter over a sequence of frames are detected by the discrete operators (4) and (3), respectively, in order to compute a ``hippus measure" defined as the coefficient of variation (standard deviation divided by mean) for the fluctuating time series of these diameter ratios. Together with the accompanying elastic deformations in the iris texture itself arising either from normal hippus or from a light-driven pupillomotor response, these fluctuations could provide a test against artifice (such as a fake iris painted onto a contact lens) if necessary in highly secure implementations of this system.