FB - a Hoffman Box is something that hangs on the wall and is used to check the setup of NVG - you use it in a darkened room and it has a very low light illuminated series of graduated circles and a series of Snellen charts in it.
The Snellen charts are a series of vertical and horizontal lines at different thicknesses that correspond to visual acuity - ones for 20/20, 20/25, 20/30, 20/35 etc up to about 20/50 I think (6/6, 6/9, 6/12 etc for Europe).
On a good pair of goggles, correctly adjusted, you should see 20/35 clearly.
The circles go from light to dark shading in sectors and the more sectors you can discern, the better the goggles.
I will take some photos tomorrow at work and post them.
Using the Hoffman Box sets a base level by which you can assess poor tubes and note excessive scintillation, honeycombing and black spots.