Hi there

I use a vision application to identify different objects, and their size is measured in Pixels.
I have a number of circular finds that I need to convert from pixels to MM squared.
They are 9, 40, 55, 71, 107 and 191 pixels (area).
Now, it also works out that in the future, I am going to have to find objects with area's from 3 pixels to 400 pixels. Is there a way that I can convert these figures from Pixels into MM squared?
I have measured, and a straight line distance of about 45mm is 200 pixels.
I know its a long shot, but consider it a challenge!!
Thanks in advance
Jon