Detecting a shape in an image

[nullschritt]

Well recently i wrote a script that compares two images, and I though.

How cool would it be to use this to compare two irises, for a kind of retinal scanner.

My problem so far is I have not yet been able to think of a way to find the iris of the eye.

What im basically trying to do is find a circle inside a circle in an image.

like the following shape

 

Then i could get the color data from the iris to compare.

any ideas?