I vaguely recall reading a while ago about how one of the limitations of active infrared systems was that the searchlight would be visible through red glass, possibly specially treated. Of course, the systems they were talking about were WWII systems, as in pretty much the first ones ever made, and by "active" it means "illuminated by an infrared searchlight (reflected light from it) because we don't have anything sensitive enough to detect anything short of that." And "searchlight" could mean as much as a foot in diameter, or possibly 6 inches. All it would let you see (I 'think') is the searchlight and not light reflected from it.
What I'm wondering is how exactly it would work, and whether it would work with more modern active infrared systems. Possibly a combination of active systems working in the near infrared and treated glass that changes the wavelength enough to be in the extreme upper end of the visible spectrum? It wouldn't give you night vision or anything, just (dimly?) reveal the active components. And possibly requiring the active system to be on the cheep side?
EDIT: I also remember the countermeasure being cheep and simple, not anything remotely like a full IR system.
Seeing (active) infared through red glass?
What I'm wondering is how exactly it would work, and whether it would work with more modern active infrared systems. Possibly a combination of active systems working in the near infrared and treated glass that changes the wavelength enough to be in the extreme upper end of the visible spectrum? It wouldn't give you night vision or anything, just (dimly?) reveal the active components. And possibly requiring the active system to be on the cheep side?
EDIT: I also remember the countermeasure being cheep and simple, not anything remotely like a full IR system.
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