Location of the pupil-iris border in slit-lamp images of the cornea

We present results for an active contour that finds the pupil-iris border in slit lamp images of the eye. Preprocessing involves producing a variance image from the original image and then locating the annulus, of a given size, which has the lowest mean variance. The centre of this annulus falls inside the pupil, giving a starting position for a discrete circular active contour (DCAC). The DCAC is moved under the influence of two forces-external and internal. The external force is based on the grey-scales immediately inside and outside of the contour, at each vertex, in both the original and variance images and pushes the vertices inwards. The internal force acts to move the contour towards a perfect polygon, δ larger than the current polygon. Repeated trials with decreasing values of δ are performed until equilibrium is reached between the two forces and the pupil/iris border has been found