The other day I took my kids to Marwell Zoo; we went into the bat house, where it was warm and very dark indeed - the habitat was very dimly illuminated by yellowish lights, the area where the public walk was almost completely dark.
Immediately, I lost track of my two children and, as the place was quite busy, feared a little for their safety. “Not to worry”, thought I , and whipped out my keychain, upon which I have a very bright little white LED torch; aware of the fact that use of this light could cause problems for the bats, I shielded it carefully with my hand as I switched it on.
A strange thing happened; it cast hardly any light at all upon the area; I eventually tracked down the kids by voice, touch and blind luck, but even then, shining the light onto the back of their coats hardly illuminated them at all. Thinking that perhaps the batteries were just going flat, I foolishly looked into the beam, which was an experience as dazzling and painful as ever.
The light usually (both before and since) casts a moderately bright illumination on anything - I use it to find my way up the stairs each night in total darkness and I use it for inspecting various things in tight corners where the light is bad, but in the bat house, it simply didn’t seem to illuminate anything - What could possibly have caused this phenomenon?