1. Field of the Invention
This invention relates to iris imaging, as may be used for biometric identification.
2. Description of the Related Art
A biometric iris imaging sensor can rapidly deliver more consistently high quality images by incorporating tip-tilt (tracking) and focus correction. At very long standoff distances, or under adverse conditions, some higher order aberration correction may also be performed. Deriving the signal to control tip-tilt and focus requires some sort of wavefront sensor (“WFS”). Sensing higher order aberrations puts more demands on the wavefront sensor and restricts design choices.
The traditional approach to wavefront sensing in these sorts of imaging applications is to carry out contrast sensing on the image to determine focus, and correlation tracking to determine tip-tilt. This approach is typically quite slow, and requires a strong signal to noise ratio for the image, which may be difficult to obtain using short exposures. The effectiveness of such techniques also depends on the contrast of the underlying image. Obtaining higher order wavefront signals from analysis of an unknown image is difficult, and requires very high signal to noise data.
In the case of biometric iris imaging we can arrange the illumination sources such that a strong glint can be produced on the surface of the cornea, and/or a strong retinal retro-reflection can be produced. In the case of the glint, the image is a diffraction-limited, or near diffraction-limited point source. By using on-axis or nearly on-axis illumination, the glint image of the illumination source appears near the center of the pupil. The glint image is produced by specular reflection from the surface of the cornea which largely maintains polarization. Due to the extreme curvature of the cornea, the glint image of the source is highly de-magnified, and will appear as a very compact or even diffraction limited source. In the case of the retinal retro-reflection the sharp edge of the iris border provides a near diffraction limited edge, which guarantees that high spatial frequency information can be measured. Both of these sources make excellent input to a wavefront sensor because they are compact with predictable high frequency contents.