Wikipedia night vison googles and scroll to the bottom...
I'll post the link later but here's the jist.
Figure of Merit
Though image intensification technology employed by different manufacturers varies, from the tactical point of view night vision system is an optical device that enables vision at low light. The US government itself has recognized the fact that technology itself makes little difference as long as an operator can see clearly at night. Consequently the United States bases export regulations not on the generations, but on a calculated factor called Figure of Merit (FOM). The method of FOM calculation and its implications for export are briefly described in a National Defense University document called “The NATO Response Force” authored by Jeffrey P. Bialos, the Executive Director of the Transatlantic Security and Industry Program at the Johns Hopkins University and Stuart L. Koehl, a Fellow at the Center for Transatlantic Relations of the same university.
... beginning in 2001, the U.S. implemented a new figure of merit (FOM) system for determining the release of night vision technology. FOM is an abstract measure of image tube performance, derived from the number of line pairs per millimeter multiplied by the tube's signal-to-noise ratio.
US-made tubes with a FOM greater than 1400 are not exportable outside the US.