After pondering the infra-red question for a week, I decided to try and get some answers in the field. I decided to pay another visit to the sett, to observe the badgers as best as I could without using the infra-red and then, once I was sure that the badgers were comfortable and that there was nothing I was doing that was disturbing them, I would turn on the IR and observe any reactions.
It was a great plan. The problem is, to paraphrase Helmuth von Moltke, no plan survives contact with badgers. I made my way through the wood as stealthily as possible and arrived at the sett by 8.00pm. Unlike last week, when the weather was very clear, the night was quite cloudy. This meant that there was more of a glow in the sky – the horrid orange reflections of the streetlights in distant towns. This glow was enough to make it possible to use the night vision scope in passive mode, without the infra-red illuminator. There was just enough light for it to work properly – I could see trees, undergrowth and the spoil heaps of the sett.
Unfortunately I couldn’t see any badgers. I waited for 40 minutes but saw and heard nothing. If the badgers had come out I would have seen them. Maybe they were frightened by my approach, but I don’t think so. Maybe they had left already and were out foraging. Maybe they didn’t emerge until after I had left.
It was a pleasant enough evening, listening to the lambs in the field and the tawny owls in the wood behind me, but I didn’t get to test my theory. I’ll try again next week and see what happens.
Tales from the Wood - The Diary of a Badger Watching Man 
I saw this entry at the bottom of an Ebay listed 1st Gen Night Vision scope:
“A slight high-pitched whine when the unit is on. The image you see may be slightly blurry around the edges. When you turn a 1st gen off it may glow green for some time. These are inherent characteristics of a 1st gen and are normal”
Perhaps a little more evidence for the ‘sound’ theory.
Pablo.
I agree with Pablo and yourself on the Sound theory with first gen night vision gear. We used to get problems with Foxes hearing the high pitch. The Forestry Commission use military spec Kite sights now and thermal cameras on counts. I have watched Deer,Badger and Fox at very close quarters without any reaction with that gear.
I have read some studies that say Foxes won’t tolerate Red filtered light but have no problem with white light.Thats what we use here. I have used red filters on local Badgers and on Bobcat ,Raccoon,Fox and coyote in the U.S with no problem. It has a lot to do with how you use the light. I may do some testing myself this Spring.