This is a light interference filter for night vision devices commonly referred to as a LIF. This filter fits over the lens of the night vision goggles. This filter measures about 30mm.
It prevents damage from green light wavelengths - blocks green laser wavelengths completely,
It will help slightly protect some image intensifiers - usually non-autogated image intensifiers and some older gen3 image intensifiers by reducing the residual light entering the device.
This filter is compatible with the following systems ONLY. This will not fit other devices:
AN/PVS-14 - Also any variants of binocular (dual) nightvision units that use PVS14 style optics.
The small carry cases that these come in also act as a clever screw tool to thread into and out from the front of the objective lense of your device.