replacing georgian leaded double glazed units?

Hi,
several double glazed units in my house which are georgian leaded have failed and need to be replaced,
how do I measure the glass unit which is leaded to match in with the rest of the leaded units which are in the same window frame,ie; one window has 4 units 2 of which have failed and need to be exactly the same lead pattern to match in with units that are ok

Rob
July 2012