How to measure the diameter of a pupil which is shrinking?

Currently working with some eye clips which feature pupil constriction. I am curious as to how one would measure the size (diameter) of the pupil as it shrinks. I would like the program or command to measure the size of the pupil by frame and input those values into an Excel file. I would appreciate any help possible!

[jonas]

You should provide an example image

[Image Analyst]

You could use image analysis. That's about all I can say until after you read this link