How to calculate RMS of ring and center of dark area

Hello, I would like to seek your help. I have an ellipse beam shape. How can I calculate the RMS of ring shape and RMS of center of dark ellipse area by using matlab program ? Thanks a lot.

[Image Analyst]

What does that mean? Do you somehow want the RMS of the shape (x-y coordinates), or do you want the RMS value of the gray levels within the shape? And there are 3 or 4 ring shapes - which one are you talking about?

[Tina Hsiao]

I think I would like to know the RMS value of the gray levels within the shape. The ring shape created by the Elliptical Laguerre-Gaussian (P=0, L=5). So far as I know, the ideal beam ring shape intensity is uniform,

but the data does not uniform. I would like to know how uniform of ring intensity shape is. So I try to give a RMS as an index. Could you give me a help? Thanks a lot