Pupil can not be extracted as a similar standard ellipse because of some interferences caused by reflections

eyelashes and eyelids

which have some difficulties to measuring pupil accurately. In order to solve the problem of eyelid occlusion

an improved method of pupil measurement under eyelid occlusion is presented. It acquires the region of interesting on pupil image and removes reflections by filtering and Sweepline based on threshold segmentation. Then

the center of the pupil is determined using inscribed parallelogram after rotating according to the dividing line for the eyelid. A proposed algorithm of center correction based on circle detection can detect the intersections of two circles

which can be used as the intersections between the pupil and eyelid in order to compute the angle of the pupil and refine the center. Furthermore

the initial parameters of the pupil ellipse obtain from the five-point method and the ellipse is optimal in the sense of non-linear least squares by minimizing the Euclidean distance between fitting ellipse and edge points. Experimental results demonstrate

this method achieves good performance in terms of robustness and accuracy and obtains accurate locations and boundaries in the case of occlusion no more than half of the pupil area.