This paper presents an active contour model to accurately detect pupil boundary in order to improve the performance of iris recognition systems. The contour model takes into consideration that an actual pupil boundary is a near-circular contour rather than a perfect circle. Two types of controlling force models, introduced as internal and external forces, are designed to properly activate the contour and locate it over the pupil boundary. The internal forces are designed to smooth the curve as well as to keep it close to a circular shape by pushing the contour vertices to their local radial mean. The external forces, which are responsible for pulling the contour vertices toward the pupil boundary, are designed based on a circular-curve gradient measurement with a proper angular range with respect to the contour center. In addition, an iterative algorithm has been developed in order to capture limbus and eyelids. The developed algorithm iteratively searches the limbus and eyelids boundaries and excludes the detected eyelids areas that cover the iris. Excluding the eyelids leads to a more precise search for limbus in the next iteration and the search is completed when the circular parameters of the limbus converge to fixed values. The eyelid contours are modeled as elliptic curves considering the spherical shape of an eyeball and the search is based on the expected contour in different degrees of eye openness.