How to find the lower eyelid in an image?
There is a section in this paper¹ that deals with eyelid contour approximation. It should help you out.

¹ Pernek, Ákos and Hajder, Levente (2014) Precise 3D Pose Estimation of Human Faces. In: VISAPP 2014. Proceedings of the 9th International Conference on Computer Vision Theory and Applications. Vol. 3. SciTePress, Lisszabon, pp. 618-625. ISBN 978-989-758-005-5