1. A method for determining blood velocity in a conjunctiva of an eye, comprising: acquiring a series of temporal frames with registered images of at least one vessel in the conjunctiva of the eye, wherein the registered images are registered to compensate for eye motion; identifying at least one blood cell present in each of a plurality of frames; calculating blood velocity within the vessel based on a distance travelled by the blood cell in the vessel between frames; and compensating for involuntary target movement by tracking eye movements.