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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.

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    This question appears to be off-topic because it is about industrial optics. Unless this information can specially be used in still photography, it is off topic. See: Are questions about industrial optics on topic? – dpollitt Nov 7 '13 at 20:24
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    I think we can use this is still photography. I'm about to buy some filters, including an 850nm, for a consumer camera modified to pass IR. – Paul Cezanne Nov 7 '13 at 23:34
  • what are you imaging? what is the spectrum of your light source? are there any windows? what is the exposure time? have you turned off the light source to check for background illumination? see here: ledmuseum.candlepower.us/led/spectra7.htm there are some spikes up there in NIR. – Michael Nielsen Nov 8 '13 at 6:43
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I can think of exactly three possibilities:

  1. Your filter is allowing more frequencies than the specification says
  2. Your fluorescent light source actually emits some amount of light in the IR range
  3. There is actually another, small source of light which you weren't aware of

And there's basically no other logical options.

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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?

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