Why is it that with my camera (an old DVT Legen) and an 850nm IR pass filter, I see light in my images even when no IR light is present? Only light source is a fluorescent light.
I can think of exactly three possibilities:
- Your filter is allowing more frequencies than the specification says
- Your fluorescent light source actually emits some amount of light in the IR range
- There is actually another, small source of light which you weren't aware of
And there's basically no other logical options.
I think this is more of a physics/science question than a photography one, but fluorescent lamps emit light in the visible spectrum, along with heat, so I'd expect you'd get lower wave lengths in the IR range, not to mention radiation from your subject. Is the light you're seeing directly from the light source, or radiated from the background or subjects?