Minus Blue filters for cockpit lighting compatibility
Iefan,
look for MIL-STD-3009. It should be available on the net. It will give you some idea what is it all about. Long story short, you are adapting the goggles AND the cockpit making them compatible together, by filtering the NVD and the cockpit lighting. The filter on the goggles is your "minus blue" filter.
GaAs Gen III tubes are better here, as anyway the are not very sensitive below 600 nm, and by filtering them you are loosing less light than in the Gen II case.
Look for information about Class A and B differences.
Sometimes the cockpit will not be enough (some nasty reflections from outside elements of the helicopter). Another question - just the cockpit or also the external lights?
Arrakis